This invention relates generally to a light source and more particularly to a vacuum ultraviolet and x-ray light source and to a short wavelength laser pumped by said light source.
The source of vacuum ultraviolet and soft x-ray radiation now in use include the hydrogen glow discharge, rare gas continua, hot plasma sources such as the duoplasmatron and the laser produced plasma, and the synchrotron. The hydrogen glow discharge in conjunction with a LiF window is most effective at wavelengths longer than about 1200 A. The rare gas continua are perhaps most effective in the spectral region between 500 A and 1200 A. At still shorter wavelengths, for example, between 80 A and 500 A, the radiation from low pressure sparks, for example, the BRV spark source, is used. Of perhaps most importance the synchrotron provides a source of calibrated, relatively well collimated radiation, useful to the region of at least several angstroms.
The radiation from all of the above light sources covers a wide spectral range. Often it is a continuum of several hundred angstroms in width, while in other cases it consists of a large number of discrete spectral lines characteristic of the media. For most, or at least many, spectroscopic and photochemical applications it is thus necessary to first pass the radiation through a monochromator or spectrometer. With the exception of the synchrotron, the above light sources all emit into 4.pi. steradians. Due to the restricted angular acceptance and relatively poor reflectivity of vacuum ultraviolet spectrometers, an intensity loss of greater than four orders of magnitude is often experienced when using a combination of light source and spectrometer.
The length of their emission in time (pulse duration) is controlled by electrical parameters and thus light sources with a pulse duration of less than about one-half nanosecond, useful in the measurement of fluorescence, are generally not obtainable.
There has recently been considerable interest in the possibility of constructing vacuum ultraviolet and soft x-ray lasers by the technique of first inverting the population of a metastable species with respect to ground, and then extracting the inverted population by applying an intense laser pulse tuned to the upper level of the resonance transition. However, the operation of vacuum ultraviolet laser sources of this type await the prior development of methods of accomplishing the inversion of the metastable species with respect to ground. Referring to FIG. 2, inversion means that there are more atoms in level 2 than in level 1. The construction of x-ray lasers using metastable levels of atomic species is described by H. Mahr and U. Roeder, Opt. Commun. 10, 227 (1974); S. A. Mani, H. A. Hyman, and J. D. Daugherty, J. Appl. Phys. 47, 3099 (1976); and H. A. Hyman and S. A. Mani, Opt. Commun. 20, 209 (1977).
The generation of radiation at a wavelength of 199 A by first exciting and storing populations in a metastable level and then transferring energy to another level from which it then radiates to ground is described by A. A. Vekhov, V. N. Makhov, F. A. Nikolaev, and V. B. Rozanov in the paper entitled "Possibility of Using Metastable Helium Like Ions in Generation of Ultrasoft X-Ray Stimulated Radiation, " Sov. J. Quant. Elect. 5, 718 (June 1975). The paper describes an experiment where decaying plasma produces metastable storage in the 1s2s.sup.3 S and 1s2s.sup.1 S states of lithium II. Radiation from a xenon pulsed discharge lamp at about 9620 A was used to transfer this population from these states, level 2, to the 1s2sP state, level 3, which then radiated to ground at the wavelength of 199 A, the radiated energy being radiated by a fluorescent process.
The present invention is based upon the realization that vacuum ultraviolet light may be obtained by excitation of populations in metastable levels and the subsequent extraction of energy from these levels by spontaneous anti-Stokes scattering. The phenomena has been known for many years, and has often been used for diagnostic applications. For example, L. Y. Nelson, A. W. Saunders, Jr., A. B. Harvey, and G. O. Neely, "Detection of Vibrationally Excited Homonuclear Diatomic Molecules by Raman Spectroscopy, " J. Chem. Phys. 55, 5127 (Nov. 1971) shows the use of both Stokes and anti-Stokes emission for studying vibrationally hot nitrogen molecules produced in an electrical discharge. In the above experiment radiation is observed at the sum frequency of the vibrational storage level in nitrogen at about 2330 cm.sup.-1 and the incident laser frequency of 4800 A. Anti-Stokes emission has also been used as a spectroscopic tool to study populations of many thermally excited organic species.
P. Braunlich, R. Hall, and P. Lambropoulos, "Laser-Induced Quenching of Metastable Deuterium Atoms. Singly Stimulated Two-Photon Emission and Anti-Stokes Raman Scattering, "Phys. Rev. A 5, No. 3 (Mar. 1972), describe the use of a collimated beam of deuterium metastable atoms excited by a laser to induce two-photon and anti-Stokes scattering in the vacuum ultraviolet region of the spectrum. The density of the atoms in the beam is low and the radiation is weak.