Interaction of low-energy electrons (1—30 eV) with condensed molecules: II. Vibrational-librational excitation and shape resonances in thinN2and CO films

Abstract

Low-energy [(1-30)-eV] electron scattering in multilayer films of ${\mathrm{N}}_2$ and CO, and in Ar matrices containing ${\mathrm{N}}_2$, was investigated using high-resolution electron-energy-loss spectroscopy (HREELS) and one-dimensional multiple scattering theory. The films were formed by condensing the gases on a polycrystalline platinum substrate held near a temperature of 20 K. Comparisons of experimental electron-energy-loss spectra in the range 0-2.4 eV with those generated theoretically indicated that the former are composed of peaks which result from single and multiple vibrational losses broadened by multiple phonon losses. The excitation function of the $v=1-3\mathrm{}$ vibrational levels of ground-state ${\mathrm{N}}_2$ and CO were measured under different film conditions and angles of incidence of the primary beam. In CO the excitation functions exhibited broad peaks at 2.5 and 20 eV due to the formation of ${}_{}{}^2{}_{}{}^{}\Pi$ and ${}_{}{}^2{}_{}{}^{}\Sigma$ transient anions, respectively, similar to those previously found in ${\mathrm{N}}_2$ films. From measurements of the intensity of the fundamental vibrational loss in ${\mathrm{N}}_2$ as a function of film thickness, the total cross section for excitation of all possible vibrational levels of ${\mathrm{N}}_2$ was found to be 3.3 × ${10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$ at resonance, and the sum of all phonon cross sections was found to be about 5 × ${10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$.