Extended Theory of Ionic–Covalent Coupling in Collisions of Rydberg Atoms with Neutral Targets

Abstract

The work is devoted to studies of ion-pair formation and resonant-quenching processes in slow collisions between the highly excited atoms and the ground-state atoms with small electron affinities. We elaborate a general approach for the calculation of transition matrix elements between the ionic and Rydberg-covalent states of a diatomic quasimolecular system by using the momentum representation for the highly excited electron wave function and a technique of the nonreduced tensor operators. The key point of our approach is that it allows us to correctly describe the variation of the Rydberg atom wave function in a wide range of electron coordinates, which is determined by the average size of the weakly bound anion wave function. The theory developed provides an efficient way for exact evaluation of the Rydberg-covalent–ionic coupling terms and generalizes the results of previous theories to systems with large characteristic sizes of the interaction potential between the outer electron and the perturbing electron-attaching particle. It is shown that the general expression of the present theory for the ionic–covalent coupling parameter contains some available analytical results of previous works as special cases. Particular attention is paid to the investigation of the effects associated with the long-range part of electron–perturber interaction in the charge transfer processes involving strongly polarizable perturbing atoms with small electron affinities. The results obtained are illustrated by numerical calculations of the ion-pair production processes in slow collisions of Rydberg { Ne}(ns) and { Ne}(nd) atoms with the ground-state alkaline-earth atoms Ba, Sr, and Ca.