Odyssey and oddity: Photo- and collision processes you would not expect

Abstract

A long-standing investigation of one intriguing collision process, Penning detachment, involved a series of theoretical studies and then experiments intended to exhibit the phenomenon. After some exploratory studies, the experiments, to our surprise, produced clear, unambiguous results that were due to an altogether different process. Identifying and pursuing that process required further theory as well as more experiments. The outcome, reviewed here, has been (a) predicted cross sections for Penning detachment of electrons from negative atomic and cluster ions; (b) predictions of cross sections for resonant two-photon and above-threshold three-photon ionization, (c) measurements of cross sections for superelastic collisions of slow electrons with excited atoms, and, still in progress, (d) theoretical values for cross sections of such superelastic collisions. The experimental method offers a new way to carry out studies of collisions of slow electrons with very high energy resolution.