MULTIPHOTON PROCESSES IN AN INTENSE LASER FIELD .6. 2-COLOR IONIZATION WITH INCOMMENSURABLE FREQUENCIES

We present results of Floquet calculations of rates and yields for ionization of atomic hydrogen, in its ground state, in the presence of a relatively weak high-frequency field and a relatively strong low-frequency (1064-nm) field. We assume that the two frequencies are incommensurable, or, if they are not, we average over the relative phase. We explore three processes: (i) Resonance-enhanced ionization, via levels that are ac-Stark mixed or/and very strongly shifted, (ii) laser-induced continuum structure, and (iii) nonresonant ionization in a regime where, despite the fact that the infrared field significantly shifts the ground state relative to the continuum, it only redistributes the photoelectrons over the low-frequency channels. We also illustrate the sensitivity of the ionization yield to the spatiotemporal configuration of the two fields.