Resonance Collisions of Electrons with Molecules and in Solids

Abstract

The resonance model describes an enhancement of the amplitude of an electron at a collision target, and usually also an increase in the collision time. The importance of the model comes from the reactions made possible by these two effects, reactions which often could not occur in direct collisions.

The resonance model became established in the years 1960–1980, mainly because of its ability to explain vibrational excitation in molecules. In recent years, it has been successfully tested on collisions involving the excitation of very large numbers of vibrational quanta.

The model is still controversial in its application to threshold peaks like those observed in HCl, where the basic mechanism of the amplitude enhancement is not generally agreed. Nevertheless, successful accounts have been given of vibrational excitation and dissociative attachment in HCl near threshold.

Some of the most exciting developments on resonance collisions are happening in solid state physics. Experiments on electron collisions with molecules adsorbed on solid surfaces show vibrational excitation which resembles what we see with the same molecules in the gas phase, yet with intriguing differences which can be traced to the effect of the surface on the trapping barrier.

Resonance collisions of the valence electrons within a metal with the lattice of positive ions are particularly important in the rare earth elements, which have vacant inner atomic f-orbitals very close to the Fermi surface. Like the shape resonances of electrons in small molecules, these resonances are accompanied by a strong coupling between electrons and vibrations. Through this coupling, the resonances in rare-earth atoms seem to lead to an important contribution to acoustic attenuation, whose physical character is quite different from the traditional direct mechanism due to Pippard.