Predissociation versus Autoionization for High Rydberg States of H2

Abstract

Dissociative Recombination (DR) has been a subject of research for more than 50 years.1 In a DR event, a molecular ion captures an electron forming an intermediate electronically excited neutral molecule that rapidly dissociates into fragments: AB++e(E)→AB**→A+B*+KER. The symbol ‘KER’ stands for the kinetic energy released to the fragments during the reaction and E is the electron’s energy. Among the numerous species studied so far, the molecular ions H2 +, H3 +, and HeH+ and their isotopomers have been studied in most detail. DR of molecular hydrogen serves as the prototypical DR reaction. In spite of its apparent simplicity, being a one-electron system, DR of H2 + is theoretically challenging. Although theory is making progress,2,3,4 theory cannot yet be used as an a priori reliable standard. In H2 the direct process, as formulated by Bates, is prominent. The lowest neutral doubly excited state, Q1 1Σg +, crosses the ionic state close to the outer turning point of the ground vibrational level. The lowest triplet doubly excited state, Q1 3Πg, does not have such a favorable crossing.5 As a consequence, theory generally assumes that the direct process is very slow for triplet collisions. In systems in which the direct process is impossible, such as HeH+, an alternative mechanism has been formulated in which the ion plus free electron is coupled with bound Rydberg states through the radial part of the kinetic energy operator.6,7 In HeH+, this mechanism explains a substantial DR cross section at low electron collision energies.