Experimental and theoretical studies of the Bi-excited collision systems He*(23 S) + He*(23 S, 21 S) at thermal and subthermal kinetic energies

Abstract

We have carried out a comprehensive experimental and theoretical investigation of the autoionizing collision systems He*(23 S, 21 S) + He*(23 S). We present high resolution electron energy spectra, obtained with a single He* beam (average relative collision energy 〈E rel〉=1.6 meV) and with crossed He* beams (〈E rel〉> =61 meV). The spectra show substantial structure, and under single beam conditions fast oscillations due to the interference of incoming and outgoing heavy particle waves in the entrance channels are observed. Accurate ab initio potential curves for the seven lowest He*—He*(Σ) molecular states have been obtained from a Feshbach projection scheme, and width functions for He*(23 S) + He*(23 S) have been derived by Stieltjes imaging. Based on these ab initio data, detailed quantum mechanical calculations of the electron spectra have been carried out and provide a thorough understanding of the experimentally observed spectral features. Good overall agreement of the calculated spectra with the experimental data is observed. The close coincidence in the positions of the experimental and theoretical peaks, especially for He*(23 S) + He*(23 S), underlines the reliability of the ab initio potentials. In the He*(21 S) + He*(23 S) electron spectrum, the dominant peak is traced to be due to autoionization from the 23Σ+ g molecular state accessed via an avoided crossing. We also present a detailed discussion of the total ionization cross sections σtot and of the fraction σAI/σtot for associative ionization together with a critical comparison with previous work. The ionization probabilities for close collisions in entrance channels, from which autoionization is spin-allowed, are near unity, and therefore the absolute values and the collision energy dependence of the total cross sections simply reflect the long-range behaviour of the excited state potentials.