Resonances in Electron Scattering by Atoms and Molecules

Abstract

This chapter deals with resonances produced in electron scattering by atoms and molecules. Closed channel resonances arise when the interaction potential between the incident particle and an excited state of the target is strong enough to support a bound state. If this state lies energetically above the ground state of the target with the incident particle at infinity, and is coupled to this continuum state by terms in the Hamiltonian, it will decay with the emission of the incident particle. The interaction potential between the incident particle and the target, which may be in an excited state, must now be such that an intermediate state can be formed, without changing the quantum state of the target, where the lifetime is appreciably greater than the collision time. Potentials with this property usually have repulsive barrier tails, which are often caused by the centrifugal force. In many cases, the velocity of the ions is so low that the Massey adiabatic criterion would predict very small ionization cross sections.