text:
\section{Introduction}\label{sec:intro}
The luminescence of excited rare gases (RG) is a very well known phenomenon that is mainly studied in the
 vacuum ultraviolet (VUV) range because of many
 applications including
 the production of coherent and incoherent VUV light sources \cite{rhodes1979,ledru2006} and the detection of ionizing radiation \cite{knoll}.

The luminescence of RG excimers is also interesting because of the knowledge that can be obtained about the structure and the formation dynamics of these diatomic molecules. These pieces of information are relevant in several fields of physics, including molecular--, plasma--, environmental--, and astrophysics.

Xenon is the most accurately studied gas because of its ability to efficiently convert the excitation energy in intense VUV radiation \cite{lorents1976}. Upon excitation accomplished by using several techniques, including high-energy particles \cite{arai1969,koe1974,keto1979,leichner1976,millet1978,davide1}, electrical discharges \cite{colli1954,wieme1974,sadeghi1977}, multiphotonic absorption by means of a laser \cite{broadmann1976, broadmann1977,dehmer1986,gornik1981,gornik1982,salamero1984,moutard1986,alekseev1999,ledru2006}, and synchrotron radiation \cite{broadmann1976,dutuit1978,wenck1979,bonifield1980}, three-body collisions of excited and ground-state atoms lead to the formation of excited dimers whose decay toward the dissociative ground state yields the 1st and 2nd VUV continua at approximately 150 and 170 nm.

The 1st continuum is produced at low gas pressure, $P<20\,$kPa and is originated by radiative transitions from the vibrationally excited $(0_{u}^{+})_{v^{\prime}\gg 0}$ molecular state, correlated with the resonant 6s$\,(^{3}\!P_{1})$ atomic state, toward the dissociative $0_{g}^{+}$ ground state. (We use here the notation relative to Hund's coupling case {\it c} because spin-orbit coupling in Xe is strong \cite{herzberg}.)

 By contrast, for $P\geq50\,$kPa, collisions rapidly quench the vibrational excitation of excimers and overlapping bound--free emissions from the lowest vibrationally relaxed $(0_{u}^{-}, 1_{u})$ molecular states correlated with the metastable 6s$\,(^{3}\!P_{2})$ atomic state produce the 2nd continuum \cite{dutuit1978,wenck1979}.

The appearance of the two continua in different pressure ranges is explained in terms of the different kinetics of the processes leading to excimer formation and decay. These processes have been investigated and identified in several spectral- and time-resolved experiments in which lifetimes and rate constants are determined in the low pressure range ($P\leq 0.1\,$MPa) \cite{gornik1982,leichner1976,broadmann1977,millet1978,wenck1979,bonifield1980,raymond1984,salamero1984,moutard1986,moutard1988,museur1994,alekseev1999,ledru2006}.

The structure of the lowest lying excited molecular states has been theoretically determined either with {\it ab initio}- \cite{ermler1978,jonin2002_II} or model \cite{mulliken1970} calculations, and experimentally by analyzing spectroscopic data \cite{gornik1982,keto1979,raymond1984,castex1981,koe1995}.

In spite of these efforts, the high pressure ($P> 0.1\,$MPa) features of the VUV continua in the gas have not been extensively investigated  \cite{arai1969,koe1974,lorents1976,moutard1988,morikawa1989} in spite of the clear  connection with the problem of excimer luminescence \cite{chesno1972,saxton1974,schuessler2000,bressi2000,petterson2002} and exciton formation and emission \cite{jortner1965,rice1966,raz1970,suemoto1977} in the condensed phase, which are of both practical and fundamental interest.

It has to be noted that, in addition to this (relative) lack of interest in the high-pressure features of the VUV excimer luminescence, practically no experiments are aimed at investigating the possibility that molecular transitions lying in the infrared (IR) range may occur in the cascade of processes leading to the formation of the VUV radiating states. In particular, as far as emission processes in Xe are concerned, there is only scant indication that a broad IR emission spectrum occurs in the range 765--830 nm \cite{richmann1993}, or, more precisely, in the range 780--815 nm \cite{keto1997}. This spectrum is attributed to a bound--bound transition from vibrational levels of the molecular $0_{g}^{+}$ state, correlated with the 6p$\,[1/2]_{0}$ atomic limit (in Racah notation), toward the $B\,0_{u}^{+}$ (6s$\,[3/2]_{1}$)$_{v^{\prime}\gg0}.$

The absence in literature of further mesurements of IR molecular emission spectra might be ascribed to the fact that the potential energy minimum  of higher lying bound excimer states occurs for an internuclear distance at which the weakly bound ground-state potential is strongly repulsive and, thus, these molecular states are not easily reached by multiphoton selective excitation.

On the other hand, broad-band excitation accomplished with high-energy charged particles \cite{leichner1976,davide1} may produce excited atoms with such high kinetic energy that can collide at short distance with ground-state atoms, thus yielding higher lying excimer states, though without any control over their parity.

Actually, we have recently observed for the first time a broad IR molecular emission spectrum peaked at about $\tilde \nu_{\mathrm{m}} \approx 7800\,$cm$^{-1}$ $(\lambda_{\mathrm{m}}\approx 1.3 \,\mu\mathrm{m})$ in both pure Xe gas and in a Xe(10\%)--Ar(90\%) mixture in the quite wide pressure range $0.01\,\mathrm{MPa}<P<0.9\,\mathrm{MPa}$. The luminescence is obtained by exciting the gas at room temperature with 50 ns-long pulses
of 70 keV electrons. Details can be found in literature \cite{Borghesani2001}.

The lR emission spectrum in both of the investigated gases is believed to be produced by the deexcitation of the same molecular species, namely Xe$_{2}.$ Actually, in the Ar--Xe mixture, energy transfer from excited Ar atoms to radiative states of Xe occurs readily and the excited Xe$_{2}$ dimer is produced in heteronuclear three-body reactions of the type
\[ \mathrm{Xe}^{\star} + \mathrm{Xe} + \mathrm{Ar} \to \mathrm{Xe}_{2}^{\star} + \mathrm{Ar}\]
where $^{\star}$ labels an excited state.

The features of this IR continuum are very similar to those of the 2nd VUV continuum. At low pressure, $P\approx 20\,$kPa, its full width at half maximum (FWHM) is $\mathit{\Gamma} \approx 900\,$cm$^{-1}$ and the ratio of $\mathit{\Gamma}$ to $\tilde\nu_{\mathrm{m}}$ is $\mathit{\Gamma}/\tilde\nu_{\mathrm{m}}\approx 0.115,$  to be compared with the value 0.116 of the 2nd VUV continuum \cite{koe1974,jonin1998}.

Owing to these similarities between the IR and VUV continua, we have attributed the IR emission spectrum to a transition between one bound and one dissociative level of the Xe$_{2}$ excimer which are lying higher in the energy relaxation pathway that eventually leads to the population of the excimer levels responsible of the emission of the VUV continua.

The study of the spectrum as a function of pressure has led to the discovery that the center of the band is red-shifted as the gas density increases. $\tilde \nu_{\mathrm{m}}$ decreases linearly with increasing the gas density $N$ up to a value $N\approx 2\cdot 10^{26}\,$m$^{-3}=7.5\, N_{\mathrm{ig}},$ where $N_{\mathrm{ig}} =2.65\cdot 10^{25}\,$m$^{-3}$ is the density of the ideal gas at $T=273.15\,$K and $P=0.1\,$MPa. The red-shift is larger in the pure gas than in the mixture.

The red-shift of the emission spectrum has been explained by taking into account the action of the dense environment on the excimer. The electronic structure of a homonuclear RG excimer can be described by an ionic core surrounded by an electron in a diffuse Rydberg-type orbital much larger in diameter than the internuclear distance \cite{mulliken1970,arai1978,audouard1991}.

It has been suggested \cite{Borghesani2001} that the higher-lying Rydberg-like levels of the excimer are influenced by two density-dependent effects. The first one is the dielectric screening of the Coulomb electron--nucleus interaction
because of the presence of atoms of the host gas enclosed within the electron orbit. This solvation effect leads to a reduction of the difference between the different electron energy levels, thus always producing a red-shift of the emission spectrum.

The second effect is a consequence of the wave nature of the electron. The wavefunction of the electron in the Rydberg-like orbital is quite delocalized and can be considere similar to that of a quasifree electron moving through the gas.
Thus, the wavefunction spans a region containing several gas atoms with which it interacts simultaneously. Multiple scattering then leads to a density-dependent shift of the electron energy that is positive or negative depending on the repulsive or attractive character of the electron--atom interaction \cite{fermi1934}.

This physical picture may retain its validity in a high-pressure environment provided that the optically active electron is only weakly scattered off the gas atoms. This situation surely occurs at low pressures, up to a few MPa,  for which the electron mean free path is larger than the orbit of the Rydberg state \cite{huang1978}.

 It is, therefore, interesting to investigate the evolution of a molecular Rydberg state as the density of the environment is progressively increased. This investigation might yield useful pieces of information about the changes of the molecular properties of Wannier--Mott-type impurity states in high-density liquids which have a unique parentage to the Rydberg states of an isolated molecule in a low-density gas \cite{raz1970,messing19771,messing19772} and it also may help understanding the reasons why no IR fluorescence has been
detected yet in liquefied RG's except superfluid He II \cite{dennis1969,hill1971,keto1974b}.

We have therefore extended the range of  the previous measurements \cite{Borghesani2001,Borghesani2005,borghesani2007ps} to higher pressures, up to $3.3 \,$MPa, corresponding to a gas density $N=8\cdot 10^{26}\,$m$^{-3}= 30\, N_{\mathrm{ig}},$ in order to test the validity of the model developed for lower densities.

\section{Experimental details }\label{sec:expdet}
The experimental setup has been described in a previous paper \cite{Borghesani2001}.
We briefly recall here the main features of the apparatus and the changes we have adopted in order to improve and extend the previous measurements.

A home-made electron gun is used to inject electrons into the sample cell in order to excite the gas. The chopped light beam of a Ne--Ar--F excimer laser (Optex, Lambda Physics) impinges on a gold-coated brass photocathode with a repetition rate as high as 200 Hz. The photoelectrons can be accelerated up to 100 keV by means of a d.c. HV supply (Spellman, mod. RHR120W). Typically, the electron energy is set at 70 keV.

The 50-ns long electron bunch produced by the electron gun enters the sample cell through a 25-$\mu$m thick Kapton window. The window is thin enough that a significant amount of electrons enters the cell with an acceptable energy degradation (10 to 20 keV of energy are lost in crossing the window), yet it is thick enough to withstand gas pressures as high as 4 MPa. The amount of charge injected into the gas, $0.1<Q<10\,$nC, depends on several factors including the aging of the photocathode of the electron gun, on the optimization of the magnetic deflection and focusing lenses, and so on.

The sample cell is kept at room temperature. Before being filled with the gas, it is heated up to $\approx 250\,^{\circ}$C and evacuated down to approximately $10^{-5}\,$mbar for several hours. The research grade gas is passed through an Oxisorb cartridge inserted in the filling line to increase its purity. A final impurity content of the gas is estimated to be $\leq 1\,$ppm. The gas pressure is measured by means of a calibrated Sensotec pressure gauge (STJE/1833-35). The gas density is computed by using the appropriate equations of state of Xe and Ar \cite{rabinovich1988}. The law of ideal gas mixtures is used to compute the density of the Ar--Xe mixture \cite{guggenheim}.

