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Photoelectron energy spectra resulting by the interaction of hydrogen with two short pulses having carrier frequencies, respectively, in the range of the infrared and XUV regions have been calculated. The effects of the pulse duration and timing of the X-ray pulse on the photoelectron energy spectra are discussed. Analysis of the spectra obtained for very long pulses show that certain features may be explained in terms of quantum interferences in the time domain. It is found that, depending on the duration of the X-ray pulse, ripples in the energy spectra separated by the infrared photon energy may appear. Moreover, the temporal shape of the low frequency radiation field may be inferred by the breadth of the photoelectron energy spectra.Comment: 12 pages, 8 figure

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 highfrequency 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.