Mechanisms producing inelastic structures in low-energy electron transmission spectra

Electron-energy-loss mechanisms producing structures in the low-energy electron transmission (LEET) spectra of condensed molecular films are examined within the framework of two-stream multiple-scattering theory. A phenomenological model is developed and applied to specific processes occurring in the course of the propagation of low-energy electrons through thin films. Electronically excited states having strong excitation cross sections near threshold or oscillations in their excitation functions are found to produce sharp oscillatory features in LEET spectra. Analysis of the dc and doubly differentiated spectra of N₂ and CO films reveals that threshold effects and resonances (i.e., transient anions) present in the gas-phase cross sections of these molecules reemerge in the condensed phase provided that they involve valence orbitals.