The IR light produced by the Xe$_{2}$ excimer deexcitation exits the cell through a quartz window and is analyzed by a FT--IR (Fourier Transfom InfraRed) Michelson-type interferometer (Equinox 55, Bruker Optics). The interferometer is operated in stepscan  mode \cite{palmer1989} because the source is pulsed. For each mirror position, the signals produced by 50 to 100 excitation events are accumulated and averaged together in order to reduce the effect of the fluctuations of the source intensity.

The interference signal is detected by an InGaAs photodiode (EG\&G, mod. G5832-05) whose output is preamplified by an active integrator of 0.25 mV/fC gain and 400 $\mu$s integration constant. The InGaAs detector shows flat responsivity in the spectral range $(0.6\leq \tilde \nu\leq1.2)\cdot 10^{4}\,$cm$^{-1}$ that is adequate for low to moderate densities $N< 4\cdot 10^{26}\,$m$^{-3}.$

For larger densities, the density-dependent red-shift is so large that the excimer band moves outside the range of the detector. Therefore, we have extended the range to be investigated by using a liquid N$_{2}-$cooled InSb photodiode detector (EG\&G, mod. J10D--M204--R02M) whose sensitivity extends farther down to approximately $1800\,$cm$^{-1}.$ In this case, a transimpedance amplifier (EG\&G, mod. PA--9) is used because of the very large output capacitance of the InSb photodiode.

The output signal is fed to a shaper amplifier (EG\&G, mod. Ortec 575A) with 3 $\mu$s shaping time in order to reduce electrical noise. Finally, the output signal is digitized by a a 16-bit A/D converter and stored in a personal computer for offline analysis. Several spectra, typically ten of them, are recorded for each density and averaged together in order to further improve the quality of the measurement. All of the spectra shown in the next pictures are thus averages over 10 single spectra.

\section{Experimental results and discussion}\label{sec:res}
In this section we present the experimental results. Although the present paper deals with the behavior of the IR excimer continuum as a function of the density, especially at higher densities than previoulsy measured, nonetheless it is worth recalling here briefly the results obtained at low density \cite{borghesani2007jpb}. Actually, the recent publication of accurate potential energy curves for Xe excimer states higher in energy than the first ones \cite{jonin2002_II} has allowed to unequivocally assign the molecular states involved in the deexcitation process investigated in this paper by analysing the spectrum recorded at low density. This piece of information is required in the following when the model for the density red-shift is developed. Thus, we first show and discuss the spectrum recorded at low density.

\subsection{IR emission spectrum at low density: Identification of the molecular states}\label{subsec:lowN}

The IR emission spectrum recorded in pure Xe gas at room temperature for a pressure $P\approx 0.1\,$ MPa is shown in Fig. \ref{fig:spclowN}. The corresponding gas density is $N\approx 2.45 \cdot 10^{25}\,$m$^{-3}.$
 A detailed analysis of this spectrum can be found in literature \cite{borghesani2007jpb}.
\begin{figure}[htbp]
 \begin{center}
 \includegraphics[angle=90,width=\columnwidth]{SpectrumShapeInPureXe.eps}
\caption{\small High-resolution IR emission spectrum of Xe$_{2}$ excimers in pure Xe gas for $T\approx 300\,$K and $P\approx 0.1\,$MPa. Xe I lines are numbered. Thick line: Franck-Condon calculation.\label{fig:spclowN}}
 \end{center} \end{figure}
Here, we recall the main features of the spectrum and sketch a brief outline of its analysis by quoting the main results.

 This spectrum has been recorded at high resolution (16 cm$^{-1}$) in order to detect atomic lines that help ascertain the relationship between the excimer states and their parent atomic states.

The excimer contribution to the spectrum is the continuous structure displayed in the center of Fig.\ref{fig:spclowN} and is surrounded by several Xe I atomic lines that can be easily identified by inquiring the database of atomic spectra at the NIST website \cite{NIST}.
The assignment of the atomic Xe I lines is summarized in Tab. \ref{tab:correspondence} in which the electronic states involved in each transition are reported along with the proper reference.
\begin{table}
\caption{\label{tab:correspondence}\small Xe I lines assignment}
\begin{ruledtabular}
\begin{tabular}{lcr}
Line&Electronic states&Reference\\
\hline
  1 & 6p--5d &\cite{humphreys1973}  \\
  2 & 6p--5d  &\cite{humphreys1973}  \\
  3 & 6p--5d  &\cite{humphreys1973}  \\
  4 & 6p--5d  &\cite{hepner1961}  \\
  5 & 6p--5d  &\cite{hepner1961}  \\
  6 & 6p--5d  &\cite{hepner1961}  \\
  7 & 6p--5d  &\cite{hepner1961}  \\
  8 & 6p--5d  &\cite{mishra2000}  \\
  9 & 7s--6p  &\cite{humphreys1952}  \\
  10 & 7s--6p  &\cite{humphreys1952}  \\
  11 & 7s--6p&\cite{humphreys1952}  \\
  12 & 7s--6p  &\cite{humphreys1973}  \\
  13 & 7s--6p  &\cite{meggers1935}  \\
 14 & 6p--6s  &\cite{meggers1935}  \\
 15 & 6p--6s  &\cite{meggers1934}  \\
 16 & 6p--6s  &\cite{meggers1934}  \\
  17 & 6d--6p, 6p--6s  &\cite{humphreys1933}  \\
 18 & 6p--6s  &\cite{meggers1934}  \\
 19 & 6p--6s  &\cite{meggers1934}  \\
 20 & 6p--6s  &\cite{meggers1934}  \\
 21 & 6p--6s  &\cite{meggers1934}  \\
\end{tabular}
\end{ruledtabular}
\end{table}

Lines 1 through 8 are emitted in transitions between excited 6p and 5d electronic states of atomic Xe. In particular, line 1 is the very well known Xe I line appearing at $\lambda =3.51\,\mu$m \cite{vallee1981}. Lines 9 through 13 stem from 7s--6p transitions. These are the lines closest to the excimer band. Line 13 quite overlaps the excimer spectrum.
Lines 14--16 are due to 6p--6s transitions, whereas line 17 is an unresolved 6p--6s and 6d--6p doublet. Finally, lines 18 through 21 are  related to 6p--6s transitions.

 It appears evident from the low density spectrum that atomic states in the 6p manifold are efficiently populated in the energy degradation processes occurring in the gas after excitation by electron impact. The crowding of lines related to atomic 7s--6p transitions in the region of the excimer band suggests that the upper bound molecular state is related to the 6p atomic limit. In particular, line 13 stems from a transition from the atomic 7s$\,[1/2]^{\mathrm{o}}$state to the 6p$\,[1/2]_{0}$ state, thus leading us to reasonably assume that the bound excimer state is related to this atomic limit.

 The experimental observation of three-body quenching of the 6p$\,[1/2]_{0}$ state confirms the hypothesis that a molecular bound state can be formed from this atomic limit by three-body collision-induced association \cite{bowering1986}. The molecule can then predissociate
toward the 5d$\,[1/2]_{1}$ atomic state with a characteristic time $\tau_{p}\approx 10^{-10}\,$s \cite{dehmer1986,museur1994}, but the mean time between collisions at the density of the experiment is so short $\tau_{c}\leq 10^{-11}\,$s \cite{Borghesani2001} as to lead to a quick electronic relaxation and stabilization of the excimer.

Moreover, the excimer population rapidly attains thermal equilibrium with the surrounding gas because the excimer lifetimes for radiative decay $\tau_{1}$ and for vibrational relaxation $\tau_{2}$ are estimated to be much longer than the mean time between collisions. Actually, though unknown for the state observed in the present experiment, such lifetimes can be assumed to have values similar to those of the VUV radiating states, namely, $\tau_{1}\approx 5\,$ns for the $0_{g}^{+}$ \cite{moutard1988} state and $\tau_{1}\approx 40\, $ns  for the $(0_{u}^{-}, 1_{u})$ states \cite{keto1979}, whereas the decay rate for vibrational relaxation $k_{2}=6.47 \cdot 10^{-17}\, $m$^{3}/$s \cite{salamero1984} yields a lifetime $\tau_{2}\approx 0.65\,$ns at the density $N=2.45\cdot 10^{25}$ at which the spectrum reported in Fig. \ref{fig:spclowN} is measured.

The identification of the molecular states involved in the bound--free transition yielding the observed spectrum can be accomplished by carrying out traditional Franck--Condon calculations \cite{herzberg}.

Recently, Jonin {\it et al}.  published theoretical potential energy curves for higher excited excimer states \cite{jonin2002_II}.
The choice of the ungerade $(3)0_{u}^{+}$ state, correlated with the 6p$\,(^{1}\!D_{2}) $ atomic  limit, as the bound molecular state, and of the gerade $(1)0_{g}^{+}$ state, correlated with the 6s$\,(^{3}\!P_{1})$ atomic limit,  as the dissociative state, fulfills all the required selection rules and allows to simulate a spectrum shape that neatly fits the experimental one \cite{borghesani2007jpb}. Henceforth, let us term $V_{u}$ and $V_{g}$ the potential energy of the upper and lower states, respectively.

In the centroid approximation \cite{herzberg,telli}, the spectrum intensity is given by
\begin{eqnarray}
 I& \propto &\sum \limits_{v^{\prime} J^{\prime}}\mathrm{e}^{-\beta E_{v^{\prime}\!J^{\prime}}} \left\{ \left( J^{\prime} + 1 \right) \Big\vert\langle\epsilon^{\prime\prime},J^{\prime} + 1 \vert v^{\prime},J^{\prime}\rangle\Big\vert^{2} \right.
 \nonumber \\
 & +&\left.
 J^{\prime}
  \Big\vert\langle\epsilon^{\prime\prime},J^{\prime}-1\vert v^{\prime},J^{\prime}\rangle\Big\vert^{2}\right\}\tilde\nu^{4}
  \label{eq:fcspc}\end{eqnarray}
Primed quantities refer to the upper bound state whereas doubly primed ones refer to the lower dissociative state. $\beta^{-1} = k_{\mathrm{B}}T,$ where $k_{\mathrm{B}}$ is the Boltzmann constant and $T$ is the gas temperature.
$E_{v^{\prime}\! J^{\prime}}$ are the rovibrational energy eigenvalues of $V_{u}.$   $\vert v^{\prime},J^{\prime}\rangle $ are the rovibrational eigenfunctions of $V_{u},$ described by the vibrational and rotational quantum numbers $v^{\prime}$ and $J^{\prime},$ respectively. $\vert\epsilon^{\prime\prime},J^{\prime\prime}\rangle$ are scattering states of kinetic energy $\epsilon^{\prime\prime}$ and angular momentum $J^{\prime\prime}$ in the vibrational continuum of the dissociative potential $V_{g}.$
$\tilde \nu$ is the emission wavenumber, i.e., $hc\tilde \nu$ is the energy of the photons emitted in the electronic transition from $V_{u}$ to $V_{g}.$ As usual, $h$ and $c$ are the Planck's constant and the speed of light, respectively.
In Eq. \ref{eq:fcspc} the selection rule $J^{\prime} -J^{\prime\prime}=\pm 1$ has been used. Details of the numerical calculations can be found in literature \cite{borghesani2007jpb}.

In Fig. \ref{fig:spclowN} the simulated spectrum shape is represented by a thick solid line. The good agreement between experiment and numerical simulation lends credibility to the proposed assignment of the molecular states.

