We present a theoretical study of two-color ionization of hydrogen with keV photons at intensities ranging from ${10}^{16}$ to ${10}^{18}\mathrm{W}/{\mathrm{cm}}^2$. We consider the atom in interaction with a superposition of two electromagnetic pulses centered around two frequencies that differ by a few atomic units and we present in detail the case of the frequencies 55 and 50 a.u. We present the electron energy spectra, angular distributions, and ionization rates based on nonperturbative and perturbative calculations. Although the ejected electron energy distribution is dominated by one-photon ionization from each pulse, we are able to identify the contribution of stimulated Compton scattering, a process in which one photon is absorbed while the other is emitted, the photon energy difference being transferred to the electron. This leads to low-energy electrons, and we show in particular that it is of crucial importance to consider the retardation effects on the ionization rates and the electron angular distributions. The relative propagation direction of the two fields also plays an important role; in the case of counterpropagating fields, the ionization by stimulated Compton scattering is dominated by ${\mathbf{A}}^2$ and competes with one-photon ionization at high intensities.

• Received 28 August 2013

DOI:https://doi.org/10.1103/PhysRevLett.112.073001