Multiphoton ionization in ultrahigh optical fields: a statistical description

The potential role of multiply excited states in multiphoton ionization of atoms under high-intensity ultraviolet laser radiation in the range of 1015–1017 W/cm2 is discussed. Since the density of multiply excited states is sufficiently great to form a quasi-continuum, the coupling can be described by an average one-photon absorption cross section. A numerical fit with experimental data from xenon, produced by 193-nm radiation, assuming an autoionization rate of 1013 sec−1, yields a cross section of 4.5 × 10−19 cm2. The resulting transition rates indicate that the motion of the electrons is highly coherent, with a ratio of dephasing rate to excitation rate of ~10−2. At 1017 W/cm2, the transition rate exceeds the optical frequency for 193-nm radiation by a factor of 30. This indicates that even for the shortest optical pulses, atoms start to interact violently with the optical field long before the peak intensity is reached and that unconventional theories will have to be developed if the observed phenomena are to be understood fully.