Although several rovibrational of $V_{u}$ are thermally excited, nonetheless the main contribution to the spectrum intensity comes from the rovibrational ground state. Thus, roughly speaking, the central wavenumber $\tilde\nu_{\mathrm{m}}$ of the spectrum corresponds to the difference $V_{u}-V_{g}$ averaged over the spatial extension of the wavefunction of the vibrational ground state of $V_{u}$
\begin{equation}
\tilde \nu_{\mathrm{m}} \simeq \langle \left(V_{u}-V_{g}\right)\rangle/hc
\label{eq:numvave}\end{equation}
This piece of information will be used in the following to intepret the density dependence of the spectrum features.
 \subsection{Density dependent red--shift of the spectrum}\label{sec:alienati}
As the gas pressure is increased, the IR emission spectrum of the excimer Xe$_{2}$ changes drastically.
In Fig. \ref{fig:XEN} three spectra recorded in pure Xe gas for $P=$0.1, 1, and $2\,$MPa, respectively, are shown.
 \begin{figure}[htbp]
 \begin{center}
 \includegraphics[angle=270,width=\columnwidth]{Xe1e10e20barhighres.eps}
\caption{\small High-resolution ($16\,$cm$^{-1}$) spectra recorded in pure Xe gas for $P=0.1,$ 1, and $2\,$MPa. The thick solid lines are interpolating curves drawn for visual help. \label{fig:XEN}}
 \end{center} \end{figure}

\noindent Except for the disappearance of some atomic lines, two main effects can be easily observed  as a consequence of the pressure increase. The first one is a strong red-shift of the center of gravity of the spectrum, whereas the second one is its broadening. In pure Xe gas, shift and broadening are so large as to make the excimer spectrum increasingly overlap the group of atomic lines numbered as 1 through 8 of Fig. \ref{fig:spclowN}. Moreover, for $P\geq 0.8\,$MPa, approximately, the InSb detector is mandatorily required in order to record the spectrum in its entirety.

The same qualitative effects occur in the Ar--Xe mixture although their entity is much smaller, as can be observed in Fig. \ref{fig:ARXEN}. Also in this case, the spectrum shifts to the red side and broadens with increasing gas pressure. However, the amount of shift and broadening is far smaller than in the case of pure Xe.
 \begin{figure}[!t]
 \begin{center}
 \includegraphics[angle=270,width=\columnwidth]{ArXe1e10e20barlowres2.eps}
\caption{\small Spectra recorded in the Ar(90\%)--Xe(10\%) mixture for $P=0.1,$ 1.15, and $2.05\,$MPa. The resolution is 100 cm$^{-1}.$ \label{fig:ARXEN}}
 \end{center} \end{figure}
The three spectra reported in this figure can be directly compared with those shown in Fig. \ref{fig:XEN} because the corresponding pressures are nearly the same. The only difference between the spectra of Fig. \ref{fig:XEN} and Fig. \ref{fig:ARXEN} is the experimental resolution, $16\,$ cm$^{-1}$ in the former figure, $100\,$cm$^{-1}$ in the latter one.

Thus, the effect of a changing environment is very evident. Whereas the increase of density is expected to result in a higher collision frequency that reduces the lifetime of the excimer species and increases the width of the continuum, such large a density dependent red-shift of the spectrum is not expected on the basis of the theories of pressure broadening of atomic lines in the impact approximation \cite{margenau1936,jablonski1945,foley1946,anderson1949,chen1957,baranger1958c,byron1964}. At the same time, the red-shift difference between the pure gas- and the mixture case is hardly explained at all.

In this section we will deal with the density dependent red-shift. The wave number of the maximum of the excimer band, $\tilde \nu_{\mathrm{m}},$
is used for its quantitative characterization. $\tilde \nu_{\mathrm{m}}$ has been determined by locally fitting a cubic polynomial to the spectrum near its maximum. A cubic polynomial has been chosen instead of a parabola in order to better locate the spectrum maximum because of the slight asymmetry of the spectrum shape in its neighborhood. $\tilde\nu_{\mathrm{m}}$ is plotted as a function of the gas density $N$ in Fig. \ref{fig:XeArXeshift}.
 \begin{figure}[htbp]
 \begin{center}
 \includegraphics[angle=270,width=\columnwidth]{XeArXeshift.eps}
\caption{\small $\tilde\nu_{n} $ vs $N.$ Closed points: Ar--Xe mixture. Open points: pure Xe gas. Dashed lines: linear fit to the data. Solid lines: prediction of the theoretical model developed in the text.\label{fig:XeArXeshift}}
 \end{center} \end{figure}

The error bars on the data points are an estimate of the uncertainty with which the values $\tilde\nu_{\mathrm{m}}$ are determined by the fitting procedure we adopted.

 $\tilde \nu_{\mathrm{m}}$ strongly decreases to smaller wavenumber values with increasing $N$ in both the pure gas and in the mixture though the effect, as anticipated, is far stronger in the pure gas than in the mixture. The density dependence of $\tilde\nu_{\mathrm{m}}$ is linear in both types of gas up to the highest investigated density $N=8\cdot 10^{26}\,$m$^{-3}=30 N_{\mathrm{i.g.}}.$ The present measurements more than double the density range explored in the previous experiments \cite{Borghesani2001,Borghesani2005}. In the region for $N\leq 4 \cdot10^{26},$ common to all of the experiments, the present data agree perfectly with the previous ones. The latter have not been shown in the picture only for the sake of graphical clarity.

The behavior of $\tilde \nu_{\mathrm{m}}$ as a function of the density $N$ can accurately be described by the linear relationship
\begin{equation}
\tilde\nu_{\mathrm{m}}= \tilde\nu_{\mathrm{m},0} -A N
\label{eq:fitlin}\end{equation}
Such linear fit is represented by dashed lines in Fig. \ref{fig:XeArXeshift} and
the values of the fitting parameters are reported in Tab. \ref{tab:fitpar}
\begin{table}[!b]
\caption{\label{tab:fitpar}\small Fitting parameters of the red-shift}
\begin{ruledtabular}
\begin{tabular}{lcr}
Gas&$\tilde\nu_{\mathrm{m},0}$ (cm$^{-1}$)&A ($10^{-22}\,$m$^{2}$)\\
\hline
  Xe & $7829 \pm 26$ & $2.08\pm 0.04 $ \\
 Ar--Xe & $7864\pm 13$ &$ 0.63\pm0.02$\\
\end{tabular}
\end{ruledtabular}
\end{table}

 Within the experimental accuracy the zero-density limit of $\tilde\nu_{\mathrm{m}}$ is the same in both the pure gas and the mixture. This means that the molecular species Xe$_{2},$ which is responsible for the IR emission in the pure gas, is also produced in the mixture. This fact is not unexpected as it is well known that excitation is readily transferred from the species of larger ionization potential (Ar) to the species of smaller ionization potential (Xe) \cite{galy1992}. In this way, the electrons injected into the mixture primarily excite Ar atoms that transfer their excitation to Xe atoms. Then, though low, the concentration of Xe atoms is sufficiently high to make their
collisions with ground-state Xe atoms highly probable, thus leading to a relevant production of the Xe$_{2}$ excimer.

The most striking feature exhibited by $\tilde\nu_{\mathrm{m}}$ is its large negative slope as a function of the density whose absolute value is a couple of orders of magnitude larger than the density dependence of the shift (either blue or red) of atomic lines \cite{laporte1975,raymond1984,moutard1986,alford1992}. The slope in the pure gas is more than three times larger than in the mixture.

These experimental observations can be rationalized by a simple model previously developed by us \cite{Borghesani2001} in which two differents effects are taken into account. The description of both of them is based on the assumption that the electronic structure of an excited state of a homonuclear diatomic molecule is quite accurately described by an ionic core and an electron in a diffuse Rydberg orbital much larger than the internuclear distance \cite{mulliken1970}. Such a state can exist also in a high-pressure environment provided that the Rydberg electron is only weakly scattered off the atoms of the host gas. This condition is fulfilled for the pressures of the present experiments because the mean free path of quasifree electrons is several nanometers long \cite{huang1978}, i.e., much longer than the radius of the orbit of the Rydberg electron.
\paragraph{Dielectric screening, or solvation, effect}
The first effect is the dielectric screening of the Coulomb interaction between the electron and the ionic core because the molecule is immersed in a dielectric medium. It is assumed that the Rydberg electron is so greatly delocalized that many atoms of the host gas are encompassed within its orbit. They are polarized by the strong field of the ionic core and, hence, they reduce the Coulomb interaction between electron and core. If the gas density is large enough the dielectric screening effect can be treated within a continuum approximation through the use of the (density dependent) dielectric constant $K(N)$ of the medium.

It has been shown in the case of the hydrogen molecular ion H$_{2}^{+}$ \cite{herman1956} that the electron energy eigenvalues, which can be considered as the potential energy curves for the nuclear motion in the Born--Oppenheimer approximation \cite{haken,atkins}, are reduced by a factor $K^{-2}$ if the ion is immersed in a dielectric medium. This result can be easily generalized for any interaction potentials of electrostatic nature acting on the optically active electron that can be expressed in terms of a multipolar expansion.

 In particular, in the present case, the potential energy curves of the initial (bound) molecular state $(3)0_{u}^{+},$ $V_{u},$ and of the final (dissociative) state $(1)0_{g}^{+}, $ $V_{g},$ must be divided by the square of the dielectric constant. Hence, also their difference is reduced by the same factor.

 As a consequence of the dielectric screening, the wavenumber of the photon emitted in the transition between the initial and final electronic states of the excimer is lowered as the density is increased
 \begin{equation}
\tilde\nu_{\mathrm{m}}= \frac{\langle \left(V_{u}-V_{g}\right)\rangle}{hcK^{2}\left(N\right)}\approx
\tilde\nu_{\mathrm{m},0}\left(1-\frac{2\alpha}{\epsilon_{0}}N\right)
\label{eq:solveff}\end{equation}
where $\alpha$ is the atomic polarizability of the atoms of the host gas. $\alpha = 4.45\cdot 10^{-40}\,$F$\cdot$m for Xe and  $\alpha = 1.827\cdot 10^{-40}\,$F$\cdot$m for Ar \cite{maitland}. $\epsilon_{0}$ is the vacuum permittivity. $K$ can be calculated as a function of $N$ by using the Lorentz--Lorenz formula $(K-1)/(K+2)=(\alpha/3\epsilon_{0})N.$ Finally, in Eq. \ref{eq:solveff} we have Taylor expanded $K$ to leading order in $N$ with a relative error $<0.04\% $ at the highest density.

$\tilde\nu_{\mathrm{m},0}\approx 7800\,$ cm$^{-1}$ is determined by a linear fit to the data.
Thus, the contribution of this solvation effect to the slope of $\tilde\nu_{\mathrm{m}}$ amounts to $2\tilde\nu_{\mathrm{m},0}\alpha/\epsilon_{0}\approx 7.8\cdot 10^{-23}\,$m$^{2}$ for Xe and to $3.2\cdot 10^{-23}\,$m$^{2}$ for the mixture.  Note that, in the case of the Ar--Xe mixture, we have used the value of polarizability
relative to Ar. Actually, the Xe concentration in the mixture $(\approx 10\,\% )$ is high enough for allowing the formation of the excimer but is sufficiently low for the newly formed Xe$_{2}$ species to be surrounded, on average, by Ar atoms only.

For both gases, this contributiondue to dielectric screening only accounts for approximately 50\% or less of the observed slope. The rest can be accounted for by including the quantum multiple scattering effect.

\paragraph{Quantum multiple scattering effect}
The second, density-dependent effect that influences the slope of the center of gravity of the spectrum is a quantum multiple scattering effect. Its quantitative description is due to Fermi \cite{fermi1934} who explained the density-dependent shift of the high spectroscopic terms of the absorption lines of alkali vapors in a buffer gas \cite{amaldi1934}.

