Excitation of Atomic Hydrogen by Proton Impact in the Presence of a Resonant Laser Field Using the First Order Magnus Approximation

Abstract

We consider proton collisions from hydrogen atoms in the presence of a laser beam (taken in the electric dipole approximation) that resonantly (or nearly resonantly) excites the hydrogen atoms from the Is to the 2p state. The laser beam is linearly polarised with polarisation either parallel (longitudinal) or perpendicular (transverse) to the direction of incidence of the proton. A non-perturbative quasi-energy approach is used to describe the laser-atom interaction, while the first-order Magnus approximation is used to describe the collision dynamics in the presence of the nearly resonant laser beam. We have calculated the integrated cross section o-(2s} for the excitation of the 2s state. It is found that 0-(2s} is small for longitudinal polarisation, as compared with transverse polarisation. We have also compared our field-free results obtained by using the first-order Magnus approximation to that obtained by the close-coupling approximation. Although both methods give excellent results, the former method is quite demanding in terms of computer time.