Electron energy loss processes at subelectronic excitation energies in liquids

Abstract

The theory of the processes by which subexcitation electrons lose energy in molecular liquids is considered. It is convenient to classify such losses as resulting from 'indirect' and 'direct' interactions; the former interactions arise from the transient electric field of the electron and the latter from the short range quantum mechanical forces occurring in direct collisions. Indirect processes are considered at some length; Monte Carlo trajectories for electrons are generated and the Fourier spectrum of the electric displacement obtained from which energy loss to dipolar relaxation and infrared-active vibrations are estimated. A tight binding scheme is used to derive an expression for the rate of energy loss to single-phonon excitation in direct collisions; the same theoretical framework is used to discuss electron trapping by the molecules of the liquid. Finally, some attempt is made to relate the loss mechanisms considered to experimentally known facts. It is concluded that the 'direct' processes are probably dominant and that knowledge of elastic scattering is of primary importance for the construction of a satisfactory theory.