The optically active electron is assumed to be loosely bound to the ionic core and it is treated as if it were moving through the gas like a quasifree electron. Its wavefunction is thus quite delocalized and spans a region containing several atoms of the surrounding gas.

In this situation, the simultaneous interaction of the electron with many atoms gives origin to multiple scattering effects that lead to a density-dependent shift $V_{0}(N)$ of the electron ground-state energy. The shift depends on the electron--atom scattering properties, namely, the scattering length $a,$ and for not too high densities is proportional to the density itself
\begin{equation}
V_{0}=\frac{2\pi\hbar^{2}}{m}Na
\label{eq:v0}\end{equation}
Here, $m$ is the free electron mass and $\hbar$ is the reduced Planck's constant. More recent calculations have confirmed the validity of the Fermi's result up to moderate values of $N$ \cite{springett1968,plenckiewicz1986,boltjes1993}.

This contribution must clearly be added only to the electronic energy of the upper bound molecular state because the electronic wavepacket necessarily collapses on either  atoms of the molecule during the transition followed by dissociation, Hence, as a function of $N,$ the electronic energy of the bound state changes in the following way
\begin{equation}V_{u}\to \frac{V_{u}}{K^{2}(N)}+V_{0}(N)
\label{eq:vuff}\end{equation}
If the electron--atom interaction is attractive, $a<0$ and $V_{0}<0,$ leading to a red-shift of the spectrum. The opposite behavior (a blue-shift) is shown by gases whose interaction with the electron is repulsive, yielding $a>0$  and $V_{0}>0$ \cite{reininger1982,reininger1983,kohler1987}.

We are finally led to the following expression for the energy of the IR photon emitted in the center of the spectrum in the transition between the two electronic states of the molecule
\begin{eqnarray}
 \tilde\nu_{\mathrm{m}}& =&\frac{\langle\left(V_{u}-V_{g}\right)\rangle}{hcK^{2}(N)} + \frac{V_{0}(N)}{hc} \nonumber\\
  &\approx  &\tilde\nu_{\mathrm{m},0}\left( 1 - \frac{2\alpha}{\epsilon_{0}}N\right) + \frac{\hbar}{mc}Na\nonumber\\
 &=&\tilde\nu_{\mathrm{m},0} - \left(  \tilde\nu_{\mathrm{m},0} \frac{2\alpha}{\epsilon_{0}} -  \frac{\hbar a}{mc}
 \right)N
 \label{eq:cicci}\end{eqnarray}
This model predicts a (negative) linear density dependence of $\tilde\nu_{\mathrm{m}},$ as experimentally observed up to $N=8\cdot 10^{26}\,$m$^{-3},$ with theoretical slope $A_{\mathrm{th}}$
given by
\begin{equation}
A_{\mathrm{th}}=\left(  \tilde\nu_{\mathrm{m},0} \frac{2\alpha}{\epsilon_{0}} -  \frac{\hbar a}{mc}
 \right)
\label{eq:}\end{equation}
By using literature values of the scattering length, namely $a=-3.09 \cdot 10^{-10}\,$m for Xe and, for the reasons previously explained, $a=-0.86\cdot 10^{-10}\,$m for Ar \cite{zecca}, we obtain the values $A_{\mathrm{th}}= 1.98\cdot 10^{-22}\,$m$^{2}$ for Xe and $A_{\mathrm{th}}= 0.66\cdot 10^{-22}\,$m$^{2}$ for the Ar--Xe mixture. These values are in very close agreement with those of the linear fit of the experimental data  reported in Tab. \ref{tab:fitpar}.
The solid straight lines in Fig. \ref{fig:XeArXeshift} represent Eq. \ref{eq:cicci}. This figure confirms the excellent description that the present model gives of the experiment.

It is worth noting that the two contributions to the overall spectral shift may not act in the same direction. Actually, the dielectric screening always reduces the strength of the Coulomb interaction of the Rydberg electron with the ionic core, thus always producing a red-shift of the spectrum.

By contrast, the shift of the ground-state energy of a quasifree electron, $V_{0},$ may be either negative or positive, depending on the sign of the scattering length. Whereas $V_{0}<0$ in gases for which the electron--atom interaction is attractive, $V_{0}>0$ for repulsive gases, such as Ne and He.

We, indeed, observe that the Xe$_{2}$ emission spectrum is less red-shifted in Ar than in Xe, mainly because $V_{0}$ in Ar \cite{tauchert1977,borghesani1991} is less negative than in Xe \cite{tauchert1977}.  We thus expect that the shift would practically be absent in a Xe--Ne mixture because the electron--Ne scattering length is small but positive \cite{omalley1980hb}, yielding a positive though small contribution to the electron energy that nearly compensates the red-shift due to dielectric screening. Furthermore, in a He--Xe mixture the electron--He scattering length is so large and positive \cite{omalley1963}, yielding $V_{0}\approx 1\,$eV \cite{sommer1964}, and the atomic polarizability is so small \cite{maitland} that the multiple scattering contribution should overcompensate the dielectric screening, thus producing an overall blue-shift of the excimer emission spectrum.

In some sense, the observation of the density-dependent shift of the IR excimer emission spectrum could be considered as a technique for measuring the density-dependent shift of the ground-state energy of a quasifree electron in a dense gas in alternative to other methods \cite{reininger1990,reininger1991}.

 \subsection{Density--broadening of the excimer band}\label{sec:brodo}
This section is devoted to the discussion of the other relevant feature exhibited by the excimer emission spectrum as the gas density is increased, namely, the large broadening of the molecular continuum that can be observed in Fig. \ref{fig:XEN} and in Fig. \ref{fig:ARXEN}.

The spectrum shape is not as simple as that of an atomic line but is rather given by the complicated convolution of the contributions of several Franck--Condon factors expressed by Eq. \ref{eq:fcspc}. Moreover, it is quite asymmetric, though its asymmetry is reduced as the density is increased. For these reasons there is no easy way to quantify the spectrum broadening.
In spite of this, we characterize it by measuring its half width at half maximum (HWHM), $\mathit{\mathit{\Gamma}},$ as a function of $N.$
The raw values of the HWHM directly determined from the spectrum, $\mathit{\mathit{\Gamma}}_{\mathrm{exp}},$ are corrected for the width of the instrumental function, $\mathit{\mathit{\Gamma}}_{\mathrm{ins}},$ in the usual way
\begin{equation}
\mathit{\mathit{\Gamma}} = \left[\mathit{\mathit{\Gamma}}^{2}_{\mathrm{exp}}-\mathit{\mathit{\Gamma}}^{2}_{\mathrm{ins}}\right]^{1/2}
\label{eq:rawgamma}\end{equation}
In Fig. \ref{fig:brodo} we report $\mathit{\mathit{\Gamma}}$ as a function of $N.$

 \begin{figure}[htbp]
 \begin{center}
 \includegraphics[angle=270,width=\columnwidth]{cleanGammas2.eps}
\caption{\small HWHM $\mathit{\Gamma}$ of the IR excimer emission spectrum as a function of $N.$ Open points: pure Xe gas. Closed points: Ar--Xe mixture.\label{fig:brodo}}
 \end{center} \end{figure}

\noindent In the whole density range explored $\mathit{\mathit{\Gamma}}$ is a linear function of $N$ for both types of gas and is well fitted to a straight line
\begin{equation}
\mathit{\Gamma} = \mathit{\Gamma}_{0} + \gamma N
\label{eq:fitbrodo}\end{equation}
The values of the fitting parameters are reported in Tab. \ref{tab:fitbrodopar}.
\begin{table}[!hb]
\caption{\label{tab:fitbrodopar}\small Fitting parameters of the spectrum broadening}
\begin{ruledtabular}
\begin{tabular}{lcr}
Gas&$\mathit{\mathit{\Gamma}}_{0}$ (cm$^{-1}$)& $\gamma$ ($10^{-22}\,$m$^{2}$)\\
\hline
  Xe & $446 \pm 9$ & $3.22\pm 0.03 $ \\
 Ar--Xe & $443\pm 6$ &$ 0.52\pm0.02$\\
\end{tabular}
\end{ruledtabular}
\end{table}

Within the experimental accuracy, the low-density limit $\mathit{\mathit{\Gamma}}_{0}$ is the same in both the pure gas and in the mixture. This observation further confirms the conclusion that the same molecular species is responsible of the IR emission in the two gases. By contrast, the slope $\gamma$ is approximately 6 times larger in the pure gas than in the mixture.

Three questions, thus, are to be addressed: the absolute values of the width of the IR continuum, its linear dependence on the gas density, and, finally, the difference between the two gases.

What
can be deduced from Eq. \ref{eq:fitbrodo}? First of all, as the IR continuum is originated in a bound-free transition, $\mathit{\Gamma}_{0}$ is related to the energy range available to the final state. It is roughly determined by the steepness of the potential energy curve of the dissociative state and by the spatial extension of the wavefunction of the vibrational ground state of the bound potential. In this sense, $\mathit{\Gamma}_{0}$ does not give any further pieces of information in addition to those gathered in simulating the spectrum shape at low density.

More interesting, by contrast, is the linear dependence of the spectrum width on the gas density that suggests that binary collisions are responsible of the increase of the decay rate of the excimer. Pressure broadening theories have been developed in the impact approximation to account for the effect of foreign perturbers on spectral lines \cite{margenau1936,jablonski1945,foley1946,anderson1949,chen1957,baranger1958a,baranger1958b,baranger1958c,byron1964}. All of them lead to the prediction of an increase of the spectral width, $w,$ that is linear in the gas density
\begin{equation}
w=A N{\bar v} \sigma_{0}
\label{eq:w}\end{equation}
where $A$ is a numerical constant of order unity \cite{baranger1958a}. $\bar v \propto T^{1/2}$ is a thermal velocity of the perturbers and $\sigma_{0} $ is the total cross section (or, optical collision cross section). If $N$ is measured in m$^{-3},$ $\bar v$ in m/s, and $\sigma_{0}$ in m$^{2},$ $w$ is expressed in s$^{-1}.$ $N\sigma \bar v$ is the scattering rate.

Even neglecting the fact that these theories are concerned with spectral lines rather than with a continuum, it is not clear if they are applicable in the present case. Actually, they have been derived under the so called impact approximation, according to which the interatomic separation must be much larger than the atomic diameter. Differently stated, this condition means that the mean free time between collisions must be much longer than the time of the collision itself or, equivalently, that the men free path is much longer than the atomic diameter.

In the conditions of the present experiment, the lowest gas density is of the order of $N_{\mathrm{min}}\approx 2\cdot 10^{25}\,$m$^{-3},$ whereas the highest is $N_{\mathrm{max}}\approx 8\cdot 10^{26}\,$m$^{-3}$.
 Let us assume a typical density of order $\tilde N = 10^{26}\,$m$^{-3}.$ A rough estimate of the average interatomic distance is $d\sim \tilde N^{-1/3}\approx 20\,$\AA, which reduces to $d\approx 10\,$\AA\ at the highest density. $d$ must be compared with the atomic radii of Xe and Ar, and with the size of the Xe$_{2}$ molecule.

 Several values can be found in literature for the hard-sphere radii $\rho_{\mathrm{Xe}},$ $\rho_{\mathrm{Ar}},$ and $\rho_{\mathrm{AX}}$ of the Xe--Xe,  Ar--Ar, and Ar--Xe interaction, respectively. Namely, $(4.26< \rho_{\mathrm{Xe}}<4.42)\,$\AA\  for Xenon \cite{bobetic1983,maitland1973,maitland1973b}, $(3.7<\rho_{\mathrm{Ar}}<3.8) \,$\AA\  for Ar \cite{maitland,maitland1973}, and $(3.98<\rho_{\mathrm{AX}}<4.11) \,$\AA\  for the Ar--Xe pair  \cite{marks1988}. The internuclear distance in the $(3)0_{u}^{+}$ bound potential of the Xe$_{2}$ molecule is $R_{e^{\prime}}\approx 3.23\,$\AA\ \cite{jonin2002_II}. Thus, it is questionable whether such theories might be applicable at the densities of the present experiment because the interatomic distance is comparable with the atomic or molecular radii.

