The Classical and the Quantum Face of Above-Threshold Ionization
Above-threshold ionization (ATI) was discovered in 1979 by Agostini et al. . Its discovery marks the beginning of the investigation of extreme highly non-linear phenomena in high-frequency fields, i.e. in the visible spectral region. ATI is photoionization in intense laser fields such that an atom absorbs more photons than necessary for subduing the ionization potential. This can be seen by inspection of the photoelectron kinetic energy spectra which consist of a series of peaks separated by the photon energy. Generally, the peak heights decrease as the order is increased. In this respect, the appearance of ATI spectra is similar to that for the generation of high harmonics in gaseous media (high-harmonic generation, HHG) for comparable laser intensities . For that effect, in 1988, it was discovered that the initial decrease of harmonic yield with increasing harmonic order is followed by a flat annex, i.e. the harmonic intensity is more or less independent of its order . This plateau-like feature ends with a steep cut-off. Years later, an analogous phenomenon was measured for ATI [4, 5].
KeywordsMicrowave Argon Sapphire Xenon Delone
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