We extend previous nonrelativistic dipole-approximation theoretical studies of two-photon electron bremsstrahlung in the Coulomb field by calculations exploring the cross section that describes the emitted photons, irrespective of the direction of the scattered electron, and its dependence on the charge Z of the target, for different detection geometries. A Born-approximation equation, valid for any emittedphoton configuration, is derived. Comparisons with the most recent experimental data are presented. PACS number(s): 34.80.i 32.80.Wr

Theoretical information on dynamic stabilization ͑DS͒ of the ground state of H is still fragmentary, while its detection has remained open because of a lack of lasers with the required characteristics. The problem has been reactivated by the new light sources that are being developed, such as VUV-FEL's or attosecond pulses from high-harmonic generation, which will offer adequate frequencies, intensities, and pulse durations. We are now presenting a mapping out of DS over an extended range of high frequencies, for pulses with various envelopes, peak intensities, and durations. We find prominent DS under conditions where our nonrelativistic, dipole approximation, and calculation should be valid. There is a marked dependence of DS on the pulse shape. The effect of quasistationary stabilization ͑of the rates͒ on DS is analyzed. We comment on the impact of our results on the detection possibility. We conclude that ground-state DS for H should be observable with the new light sources in a state-of-the-art experiment.

This paper addresses the problem of above-threshold ionization (ATI) of hydrogen interacting with a short and intense x-ray electromagnetic field. We consider the weakly relativistic regime where the speed of the photoelectron stays well below the velocity of the light c. We solve the time-dependent Schrödinger equation (TDSE) using a spectral approach with a basis of one-electron orbitals, calculated with L 2 -integrable B-spline functions for the radial coordinate and bipolar spherical harmonics Y lm for the angular part. Retardation effects are included up to O(1/c); they induce two extra terms leading to electric quadrupole and magnetic dipole transitions. These latter terms depend on the polarization and photon momentum directions, forcing the resolution of the TDSE in a three-dimensional space. Relativistic effects [of O(1/c 2 )] are fully neglected. Photoelectron energy and angular distributions are obtained for photon energies ranging from 200 eV to 3 keV. We study the lower energy region of the ATI spectrum and we analyze nondipole effects, through their (l,m) components, in particular, for the lower two peaks. Although these effects are small, we show that, increasing the photon energy, the photoelectron angular distributions begin to differ significantly from the ones obtained in dipole approximation.

We present a theoretical study of two-color ionization of hydrogen with keV photons at intensities ranging from 1016 to 1018  W/cm2. 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 A2 and competes with one-photon ionization at high intensities.

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