 However, in a less sophisticated approach, we can resort to using the old Lorentz theory \cite{lorentz1906} in which atoms are assumed to behave as hard-spheres. When a perturber collides with a radiating atom, the radiation process is interrupted completely (or the phase is changed by an arbitrary amount \cite{foley1946}), the vibration energy being converted into kinetic energy. This theory actually predicts the same dependence of the broadening on the density as given by Eq. \ref{eq:w}. Thus, we will use Eq. \ref{eq:w} with $A=1.$ Though $\sigma_{0}$ may not necessarily be the kinetic cross section, it is expected to be of the same order of magnitude \cite{margenau1936}.

The broadening of the spectrum expressed in wavenumbers is simply related to the scattering rate in such a way that the slope $\gamma $ is directly related to the scattering cross section as
\begin{equation}
\gamma =  \frac{1}{2\pi c}\sigma_{0} \bar v
\label{eq:gamma}\end{equation}
The thermal velocity is $\bar v = (3k_{\mathrm{B}}T/\mu)^{1/2},$ where $k_{\mathrm{B}}$ is the Boltzmann constant and $\mu$ is the reduced mass of the Xe$_{2}-$Xe or Xe$_{2}-$Ar systems, respectively. The cross section is calculated from the experimental values of the slope $\gamma.$ We note that, owing to the very large mass of Xe$_{2},$ the broadening due to the Doppler effect is estimated to be $\mathit{\Gamma}_{D}\approx 2\cdot 10^{-2}\,$ cm$^{-1}$ in the band center, and is thus completely negligible in comparison to the broadening due to collisions \cite{Borghesani2005}.

In Tab. \ref{tab:crs} we report the values of the cross sections determined for the pure gas and for the mixture for $T=300\,$K along with the values of the hard--sphere radius $\rho = (\sigma_{0}/\pi)^{1/2}. $%

\begin{table}[!b]
\caption{\label{tab:crs}\small Physical parameters of the spectrum broadening}
\begin{ruledtabular}
\begin{tabular}{lcr}
Gas& Xe&Ar--Xe\\
\hline
$\bar v \,$(m/s) & $292$ &$ 465$\\
$\sigma_{0}\,$(nm$^{2}$) & $208
 $& $21
 $\\
$\rho\,$ (\AA)& 128&41\\
\end{tabular}
\end{ruledtabular}
\end{table}

  It is known that the optical collision cross sections and diameters depend on the nature and strength of the molecular forces, as well as upon the intrinsic properties of the radiating molecule and are usually greater that the kinetic collision cross sections and diameters \cite{crane1963}. Nonetheless, the values of the cross sections and of the hard-sphere radii determined from the observed slopes and reported in Tab. \ref{tab:crs} turn out so large as not to be reasonable for a RG molecule--RG atom collision.

We further observe that the cross sections deduced from the experiment in the pure gas and in the mixture are very different from each other in spite of the fact that in both case a RG molecule--RG atom collision is occurring and that the values of the hard-sphere radius of Xe and Ar are not this different. The ratio of the cross sections is $S=\sigma_{0}(\mathrm{Xe})/\sigma_{0}(\mbox{Ar--Xe}) =9.84.$ We believe that this difference is a direct consequence of the indistinguishability of identical particles in quantum mechanics, as we will show later.

We first suggest a possible explanation for the large values of the cross sections. We note that the spectrum is given, roughly speaking, by a convolution of the contributions of many vibrational and rotational states of the upper bound state according to Eq. \ref{eq:fcspc}. Thus, the cross section determined from the slope is the weighted sum of the cross sections of all rovibrational states contributing to the overall emission.

In the conditions of the present experiment, the Xe$_{2}$ excimers are in thermal equilibrium with the atoms of the surrounding gas and many vibrational and rotational states of the molecule are occupied according to the Boltmann distribution. If the collision cross section of the molecule for each of its rovibrational states is $\sigma_{v^{\prime}J^{\prime}},$ the cross section determined from the spectrum broadening is, roughly speaking, a weighted sum of each individual contribution
\begin{equation}
\sigma_{0} \sim \sum_{v^{\prime}J^{\prime}}e^{-\beta E_{v^{\prime}J^{\prime}} }\sigma_{v^{\prime}J^{\prime}}
\label{eq:svjtot}\end{equation}
The potential energy $V_{u}$ \cite{jonin2002_II} can be very well approximated by a Morse-type potential of the form
\begin{equation}
V_{u}= T_{e}^{\prime} + D_{e}^{\prime}
\left\{ 1- \exp{\left[-\beta_{e^{\prime}}\left(
R-R_{e^{\prime}}
\right)\right]
}
\right\}^{2}
\label{eq:vumorse}\end{equation}
with $T_{e}^{\prime}= 13860\,$cm$^{-1},$ $D_{e}^{\prime}=1717\,$cm$^{-1},$ $R_{e^{\prime}}=3.23\,$\AA, and $\beta_{e^{\prime}}R_{e^{\prime}}=6.734$ \cite{borghesani2007jpb}. Its well strength is such that it can accommodate up to $v^{\prime} =34 $ vibrational levels. At the temperature of the experiment, $T=300\,$K, $k_{\mathrm{B}}T\approx 208.5\,$cm$^{-1},$ the average vibrational quantum number is $\langle v^{\prime}\rangle =3.$

The rotational constant of the Xe$_{2}$ in the $(3)0_{u}^{+}$ state is $B_{e}^{\prime}= \hbar^{2}/(2m_{r}R_{e^{\prime}}^{2})\approx 2.47\cdot 10^{-2}\,$cm$^{-1},$ in which $m_{r}\approx 1.09\cdot 10^{-25}\,$kg is the average reduced mass in a sample of natural isotopic abundance \cite{borghesani2007jpb}. It corresponds to a rotational temperature $\Theta_{r}\approx 3.5\cdot 10^{-2}\,$K. For $T=300\,$K, rotational states of high angular momentum are thermally excited. Their population is non-negligible for a rotational quantum number $J^{\prime}$ as high as 250, with average $\langle J^{\prime}\rangle = 80.$

Thus, roughly speaking, a number $N_{c}=\langle v^{\prime}\rangle\cdot \langle J^{\prime}\rangle = 240$ of different rovibrational levels, on average, contributes to the measured cross section. If we assume that the excimer size and, hence, its cross section, does not depend too much on its rovibrational state, each of them contributes an amount $\sigma$ to the total cross section $\sigma_{0}$. Hence, $\sigma_{0}\approx N_{c}\sigma .$ In this way a crude estimate for the excimer cross section is easily obtained as $\sigma \approx 8.7 \,$nm$^{2},$ that corresponds to a hard-sphere radius $\rho \approx 16.6\,$\AA\ for the pure gas, and $\sigma\approx 0.88\,$nm$^{2} $ and $\rho\approx 5.3\,$\AA\ for the Ar--Xe mixture.

If one calculates the hard-sphere radius of the Xe$_{2}-$atom interaction as the bond length of the Xe$_{2}$ molecule, $R_{e^{\prime}}$ plus one half of the Xe--Xe (or Ar--Xe) hard-sphere radius, one obtains
$\rho \approx 5.4\,$\AA\ for the pure gas case and $\rho\approx 5.2\,$\AA\  for the mixture case.

This estimate is in quite good agreement with the value deduced from the analysis of the observed broadening of the spectrum only for the Ar--Xe mixture, whereas it is nearly three times as large for the pure gas. This is an obvious consequence of the value $S=9.84$ of the cross section ratio.

We believe that the difference in the cross sections in the pure gas and in the Ar-Xe mixture is a direct manifestation of the quantum indistinguishability of identical particles.
Let us first consider the pure gas case, in which a Xe atom is assumed to be colliding with the excimer. This binary collision picture is justified by the linear dependence of the spectrum broadening on the gas density.

Just for the sake of visualizing the physical situation, let us consider a slow collision occurring along the direction of the bond axis. For $T=300\,$K, the thermal wavelength of the excimer and of the colliding atom is quite short, of the order of 0.1 \AA, much shorter than the molecular bond length $R_{e^{\prime}}=3.23\,$\AA.  Thus, the wavefunction of the approaching atom is correlated only with that of the closest atom in the molecule whereas it is not correlated over the bond length with the wavefunction of the other atom in the molecule. This predominance of the interaction of the colliding atom with the nearest atom in the molecule during a atom--diatom collision has also been shown in other systems, such as H + H$_{2}$ \cite{krstic1999a}.

For this reason, the Xe$_{2}-$Xe collision at thermal energy must be rather treated as a Xe--Xe collision. In this case the impact and recoil wavefunctions overlap and interfere and the cross section must reflect the interference pattern of the wavefunctions. This situation bears strong analogies with processes of symmetrical resonant excitation or charge tranfer in which glazing angle collisions are converted to nearly head-on collisions \cite{mb,mason}.

The indistinguishability of identical boson particles leads to the symmetrization of the wavefunctions of the colliding pair. Thus, the scattering amplitude $f(\theta)$ must be replaced by $f(\theta) +f(\theta -\pi)$ and the differential cross section $\mathrm{d}\sigma/\mathrm{d}\Omega =\vert f(\theta) +f(\theta -\pi) \vert^{2}$ is enhanced by a factor 4 with respect to the case of a collision of non-identical particles. The quantum cross section (at low energy) is 4 times larger than the classical one \cite{gasio}.

Moreover, there are two scattering channels, because the two Xe atoms in the molecule are equivalent. The incoming Xe atom can collide with either Xe atoms of the molecule. Thus, the scattering probability, hence the scattering cross section, is enhanced by a further factor~2.

The situation is completely different in the Ar--Xe mixture because now there is no longer the requirement of indistinguishability of identical particles. In this case, the incoming Ar atom scatters off the molecule that is now to be considered as a single physical entity.
As a conclusion, the scattering cross section for the Xe$_{2}-$Xe collision is 8 times larger than cross section for the the Xe$_{2}-$Ar collision.

In literature, the classical atom--diatom scattering cross section is not uniquely defined in terms of the hard-sphere radii of the participating particles and several formulas can be used. Similarly, the cross section ratio $S$ can be calculated in several different ways. For instance, we quote four different formulas that give comparable results \cite{gislason1990}
\begin{eqnarray}
 S_{1}&= & 8 \left[
 \frac{R_{e^{\prime}}+\rho_{\mathrm{Xe}}}{R_{e^{\prime}}+\left(
 \rho_{\mathrm{Xe}}+\rho_{\mathrm{Ar}}
 \right)/2}
 \right]^{2}
   \label{eq:s1}\\
 S_{2}&=  &8\left[
 \frac{4\rho_{\mathrm{Xe}}}{3\rho_{\mathrm{Xe}}+\rho_{\mathrm{Ar}}}
 \right]^{2} \label{eq:s2}\\
S_{3}&= &
8\left[
\frac{6\rho_{\mathrm{Xe}}^{2}}{2\rho_{\mathrm{Xe}}^{2}+\rho_{\mathrm{Ar}}^{2}+3\rho_{\mathrm{Xe}}\rho_{\mathrm{Ar}}}\right]
\label{eq:s3}\\
S_{4}& =& 8\left[
\frac{2\rho_{\mathrm{Xe}}}{R_{e^{\prime}}+\rho_{\mathrm{AX}}}
\right]^{2}
\label{eq:s4}
\end{eqnarray}
in which the factor 8 due to quantum effects has been explicitly written.

