Resonance and Threshold Phenomena in Low-Energy Electron Collisions with Molecules and Clusters

Abstract

A survey of resonance and threshold phenomena in low-energy electron collisions with molecules and clusters is presented. Following an introduction into the role of resonances to promote vibrational excitation and anion formation through electron attachment, we discuss recent progress in achieving high energy resolution, using optimized conventional setups and photoelectron methods (the latter achieving sub-meV energy widths in attachment studies). Discussing selected cases, we highlight threshold and resonance phenomena in electron scattering and attachment channels, as observed in recent high resolution experiments and characterized by improved theoretical models. The threshold behaviour for dissociative electron attachment proceeding through s-wave and p-wave capture is demonstrated for CCl4 and Cl2, respectively. Threshold peaks, vibrational Feshbach and outer-well resonances as well as boomerang-type oscillatory structures are discussed for the polar molecules HF, HCl, and CH3I. Narrow vibrational Feshbach resonances, observed in cluster anion formation due to electron attachment to molecular clusters of CO2 and N2O, and their size-dependent redshifts, illustrating the effects of solvation, are discussed and explained as diffuse weakly-bound electron states. We briefly address the recent observation and the relevance of vibrational resonances in positron-molecule collisions at energies below 0.5 eV, as observed in positron annihilation. We conclude with a brief summary and mention some perspectives for future work.