A TIME-DEPENDENT THEORY OF RESONANT MULTIPHOTON IONIZATION OF ATOMS BY SHORT PULSES .2. STRONG-FIELD RESULTS FOR THE 2-PHOTON IONIZATION OFCESIUM

In the previous paper, a non-perturbative time-dependent theory of res onant multiphoton ionization by both smooth and rectangular pulses was derived, and as a first step, applied to the two-photon ionization of caesium in weak fields. In the present paper, we continue the analysi s of the same system, extending the weak-field results and considering the case of stronger fields. At higher intensities the spectra exhibi t energy modulations of a different character from those predicted for low power. Here, for stronger fields, the structure arises from Rabi oscillations, which, for the smooth pulses assumed here, are triggered by time-dependent Stark shifts. In order to obtain a better understan ding of the energy oscillations, we also look at the photoelectron ene rgy spectrum produced by a rectangular pulse, a problem which is solve d exactly. We show that there are other sources of strong-field struct ure.