Converged three-dimensional calculations of above-threshold ionization: Angular-momentum constraints and the kinetic-energy distribution

We present an application of the artificial-channel method to the calculation of multiphoton ionization rates under continuous-wave illumination conditions. We use the method to treat above-threshold ionization (ATI) of hydrogen in three dimensions with linearly and circularly polarized light at field intensities ranging from ${10}^9$ to ${10}^{14}$ W/${\mathrm{cm}}^2$. The results are compared with perturbative as well as nonperturbative calculations for short pulses and with experiments. The range of validity of the dipole approximation for ATI processes is also examined. An analysis of both the angular and angular-momentum distribution of the photoelectrons at different intensities is presented. We find that ATI cross sections for circularly polarized light at 532 nm are three orders of magnitude weaker, with electronic kinetic energies peaking at higher values, than in the case of linearly polarized light. This last effect is shown to result from the different ponderomotive potential experienced by the electron, due to the drastically different angular distributions computed for the two cases. Connection is made between the angular distributions, and the angular-momentum constraints affecting it, and the kinetic energy of the ensuing electrons. It is argued that it is difficult to understand the ATI process without taking the three-dimensional aspects of the problem into consideration.