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bound WP in an atom or a molecule and a delayed IR pulse, locked in phase with the XUV pulse, is used to probe it. Coincident with the creation of the bound WP, we also create a continuum WP, which serves as a ref- erence. After a variable delay, the bound WP is ionized by the IR pulse, and both continuum WPs, created di- rectly by the XUV pulse, or by the two-step (XUV+IR) process, interfere. The analysis of the interferogram ob- tained when measuring the photoelectron spectrum as a function of delay allows us to determine the spectral components of the bound WP. This technique enables us to obtain a spectral resolution given by the inverse of the IR-XUV delay, which is typically a few tens of femtosec- onds i.e. more than a factor 100 better than the Fourier limit of the (attosecond) excitation pulse. We demon- strate the technique experimentally using an attosecond pulse with 350 as duration and central energy of 24 eV which excites a broad WP in helium, including bound (unknown) and continuum (reference) components. A few-cycle IR pulse probes the bound WP, and analyzing the electron spectra allows us to recover the composition of the WP. 

In the experiments, performed in Milan, a linearly po- larized phase stabilized 5-fs IR laser pulse is divided into a central and annular part using a mirror with a hole in the center. The polarization gating technique is used on the central part to obtain a laser pulse with a temporal window of linear polarization with duration of less than half an optical cycle [14]. This laser beam is focused into a xenon gas cell to generate XUV radiation via high order harmonic generation. The low order harmonics and the collinear IR radiation are removed using a 100 nm thick aluminum filter. This metallic filter also provides partial dispersion compensation of the intrinsic positive chirp of the emitted pulse [15], compressing it to a duration of 350 attoseconds. The attosecond pulses are focused using a grazing incidence toroidal mirror into the active region of a velocity map imaging spectrometer (VMIS) [16], used to record the photoelectron momentum distributions. 

The attosecond pulses have a central frequency of 24 eV with a bandwidth exceeding 10 eV and excite he- lium from its ground state to a coherent superposition of bound and continuum p states. At a controllable time-delay the bound WP is ionized by the probe IR laser (bandwidth 0.53 eV), which is collinearly recom- bined with the attosecond pulse using a second mirror with a hole in the center and which is focused by a spherical mirror. The IR intensity was close to 1 × 10 13 W/cm $^{2}$, well below that necessary to tunnel ionize He in its ground state, but high enough to induce “streaking” when both IR and XUV pulses overlap [3, 19], allowing us to determine the delay between the two pulses. The XUV and IR beams are crossed with an effusive He gas jet emerging from a capillary incorporated into the repeller electrode of the VMIS [17]. Using a set of electrostatic lenses, the electrons emitted in the two-color photoion- 

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ization process are accelerated onto multichannel plates coupled to a phosphor screen detector. The effusive gas jet allows us to obtain a gas density of ∼ 3 × 10 15 cm − 3 in the interaction region, while keeping a low enough pres- sure close to the multichannel plates. Two dimensional images are acquired with a CCD camera and used for the retrieval of the 3D initial velocity distribution [18]. 

A scan of the photoelectrons emitted in a small angle around the polarization axis in the upward direction is shown in Fig. 2. Both fields are vertically polarized. The color indicates the photoelectron intensity S$_{exp}$ ( E, t ) as a function of observation energy E and delay t . When the two pulses overlap temporally (at 0 fs delay), the photo- electron spectra are streaked, indicating photoionization by an isolated attosecond pulse in the presence of a laser field [3, 19]. In the more interesting region where the at- tosecond pulse precedes the IR probe, interference fringes are observed in the low energy region of the spectrum, up to about 2 eV. As explained below, interferences fringes are expected also at higher energy, but the spectrometer resolution in this region prevents their observation [20]. This interference pattern is due to the multiple pathways leading to the same final continuum energy. During the delay, t , a continuum state with energy E and a bound, stationary state with energy E$_{i}$ accumulate a phase dif- ference ( E − E$_{i}$ ) t/ ¯ h . The interference fringes, defined as the curves of constant phase difference, are therefore hy- perboles, which become more closely spaced as the delay increases. 

In Fig. 3, we present quantum calculations based on the single active electron approximation [6] under con- ditions that are close to those of the experiment. The probe has a 6 fs duration with a cos 2 shape and an in- tensity of 1 × 10 13 W/cm $^{2}$, while the duration of the at- tosecond pulse is 180 as which is shorter that the 350 as in the experiment but chosen to match the experimental bandwidth [20, 21]. The theoretical spectra as a func- tion of delay, S$_{theo}$ ( E, t ), shows electrons emitted in the direction of the IR and XUV polarization axis. Interfer- 

