Theory of the orbital angular momentum and energy transfer processes in collisions involving Rydberg atoms

Abstract

A detailed semiclassical study of the orbital angular momentum and energy transfer processes induced by collisions of Rydberg atoms with atomic particles is reported. Basic equations of the unitarized perturbation theory for the probabilities and cross sections of the inelastic transitions are derived using the impact-parameter method combined with the JWKB approximation for the wavefunctions of highly excited states. Analytical formulae are given for the quasi-elastic l-mixing cross sections and rate constants averaged over the Maxwellian velocity distribution. Unlike the well known formulae of the Born and impulse approximations they are valid for both the weak- and close-coupling regions and may be used at high, intermediate and low enough n. The contributions of the short- and long-range interaction between the Rydberg electron and perturbing atom are explicitly expressed as functions of the principal quantum number and the inelasticity parameter of the process. Theory is applied for calculations of quenching processes in thermal collisions of the Rydberg , and atoms with the ground-state atom and for the comparison with experiment. It is demonstrated that the orbital angular momentum mixing processes are appreciably dependent on the energy transferred to the Rydberg atom even at small quantum defect of the initial nl-state. This work points out that the l-mixing collisions with large values of the transition energy defect behave essentially like inelastic n,l-changing processes and cannot be described within the framework of available theoretical models for the orbital angular momentum transfer. The observed effect, of a strong drop of the l-mixing cross sections in the collisions as compared with the quasi-elastic process involving the -state, is quantitatively explained using the developed theory.