Two-frequency multiphoton ionization of hydrogen atoms

The theory of two-frequency multiphoton ionization of hydrogen is reconsidered with particular emphasis on gauge aspects. The analysis is specialized to the case when a relatively weak high-frequency radiation field causes the ionization, while an intense low-frequency radiation field produces structured continuum states for the ionized electrons. Previous results, available in the literature, are recovered as limiting cases of the expressions derived in the present paper. Angular distributions, total cross sections as a function of the low-frequency field intensities, and photoelectron spectra are calculated. Several new features are found and compared with the available information