Quantum mechanical theory of collisional ionization in the presence of intense laser radiation

Abstract

We present a quantum-mechanical formalism for treating ionizing collisions occurring in the presence of an intense laser field. The theory rigorously takes into account both the intense laser radiation and the internal electronic continuum states associated with the emitted electrons. We accomplish this essentially by combining discretization techniques, used in a recent study of fieldfree collisional ionization, with expansions in terms of so-called electronic-field representations for the quasi-molecule-plus-photon system. This leads to a coupled-channel description of the heavy-particle dynamics which involves effective electronic-field potential surfaces and continua. We also discuss qualitatively characteristic features of ionizing collisions accompanied by intense lasers, drawing comparisons with their fieldfree counterparts. Our remarks are designed to encourage experimental investigation of collisional ionization in the presence of intense lasers, and to stimulate further theoretical work. Because the electronic continuum meets requirements of exact energy resonance for absorption of a photon over large ranges of the internuclear separations, collisional ionization in an intense field should occur much more readily than other field-influenced inelastic collisions, in which photon absorption is resonant only near potential surface pseudocrossings. We therefore suggest laser-influenced ionizing collisions as very good candidates for experimental verification of the effects of intense laser radiation on inelastic collisional processes. We describe the anticipated behavior of the energy distribution of electrons emitted due to radiative coupling. Our comments are based on some physically reasonable assumptions about the electronic transition dipole matrix elements between discrete and continuum electronic states. Actual calculations of such matrix elements involve special electronic structure considerations, and these are outlined in some detail in an Appendix.