Gauge aspects in multichannel multiphoton ionization

Gauge-consistency aspects in multiphoton ionization by a strong single-mode laser field are considered. Differential and total cross sections of multiphoton ionization of the hydrogen atom are calculated in both the radiation (A gauge) and the electric field gauges (E gauge) within the framework of an S-matrix treatment, in which the wave function of the ejected electron is approximated by a Volkov-Coulomb wave function. The coupling between the initial atomic state and all the partial waves entering the expansion of the Coulomb function, resulting in a natural way from the requirements of gauge consistency, affects strongly the angular distribution of the ejected electrons, as compared with cases in which hybrid procedures are adopted