By inserting the literature values for the hard-sphere radii into the previous equations, the  calculated cross section ratio turns out to be comprised in the range $8.4\leq S\leq 12.$ The experimental value $S_{\mathrm{exp}}=9.84$ is well within this range \cite{mogentale2007}.

 We note that this picture is very crude as we have not considered, for instance, any asymmetry of the atom--diatom interaction potential that may appear if the collision occurs along a direction forming an angle with the bond axis. However, we believe that asymmetries, if any, should be washed away by the averaging effect of molecular rotations.

In spite of the crudeness of this picture, the agreement of its prediction with the experimental outcome lends credibility to the hypothesis that the different strength of the collision broadening in the pure gas and in the mixture is actually caused by the quantum indistinguishability of identical particles. At the same time, this is a rather surprising result because one is na\"{\i}vely led to expect that such effects manifest themselves at low temperature rather than at room temperature.

 \section{Conclusions}\label{sec:fine}
The present new measurements of the IR emission luminescence of Xe$_{2}$ excimers carried out in an extended density range have confirmed the previous results obtained in a more restricted density interval.

The correct assignment of the molecular states involved in the bound-free transition and the extension of the investigated density range has allowed to put on firmer theoretical ground the heuristic model developed to explain how cooperative- and multiple scattering effects continuously develop upon increasing the density of the environment of the excimers. In particular, the density dependence of the shift of the luminescence spectrum could be used to obtain an independent measure of the ground-state energy of a quasifree electron in a dense gas.

Quite surprisingly, the analysis of the differences in the shape of the Xe$_{2}$ spectrum recorded in the pure Xe gas and in a mixture with Ar has put into evidence that quantum effects due to the indistinguishability of identical particles are present even though the temperature of the experiments is fairly high.

In spite of the success of the model we have developed for interpreting the density dependence shift of the spectrum, several problems are still open. In particular, a better understanding of the broadening of the spectrum is needed to determine reliable values of the optical collision cross section of the excimer.

Moreover, the future developments of the experiment will pursue the possibility of still increasing the pressure in order to bridge the gap with the liquid. We will also investigate the behavior of the Xe$_{2}$ excimers in mixtures with different RG gases for which the electron--atom interaction is repulsive in order to test the validity of our heuristic model.

\section*{Acknowledgments}We acknowledge the invaluable support of D. Iannuzzi at the  Vrije Universiteit, Amsterdam, The Netherlands.

redpajama_set_name: RedPajamaArXiv


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Budget cuts annou…
An NPM has been established
yes, in 2011
Number and percentage of female prisone…
5.5 %
/ World Prison Brief, ICPR
Incarcerated minors
Untried prisoners
29.2 %
Death penalty is abolished
yes, since 1952
Security, order, and discipline
Contact with the outside world
Admission and evaluation
Physical integrity
National Preventive Mechanisms and other external control b…
Sentence adjustments policies
All inmates are admitted to prison with a valid commitment order
Prisoners can inform their families about their imprisonment
All inmates have the right, if they so wish, to call a relative to inform them of their imprisonment.
The prison service is not allowed to disclose an inmate's imprisonment without their permission.
There is a reception area for arriving prisoners
in most facilities
A copy of the prison regulations is made available to the prisoners
Admission staff are trained to identify prisoners at risk of harming themselves or others. They must report these prisoners to all departments during weekly meetings. At-risk prisoners are visited by a nurse and a psychologist.
Prison staff draw up an individual sentence programme for each prisoner.
This programme sets out the organisation of daily life: work, recreational and sports activities, learning, and visits. Prisoners usually work for half of the day. The other half is devoted to other activities.
Prisoners can be assisted by a lawyer throughout their incarceration
All prisoners have access to a lawyer of their choice.
Legal representation during the trial is guaranteed by the Legal Aid Commission. (Raad voor Rechtsbijstand). Article 34 of the Legal Aid Act (Wet op de rechtsbijstand) states that free legal representation is guaranteed to prisoners whose annual income, as of 1 January 2018, is less than or equal to €26,900 for a single person and less than or equal to €38,000 for persons living as a couple.
Prisoners have access to a legal aid centre
Legal aid centres, run by students, are set up in some facilities (e.g. Nieuwersluis prison in Utrecht, Zaanstad in Amsterdam, and Nijmegen prison).
Inmates do not receive legal aid for administrative matters (divorce, social benefits, etc.).
Prisoners may receive visits from their lawyers on any working day. The visit must be scheduled at least one day in advance (Article 3.8.2 of the Internal Regulations of Penitentiary Institutions).
Prisoners are not permitted to receive visits from their lawyers during hours of work, unless there is an emergency or special permission is granted by the governor. Visitation time during working hours is exempt from remuneration.
Deaths in custody are logged in a register
The number of deaths in custody are published once a year.
Number of deaths in custody
/ Council of Europe, "Space I – Report 2018", p.99.
Variation in the number of deaths in custody
decreased by 44%
32 inmates passed away in detention in 2015.1
Council of Europe,"Space I - Report 2016", p.115. ↩
Number of deaths attributed to suicide
Council of Europe, "Space I – Report 2018", p.99.
Variation in the number of suicides
decreased by 9 %
There were 11 suicides in 2015.1
Council of Europe, "Space I - Report 2016", p.115. ↩
Death rate in custody (per 10,000 prisoners)
Suicide rate in custody (per 10,000 prisoners)
National suicide rate (per 10,000 inhabitants)
The suicide rate per 100,000 inhabitants in the national population is 9.6.
/ World Health Organization,"Suicide rate estimates".
The prison service must notify a judicial authority for
all deaths
Staff must systematically inform the Supervisory Committee representative (Kenniscentrum Commissie van Toezicht) of any death that occurs in the facility.
The prison governor informs the deceased's relatives as soon as possible.
The governor is required to discuss administrative matters related to the death with the next of kin. The governor also grants them access to the place of death.
The chaplain of the prison makes him/herself available for the deceased's next of kin.
Suicide prevention policies are implemented
Dutch criminal law does not punish suicide attempts. However, some preventive measures, such as solitary confinement or the use of mechanical restraints, are perceived as a form of punishment.
Since 1999, all facilities have had a suicide prevention policy in place. Prison officers receive training on the measures to be taken.
Prevention includes the following measures1:
Medical examination upon arrival. The medical file may be shared with surveillance staff in case of risk. Such sharing is not mandatory.
Provision of a direct phone line within the cell. Staff are required to respond within 10 minutes.
Placement in solitary confinement. This measure may only last a maximum duration of two weeks. It may be repeated an unlimited number of times. All objects in the prisoner's possession are removed, and they must wear suitable anti-suicide clothing. Video surveillance may be imposed if necessary. Use of mechanical restraints is also possible. A medical examination must be conducted on a daily basis. The General Inspectorate of Justice and Security (Inspectie Justitie en Veiligheid) notes that inmates in solitary confinement do not usually receive visits from medical staff at weekends.
Thoonen, E., "Death in State custody", 2017, p. 263-281. ↩
The prohibition of torture is enshrined in the Constitution and the legislation
only in the legislation
Article 8 of the Dutch International Crimes Act makes torture a crime. This definition is in line with the Convention against Torture and the Rome Statute of the International Criminal Court.
The United Nations Convention against Torture (UNCAT) was
ratified in 1988
(signed in 1985)
All allegations and suspicions of ill-treatment inflicted on prisoners are logged
If so, the prison service must notify a judicial authority
The principal policies implemented to prevent abuse are:
training of prison staff
possibility of lodging a complaint with an independent body (Supervisory Committee)
continual access to a lawyer
In accordance with the Istanbul Protocol, medical staff undergo training to identify signs of torture or ill-treatment.
According to the testimonies of medical staff gathered by a delegation from the European Committee for the Prevention of Torture (CPT) in 2016, cases of traumatic injuries are not duly recorded.
Updated on: 9/21/20
/ CPT
Each prison facility keeps an updated record of violence between inmates
Acts of violence between prisoners are investigated
Members of the medical staff informed a CPT delegation in 2016 that cases of violence, particularly altercations between prisoners, were not systematically reported to the investigative authorities.
Number of complaints filed by prisoners against the prison service
/ Pauline Jacobs and Anton Van Kalmthout, "Chapter 4 : The Dutch complaint and appeal procedure for prisoners in the light of European standards" in Monitoring Penal Policy in Europe, 2017, p.14
Articles 60 and 61 of the PBW provide for the right to file a complaint against any decision made by or on behalf of the facility director.
There are no formal requirements for the complaint. Any identifiable written material is treated accordingly. It must be filed within seven days of the disputed decision. The complaint may be written in Dutch or in any other language (Article 61 of the PBW).
Standard procedure guarantees confidentiality: prisoners file their complaints in dedicated mailboxes, accessible only to the Supervisory Committee.
The complainant may be assisted by a lawyer at no cost.
The Complaints Committee (Beklagcommissie) is the specialised body for receiving complaints. It is appointed by the Supervisory Committee (Beklagcommissie van Toezicht) (see "National preventive mechanisms and other external control bodies").
The Complaints Committee consists of three members of the Supervisory Committee and a secretary (Article 62 of the PBW). The chairman is ideally a magistrate.
The Complaints Committee may order the implementation of certain measures, conflict resolution, or the awarding of compensation.
Inmates and the prison service may appeal against the decisions of the Complaints Committee. The appeal must be lodged within seven days of the decision. It is handled by the Appeals Committee (Beroepscommissie) of the Council for the Application of Criminal Law and Youth Protection (Raad voor Strafrechtstoepassing en Jeugdbescherming).
The Dutch complaints procedure is considered by some experts to be an effective remedy for inmates' grievances.1 It is one of the few complaint mechanisms that are regarded highly by the European Court of Human Rights and the CPT. Prisoners are, however, constrained by the short time limit for lodging complaints.
Pauline Jacobs and Anton Van Kalmthout, "Chapter 4: The Dutch complaint and appeal procedure for prisoners in the light of European standards", in Monitoring Penal Policy in Europe, 2017, p. 15. ↩
The Complaints Committee may refer a complaint to the Supervisory Committee. The complaint is then dealt with more formally, with the Supervisory Committee acting as a mediator between the prisoner and the head of the facility (Article 63 of the PBW).
National Preventive Mechanisms and other external control bodies
The Optional Protocol to the Convention against Torture and Other Cruel, Inhuman or Degrading Treatment or Punishment (OPCAT) was
Name of the NPM
Nationaal Preventie Mechanisme
The NPM has come into office
The NPM was appointed by
the executive power
Structure of the NPM
several institutions
The Dutch NPM is a network comprising several institutions, managed by the Inspectorate of Justice and Security (Inspectie Justitie en Veiligheid). It is "an open-ended advisory body for institutions".1 These institutions carry out their activities in accordance with their statutory duties. They include:
Health and Youth Care Inspectorate (Inspectie Gezondheidszorg en Jeugd)
Council for the Application of Criminal Law and Youth Protection (Raad voor Strafrechtstoepassing en Jeugdbescherming)
Guidance group of the Custodial Institutions Supervisory Committee (*Klankbordgroep Commissie van Toezicht justitiële inrichtingen *)
National Supervisory Committee for in-custody care (Landelijke Commissie van Toezicht Arrestantenzorg)
Supervisory Commission (Commissie van Toezicht)
Supervision of the NPM was assigned to the National Ombudsman until 2014, at which time it decided to withdraw. The main reasons cited for the ombudsman's withdrawal were the lack of sufficiently independent inspections and the absence of clarity regarding its duties.
The network structure of the NPM was a political choice made by the Ministry of Justice "which saw no added value for the Netherlands in the Protocol compared to the monitoring systems already in place, whether provided by the Committee for the Prevention of Torture (CPT) or by national bodies".
Jacobs, P., "Supervision / Inspection of deprivation of liberty in the Netherlands", 2015, p.4. ↩
The NPM reports are made public
The NPM publishes an annual report. It is its only publication.
Each of the member institutions produces reports corresponding to its remit. These are usually published, with a few exceptions.
The Health and Youth Care Inspectorate publishes its reports on its website (in Dutch).
The legislation allows the NPM to carry out unannounced visits
The NPM can monitor all prison facilities, units and premises
Prisons in the Dutch Caribbean do not, however, fall within the scope of OPCAT/NPM. The Council for Law Enforcement (Raad voor de Rechtshandhaving) is not part of the NPM. It oversees prisons in the Caribbean (Bonaire, Curaçao and Sint Maarten). Aruba does not fall within its jurisdiction. Aruba has on occasion asked the Inspectorate of Justice and Security (Inspectie Justitie en Veiligheid) to review the conditions of its prison.
The NPM recommendations are effectively implemented
The bodies that make up the NPM do not issue recommendations. They are "*not tasked with advising on proposals for relevant future policies and legislation"1.
Jacobs, P., "Supervision / Inspection of deprivation of liberty in the Netherlands", 2015, p. 4. ↩
A regional body monitors the places of deprivation of liberty
yes, the Committee for the Prevention of Torture (CPT)
The CPT has highlighted the lack of independence within the NPM. It does not have its own staff or budget.1
Jacobs, P., "Supervision / Inspection of deprivation of liberty in the Netherlands", 2015, p. 4-5. ↩
Its reports are made public
The CPT made visits to the Netherlands in 1992, 1994, 1997, 1999, 2002, 2007, 2011, 2014 and 2016.
Read the reports published by the CPT following its visits to the Netherlands here.
The Subcommittee for the Prevention of Torture (SPT) has visited the country
yes, from the 28 to the 31 July 2015
The purpose of the visit was to evaluate the work carried out by the NPM.
Its report was made public
Read the report published by the SPT following its visits to the Netherlands here.
Each facility has a Supervisory Committee (Commissie van Toezicht). This is made up of members of the public, and must "*enable supervision by persons outside the prison, offering the outside world a view of the entire facility"1.
It participates in NPM meetings as an "associate".
Supervisory Committees are set up for each establishment.
The Netherlands Institute of Human Rights (Studie- en Informatiecentrum Mensenrechten, SIM) also monitors conditions of detention.
Jacobs, P., "The Development of Rechtsburgerschap of Prisoners: A National and European Perspective", 2015, p. 386. ↩
The law provides for a sentence adjustment system
A new measure has allowed certain inmates to be released under electronic supervision during the Covid-19 pandemic. There are not enough bracelets available to meet the demand and the minister for justice and security has ordered 1,500 more. A total of 200 prisoners are believed to have been released and placed under electronic supervision.
Updated on: 10/9/20
/ Bloomberg
Sentence adjustment
The Public Prosecutor's Office is responsible for decisions on sentence adjustment. The judge and the facility director have residual jurisdiction in this matter.
Temporary release
The Public Prosecutor's Office may be consulted on the temporary release of inmates. The final decision is made by the prison management.
Retraction of alternative sentences
The public prosecutor's office may decide on incarceration if alternative measures, such as community service, are not adequately executed.
Sentence adjustments can be granted during the incarceration
Prisoners serving a one-year prison sentence may apply for parole after serving six months and one third of the remaining sentence.
Prisoners serving a sentence of more than one year's imprisonment must serve two thirds of their sentence.
Parole finishes at the end of the initial sentence (minimum of one year).
Prisoners can contest a negative decision of sentence adjustment
The law provides for a temporary release system
The decision to grant temporary release lies with the facility director. The counsel of the Public Prosecutor's Office may be sought.
Only prisoners who have been tried and convicted or who have been convicted on appeal may be granted temporary release (Article 26 of the PBW).
Prisoners have been temporarily denied leave from 13 March 2020 onwards due to measures to stop the spread of Covid-19.
/ AD
Temporary release is granted in accordance with the length of the sentence.
It is arranged by the governor at the point of incarceration and is spread over the entire length of the sentence. The maximum frequency is once a month. Each release is limited to 52 hours.
Prisoners may be granted temporary release, for good behaviour, at Easter, Pentecost, Christmas and New Year.
Temporary release is also granted for personal reasons (birth, illness, death of a close relative; specialised medical care; preparation for release, job interviews, education or training). In such instances, temporary release may be supervised (Article 21 of the EAA).
The law provides for a sentence adjustment for medical reasons
The sentence may be suspended on "compelling physical or mental grounds, provided that they are incompatible with continued detention". Prisoners may also apply for a pardon if they are terminally ill.1
hoonen, E., "Death in State custody", Maklu Uitgevers N.V, 2017, p. 181. ↩
Number of prisoners who have been granted a presidential pardon or amnesty during the year
/ Council of Europe,"Space I - Report 2016", p.34.
Royal pardons are regulated by a law (Gratiewet).
Article 2 provides two grounds for pardon:
The court did not take into account circumstances that would have justified a different sentence or no sentence at all
Execution of the sentence no longer serves the purpose for which it was issued
Pardons may lead to partial or total reduction of the sentence, to its suspension or a transfer to an alternative measure.
The prisoner, a family member, his/her lawyer or a member of the parole staff may apply for a pardon.
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text:
PAPAMOA SAND SCUTTLE
Papamoa Community
Surf Rescue Base Trust
The Papamoa Sand Scuttle is a fundraiser for Papamoa Surf Life Saving Club's new club building.
The current building was built in the 1990s when the Surf Club had a membership of 150. The building was subsequently extended in 2001 when the membership was at 250, but even then, no one could have foreseen the significant population growth that was about to occur in Papamoa. Today the Surf Club has membership of over 750 and this figure is forecast to grow to well over 1000 members by 2020.
The new building will be environmentally and aesthetically pleasing, and instantly recognisable as a rescue hub. It will be fit-for-purpose and functional for our immediate needs, and future-proofed for tomorrow's needs – with secure space for rescue equipment storage, education and training areas, improved communication systems, first aid facilities, and duality for broader community use.
Keep up to date with the new build project here
Coast Care
Everyone who enters the Papamoa Sand Scuttle is helping to look after our coastal environment.
For every participant, a dune plant will be planted along Papamoa beach, close to Papamoa Surf Club.
Coast Care is a partnership programme between the Local City and District Councils, DOC and Bay of Plenty Regional Council. Supporting local communities, care groups and schools to restore and protect the sand dunes along the Bay of Plenty beaches.There are 25 groups throughout the Bay of Plenty who all help to make sure sand dunes are not lost to erosion, weather or careless management.
The popularity of our beaches, changing climate, pests and development pressure are all affecting the health of our coast, particularly dunes. This makes the work of Coast Care groups even more important in making sure our beaches remain for many more years to come.
Find out more about Coast Care here
In association with Papamoa Surf Life Saving Club
Sign up to our mailing list:


