Dynamics of Low Energy Electron Collisions with Molecules and Clusters

Abstract

The dynamical behaviour of slow electrons traversing ases is to a large extent determined by two effects: the energy dependent evolution of the scattering phases for the relevant partial waves and the influence of temporary negative ion states (resonances)1. For quite a few atoms and molecules, special behaviour of the s - wave (L = 0) phase shift leads to a deep Ramsauer-Townsend minimum in the scattering cross section between 0 and 1 eV which strongly affects the electron mobility in these gases. Even more importantly, resonances (compound states of the electron-molecule system with lifetimes ranging typically from 10^-15 to 10^-11 s) are often found to dominate the dynamics of electron-molecule collisions over the energy range 0 to 10 eV. The extended time interval (compared with the direct transit time which is below 1 fs), spent by the incoming electron close to the target while in the resonance state (lifetime τ = ħ/Γ, Γ= resonance width), has profound effects especially on collision channels which involve a reaction of the nuclear framework, i.e. on vibrational excitation (VE) and on dissociative attachment (DA). Apart from well-known shape resonances such as H₂ ^-(²∑_u), N₂ ^-(²∏_g), CO^-(²∏), O₂ ^-(²∏_g, v ≥ 4) which are located below the lowest limit for DA and owe their lifetime to the centrifugal barrier of the electron, repulsive anion states above the DA limit are important for VE as well as DA. The importance of resonances for vibrational excitation (VE) as well as negative ion formation via dissociative attachment (DA) is illustrated in Fig. 1.