Atomic photoionization by intense, short laser pulses

The Strong Field Approximation (SFA) (1) method is applied to a hydrogenic atom interacting with an intense, short-pulse, circularly-polarized laser field. The field is taken to be in the form of a one-cycle wave-packet pulse whose amplitude has a Gaussian temporal dependence. The electron, which is initially bound to the hydrogen nucleus, is described by a wave function which is a solution to the Schrodinger wave equation. The SFA is an S-matrix method which uses the time-reversal symmetry property of quantum mechanics to place the focus of field interaction on the final state rather than the initial state. This fully interacting final state is approximated by a Volkov solution, in which the binding potential of the atom is neglected and only the field-interaction effects are included. Numerical procedures are used to integrate over those domains in time and energy which make the dominant contributions to the ionization process. Results from calculations which determine the probability of ionization and the energies of the final-state electrons are presented. Transient probabilities that follow the progress in time of the electron show that, during the pulse, the electron has a large probability of reaching a Volkov state and then revisiting the atom. This is attributed to the interference between the terms of the interaction Hamiltonian. Unlike the final-state ionization probability in the long-pulse case, the probability in the short-pulse case exhibits dependence on the azimuthal angle. Spectral calculations of photoelectron energy present peaks seen in both long-pulse theory and in experiment.