Collisional ionization in the presence of intense laser radiation: Quantum-mechanical calculations for He* (3S)+Ar±h/ω→He+Ar+(2P)+e−

Abstract

We present a quantum-mechanical coupled-channels approach for studying laser-assisted collisional processes leading to transitions between discrete and continuum electronic states. This coupled-channels approach is an approximate application of our more general quantum-mechanical formalism for treating bound-continuum collisional transitions assisted by intense laser radiation, and uses discretization procedures to deal with the continuum states, and so-called electronic-field representations to account for the presence of intense laser fields. We discuss in detail all approximations leading to our coupled-channels approach. We then use our approach to calculate laser-assisted Penning ionization of Ar by He* (1s2s,3S) for the specific cases of radiation from the 10.6 μm line of a CO2 laser of intensity near 1.6×108 W/cm2, and the 1.315 μm line of an iodine laser of intensity near 109 W/cm2. We obtain ionization cross sections (per unit emitted-electron energy ε) dσ/dε as a function of ε. dσ/dε reflects a spectrum of emitted-electron energies that exhibits pronounced laser-modified collisional effects. The usual field-free spectral region around ε0(?0.149 hartree) is augmented by new spectral regions governed by radiative coupling and located near ε+(−)?ε0+(−) h/ω, where h/ω is the laser’s photon energy. Compared to their field-free counterparts, these laser-modified spectral contributions are more sharply peaked and favor Penning ionization over associative ionization more strongly. In carrying out these calculations we treat the radiative coupling in the dipole approximation. By analyzing this radiative coupling for He* (1s2s,3S)+Ar Penning ionization, we show that it rigorously vanishes asymptotically. We use this fact to propose a physically reasonable functional form for such radiative coupling between bound and continuum electronic states.