Finished final leg of Rally Novice Title March 2015!
Best Female, Best of Breed!
Candy's first Rally Trial (September 20, 21, 2014) where she received Rally Novice B with scores of 99 and 98! One more leg to go for her title!


text:
Articles tagged learning to code (2)
Mark CoddingtonOctober 25, 2013
This Week in Review: Demystifying coding for news, and the AP's firings under fire
Plus: Tech billionaires in journalism, David Pogue's jump to Yahoo, and the rest of this week's news in the journalism and tech worlds.
Jake LevineMarch 13, 2013
Jake Levine: Why learning to code isn't as important as learning to build something
"Programming is a means to an end, not an end in itself. You should be trying to do as little of it as possible to make the thing that you want."
Levine, Jake. "Jake Levine: Why learning to code isn't as important as learning to build something." Nieman Journalism Lab. Nieman Foundation for Journalism at Harvard, 13 Mar. 2013. Web. 30 Jan. 2023.
Levine, J. (2013, Mar. 13). Jake Levine: Why learning to code isn't as important as learning to build something. Nieman Journalism Lab. Retrieved January 30, 2023, from https://www.niemanlab.org/2013/03/jake-levine-why-learning-to-code-isnt-as-important-as-learning-to-build-something/
Levine, Jake. "Jake Levine: Why learning to code isn't as important as learning to build something." Nieman Journalism Lab. Last modified March 13, 2013. Accessed January 30, 2023. https://www.niemanlab.org/2013/03/jake-levine-why-learning-to-code-isnt-as-important-as-learning-to-build-something/.
| url = https://www.niemanlab.org/2013/03/jake-levine-why-learning-to-code-isnt-as-important-as-learning-to-build-something/
| title = Jake Levine: Why learning to code isn't as important as learning to build something
| last = Levine
| first = Jake
| date = 13 March 2013
| ref = {{harvid|Levine|2013}}


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Villa away leg - SOLD OUT
The final 1,500 away tickets for City's forthcoming Capital One Cup Semi-Final Second Leg at Aston Villa's Villa Park have now been SOLD OUT.
With some supporters queuing from as early as 3:00am this morning to get their hands on the final batch tickets, it means Phil Parkinson's men will now be cheered on by over 6,300 City fans in just under two weeks time.
The football club would like to remind supporters who have already purchased tickets over the phone or online that they can be collected from the Ticket Office at the Coral Windows Stadium at any point until 22 January.
If you would like your tickets posted please contact the Bantams Ticket Office directly.
Aston Villa still have a number of banqueting packages available and these are available directly from Villa Park.
The club are finalising the details for coach travel so please keep an eye on the website over the next few days for further details.


