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The recently observed generation, by samples of atomic rare gas, of high-order harmonics of the fundamental frequency of an infrared laser with orders N greater than 100, opens the possibility to have at one's disposal a table-top source of coherent and pulsed VUV, (or even soft-x-ray), radiation. The characteristics of such sources, (very high brightness, sub picosecond duration, . . .), as compared to conventional UV and soft x-ray sources, has recently attracted a considerable attention. Their usefulness has indeed been demonstrated in recent solid state inner-shell spectroscopy experiments, and also to determine the lifetimes of atomic autoionizing states in He. We discuss here, via a theoretical study involving the numerical resolution of the Time-Dependent Schrodinger Equation (TDSE), the main characteristics of a new class of two-color photoionization processes, dubbed Laser-Assisted Single-Photon Ionization (LASPI). The idea is to expose simultaneously an atomic target to the field of, on the one hand, a high order harmonic radiation with frequency (omega) H, and on the other hand, the fundamental of the laser, with frequency (omega) L, which has been used to generate the harmonic field. Then, the radiation pulses are highly correlated and one expects that they have similar durations, which conveniently ensures that the target experiences the two fields simultaneously. Then, if the harmonic frequency is high enough, i.e. is such that: (omega) H greater than EI, where EI is the ionization energy of the atom, LASPI can be observed (atomic units will be used throughout, unless otherwise mentioned). As we show, our simulations indicate that one can find a regime for the respective intensities of the fields, in which the ionization of the target results in fact of the simultaneous absorption of one single photon from the high frequency field and of the exchange of a number of laser photons. The first experimental observation of this process has been reported at this conference. Note also that the observation of a similar process, namely laser-assisted Auger transitions, has been also recently reported.
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