MULTIPHOTON PROCESSES IN AN INTENSE LASER FIELD .5. THE HIGH-FREQUENCY REGIME

We present results of Floquet calculations of shifts and widths of the 1s and 2s energy levels of atomic hydrogen irradiated by intense linearly or circularly polarized light whose frequency omega is above the (weak-field) threshold omega-thr(i) for one-photon ionization from state i. We have studied the dependence of the shifts and widths on omega and on the intensity I. Where possible, we compare our results with those obtained from a high-frequency theory [M. Pont and M. Gavrila, Phys. Rev. Lett. 65, 2362 (1990)] that yields shifts that depend only on alpha-0 (proportional square-root I/omega-2), the excursion amplitude of a free electron, rather than on I and omega separately. As I increases, with omega fixed, the width reaches a maximum value GAMMA-max at an intensity I(max) for which square-root (h-omega/2P) almost-equal-to 1, where P = 2-pi-I/mu-c-omega-2, the ponderomotive shift. As I increases beyond I(max), the width decreases toward zero, in accord with the high-frequency theory, and the shift approaches the result of that theory. (For different fixed omega, the shifts first cross the omega = infinity curve as alpha-0 increases, and they intersect, almost at a common value of alpha-0, before approaching the omega = infinity curve.) As omega increases, I(max) increases as roughly omega-3, and GAMMA-max decreases. If omega is sufficiently large, we find that (2-pi/omega)-GAMMA-max/h < 1, so that, for very high frequencies, ionization takes place over more than one cycle even at large intensities. At frequencies below omega-thr(i), we detect states that emerge from "shadow" states; these states allow for a continuous variation of the shift and width across the threshold. Furthermore, we conjecture that the rise and fall of the width, in the vicinity of I(max), occurs through an interaction between shadow and real (or "dominant") states.