text:
FAITH DOING JUSTICE
Vol 29 No 14
Kindness stories that are good for the spirit
Julie Perrin, Tender: Stories that lean into kindness, MediaCom Education Inc., ISBN 781925722192
Every generation throws up popular reflection on how best to live. Much of it comes from within religious traditions. It is designed to help people reflect on their lives by building bridges betwee religious traditions and the world in which the reader lives. It is instructive to note how its style and emphases vary with changes in the religious and broader culture.
Julie Perrin's recent book Tender, a collection of short reflections, grouped thematically, is splendid in its own right. It is spare and elegant in its writing, inclusive in its address and open-ended in its invitation to reflection. It also offers an occasion to reflect on the ways in which spiritual writing has changed.
Seventy years ago much spiritual writing was based in the language, ritual practices and shared faith of religious communities. Readers outside the community would normally feel themselves made outsiders. A reader wandering along shelves of spiritual books would look first for their denominational origin to see if they were Anglican, say, or Catholic. If readers drew on their own experience or on contemporary news, they usually did so simply to illustrate a point already made conceptually.
Fifty years ago many churches were becoming less tribal, ascription to faith was less certain, and people looked to a range of authorities when seeking wisdom. Though spiritual writers generally worked within a particular tradition, they also drew on other Christian and religious traditions and appealed to psychology and other secular sources. The writing was often exploratory rather than didactic. Readers measured it, not by its denominational solidity, but by its spiritual depth.
Today another genre coexists with those earlier ones. In it, writers with a strong and questioning faith address a general audience, using anecdotes, inherited wisdom, contemporary social debates and a broad cultural reference to encourage their readers to move beyond inherited prejudice and ask themselves what really matters. The heart of the exercise is to find the right words that will disclose the depth in ordinary experience. The writing encourages readers to pay attention to the world around them, to wonder, and to celebrate God's presence there. Readers evaluate it by the quality of the writing and its attentiveness to the depth of ordinary human experience.
Among writers familiar in Australia who write in this vein are Michael McGirr, Terry Monagle, Pádraig Ó Tuama and the much missed Brian Doyle. Their writing does not merely describe but evokes and creates a world, and shapes a human response that respects its variety and mystery. Significantly, too, these writers are masters of short anecdotal pieces which invite us to see the great in the small. They also write fastidiously, choosing words that disclose freshly aspects of reality which the language of traditions conceals at first reading. Following the age-old advice to writers their words don't merely tell but show.
These qualities are evident in Tender. Even the title is evocative. Not Tenderness, a lovely word, but sometimes tainted with sentimentality, the adjective Tender suggests at once rawness, vulnerability, affection and offering. In the subtitle — stories that lean into kindness — lean intimates the delicacy of an invitation to follow a direction without compulsion. It also suggests the intimacy shown when we lean on another's shoulder, and perhaps trust that the reader will respond to the gentle invitation. Kindness evokes both generosity and our shared humanity.
"Human cruelty and human kindness, they sit closely together." — 'Bank Teller', by Julie Perrin
The encounters and reflections that compose the book display the same sharp attention to the rawness and pathos of reality and the possibilities of words. Behind the initial detail of description of place, time and ordinary actions, a deeper reality is disclosed that leads to reflection inviting readers to ask themselves what matters. In 'Bank Teller', for example, the opening paragraph sets the scene through simple, carefully observed detail:
'In Melbourne, I'm walking along Sydney Road, Brunswick, the traffic is stretched bumper to bumper. There is a taxi at the kerb outside the bank. As I enter the foyer through the glass doors a young man is walking towards me. His arm is outstretched as if to make a space ahead of him. With his other arm he is guiding a woman dressed in long flowing robes and hijab.'
The succeeding paragraphs fill out the description. The young man is corporately dressed, sharp, but in his attention there is something unexpectedly 'decorous, almost tender'. The woman's face is a mass of scarring, with no nose, and she seems to be blinded. As they talk together, 'he leans his head closer, so he can hear'. The final paragraph sums up the story into which the reader has been drawn and invites them to ponder a larger question:
'Human cruelty and human kindness, they sit closely together. The bank teller carved out a space for the scarred woman to walk in. Her presence and energy called something out of him, and from other witnesses to their conversation. What will it take before we can make space on our shores for people who carry their scars less visibly?'
In this writing, every word, every punctuation mark, every turn of phrase has been considered in order to draw the reader into the human reality of a simple action, to share the writer's compassionate regard, and finally to embrace a question that ripples out into widening circles. Our shores can be those of our inner life, our domestic relationships, of our workplace and, evidently, of our nation.
In this book, as in the genre of which it is representative, the writing encourages the readers to draw on their traditions, not as authorities to close conversation but as resources to open it. It displays devotion to the word fleshed out.
Andrew Hamilton is consulting editor of Eureka Street.
Recent articles by Andrew Hamilton.
The bastard subsidiarity of bushfire responses
Australia Day as a day for humility
Bushfire divisions etched in sand
Christmas in a time of fire
Greta Thunberg for person of the year
Topic tags: Andrew Hamilton, Julie Perrin, Brian Doyle
SUPPORT RESPECTFUL CONVERSATIONS IN A TIME OF DIVISION
If there's one thing that the recent election campaign and its outcome demonstrated, it's the depth of the divisions that exist in our Australian community.
Our politics is focused on point-scoring, personalities, and name-calling across party lines. The media, for the most part, don't help, driven by the 24-hour news cycle and the pursuit of advertising dollars into a frenzy of click-bait and shallow sensationalism.
What does it mean to be an Australian in times like these? What are the values that unite us?
Eureka Street offers an alternative. It's less a magazine than a wide ranging conversation about the issues that matter in our country and our world; a conversation marked by respect for the dignity of ALL human beings.
Importantly, it's a conversation that takes place in the open, unhindered by paywalls or excessive advertising. And it's through the support of people like you that it is able to do so.
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Existing comments
Really enjoyed this thoughtful and beautifully written article, and the article published by Eureka Street by Julie Perrin, 'Jack baptised himself' on 17/7. Looking forward to reading more!
Karen B | 18 July 2019
I agree that current writers of the kind identified by Andrew exercise admirable human sympathy and dexterity with language, but can't say I find them as spiritually engaging or rewarding as I do, say, writers like Graham Greene, Flannery O'Connor, Evelyn Waugh or Andrew Bullen SJ (author of the recently published poetry collection "Etiquette with Angels"). When I ask myself, 'Why?", I think the answer is that there is a closer, more explicit - but no less evocative - integration in these latter authors' works between the spirituality they explore and and doctrines which inform it, particularly when they address profound theological realities like the workings of sin and grace in human experience. The religious sensibility - indeed mystery - these writers arouse is enhanced, I suggest, by the value they place on and provision they make for the intellect in their subject matter and treatment of it: "a small red flame - a beaten-copper lamp . . . relit before the beaten-copper doors of a tabernacle. . . burning anew among the old stones" in the Marchmain chapel (Waugh), and "Who besides Dante and the damned/enters hell wittingly?" (Bullen, in his masterful Holocaust poem, "Kreis, Kreis") come readily to mind. By comparison, "the modern spiritual mode" (or should that be "post-modern"?) to me resembles an echo without a source, or perhaps a child - like much of contemporary Europe - shy of acknowledging its spiritual and religious parentage.
John RD | 18 July 2019
A thoughtfully shaped review, which understands and honours the tender observation of the original. Julie's writing makes generous space for the deep unsaid.
Janet | 21 July 2019
Poetic, John RD.
john frawley | 22 July 2019
It is satisfying to read this article by a reviewer who is prepared to 'lean' thoughtfully into the author's writing and its intent. I have enjoyed Julie Perrin's articles published in Eureka Street, in particular "James and the Four Eggs" and "The Rosella's last walk" and I am now enjoying taking time over her book "Tender".
Sally | 26 July 2019
Excellent, nuanced review of a book I found deeply engaging.
Brian Walters | 27 July 2019
I wish I could speak with the eloquence of the reviewer of Julie's exquisite book, "Tender". Julie's writing "...is spare and elegant in its writing, inclusive in its address and open-ended in its invitation to reflection." I am not a Christian but I find Julie's writing very compelling because it is not bogged down in theological exegesis but in soulful observation of fellow humans from a heart full of compassion. My eyes are opened to the miracles in the ordinary and I reflect on the world differently. Thank you for the gift of your writing Julie.
Jill H | 31 July 2019
I loved this little book. Such wonderful story telling, keen observations of everyday life and a timely reminder of the importance of kindness. Luckily I received this book as a gift and I would recommend it as a wonderful gift for family and friends.
Ann P | 20 August 2019
I thought I would just dip 'in and out' of this book, but of course, once I started I read it cover to cover. Thoughtful and insightful observations of everyday life and also times that are more challenging. I would really recommend it - a meaningful read!
Sue G | 01 September 2019
Julie Perrin's collection of vignettes and reflections, 'Tender' gently moves the reader towards a greater humanity. I am so pleased that I read this excellent book.
Andrew Calwell | 15 September 2019
Margaret Dooley Young Writers Fellowship
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Justice Ruth Bader Ginsburg says she is cancer-free, according to report
Jan. 9, 2020 at 6:50 am Updated Jan. 9, 2020 at 7:34 am
Liberals fearing yet another Supreme Court vacancy for President Donald Trump to fill have some cause for relief: It doesn't appear that Justice Ruth Bader Ginsburg is going anywhere anytime soon.
The 86-year-old justice told CNN this week she is still "cancer-free," six months after undergoing radiation to remove a cancerous tumor on her pancreas.
"I'm cancer-free. That's good," Ginsburg said in an "energized" manner, according to reporter Joan Biskupic.
Ginsburg's age and health have been top of mind for policymakers, lawmakers and activists who wonder whether Trump will have a third opportunity to appoint a Supreme Court justice — and further cement his already well-established legacy of orienting the nation's courts in a more conservative direction.
The progressive and iconic justice, lately the subject of several films exploring her life's broad impact, has fought cancer four times — and won. Before her pancreatic cancer diagnosis last year, Ginsburg underwent surgery for lung cancer in December 2018. She had surgery for early-stage pancreatic cancer in 2009 and was treated for colon cancer a decade before that.
She missed a day of oral arguments in November because of what a court spokesman described as a "stomach bug." That same month she was hospitalized over a weekend for chills and a fever and treated for a possible infection.
But it's rare for her to miss a day on the bench, and she still makes frequent outside appearances. After her August treatment, she told a crowd at the National Book Festival in Washington that "this audience can see that I am alive."
Ginsburg is the eldest and most senior of the court's four remaining liberal justices. In many recent cases, these four have dissented from the five conservative-leaning justices, who now include Trump appointees Neil Gorsuch and Brett Kavanaugh.
When asked how long she'll serve, Ginsburg usually responds that she won't retire as long as she can do the job "full steam." Yet Democrats fear that could occur before their own party controls the White House again, as another Trump appointee probably would ensure a conservative majority on the court for decades to come.
"Ginsburg's fans, many of who call her the 'Notorious RBG' and have her likeness as action figures, tattoos and on ugly Christmas sweaters, celebrated the news of her clean bill of health on social media," The Washington Post reported.
Of course, conservatives — especially anti-abortion rights activists — would love to see Ginsburg step down.
But regardless of when Ginsburg eventually steps down, conservatives already have an upper hand on the court.
The court is scheduled in early March to consider a Louisiana law requiring doctors at abortion clinics to have admitting privileges at nearby hospitals. The question is whether the law unduly burdens women's access to abortion — but anti-abortion activists and GOP lawmakers are asking the court to do something much broader: To reconsider the 1973 Roe v. Wade and 1992 Planned Parenthood v. Casey rulings, two landmark abortion rights decisions.
A group of 207 lawmakers signed an amicus brief last week asking the court to "again take up the issue of whether Roe and Casey should be reconsidered and, if appropriate, overruled." Just two Democrats — Reps. Collin Peterson of Minnesota and Daniel Lipinski of Illinois — signed on to the brief.
The brief signals the optimism conservatives feel that the court will rule with them — although Kavanaugh in particular hasn't ruled on many abortion rights cases in the past, so it's impossible to perfectly predict his stance.
And then there's another big health-care case sitting at the Supreme Court's doorstep — one that Republicans may not be so eager for it to take up.
Democratic-led states have asked it to hear a lawsuit against the Affordable Care Act, a case that could result in the dismantling of part or all of the law's patient protections and coverage expansions. The Democrats defending the law don't want that to happen — but they do recognize that a Supreme Court ruling in the middle of an election year would draw attention to the Trump administration's position against the ACA.
This story was originally published at washingtonpost.com. Read it here.


A 2-disc DVD Video of the Friday Evening performance is now available. It was filmed on 3 cameras and will be fully edited with chapter menus to easily find your favourite dances. Those dances not performed on the Friday evening were filmed for DVD before the Saturday Matinee.
2) You can see what's included on the PhotoDisc or buy instant digital downloads or order individual prints online from our new photo website posted out directly to you usually in a couple of days (by PhotoBox).
Top Tip: If you want to make a PhotoBook or album of photos, upload your chosen photos from the PhotoDisc to somewhere like PhotoBox.com where you can create your personalised record of the show!
2017 titles and earlier still available - click on the poster to order!