Laser-induced free-free electronic transitions near a resonance

Abstract

A theory of electron-atom scattering in the presence of moderately intense laser fields is applied to resonant electron-atom scattering. The radiation field is treated in the lowest perturbative order and the electron-atom interaction is approximated in terms of the N-state close-coupling model. A resonance in the elastic electron-atom cross section in the absence of the laser field at the energy ER leads to two resonances in the free-free absorption cross section when the laser field is present. These two resonances are due to resonant scattering of the electron by the target atom before or after a photon is absorbed. If the field-free electron-atom resonance occurs in the lth partial wave, and if the photon polarization vector is taken parallel to the initial electron momentum, the angular distributions corresponding to these two resonances are proportional to cos2 theta Pl2(cos theta ) or to Pl2(cos theta ). Furthermore two versions of soft-photon approximations are tested numerically on a two-state close-coupling model. For a CO2 laser these soft-photon expressions agree within an error of only 2% with the results of the exact calculations. Application of this soft-photon approach to the Ar+e-+h(cross) omega CO(2) experiment of Andrick and Langhans (1975) yields a free-free transition rate of 150 s-1. These results agree to within 2% with the numerical calculation of Geltman (1972), who employed more realistic model potentials.