Phase-dependent effects in multiphoton ionization induced by a laser field and its second harmonic

Abstract

We present calculations of ionization rates, angular distributions, and above-threshold-ionization spectra for a hydrogen atom in a strong, two-color laser field. The two lasers are first- and second-harmonic fields with the same intensity and a constant relative phase difference between them. At longer wavelengths (1064 nm) and higher intensities (>${10}^{13}$ W/${\mathrm{cm}}^2$), there is clear evidence in the phase dependence of the ionization rates that ionization takes place primarily through tunneling. Even though the total ionization rate in this regime depends only on the peak value of the time-dependent electric field, the angular distributions show additional phase-sensitive effects. In particular, there is a large forward-backward asymmetry in the emitted electron distributions that is not simply correlated with the maximum electric field. At shorter wavelengths and/or lower intensities, there is a transition to multiphoton ionization with interference between ionization paths from the two lasers evident in the total ionization rates. In all cases, we find that the total ponderomotive shift of the ionization limit is the sum of the shifts for the two individual fields.