Vibrational electron spectroscopy of molecules in solids: Effect of the conduction-band density of states

Abstract

The role of the conduction-band density of states (CB DOS) for electron scattering in the condensed phase is investigated using electron-energy-loss spectroscopy of a molecular target isolated in a host medium. As an experimental model we study ${\mathrm{O}}_{2}$ embedded in an Ar matrix, since ${\mathrm{O}}_{2}$ in the gas phase offers smooth resonant vibrational cross sections spanning most of the energy range where the CB DOS of solid Ar is known. The vibrational energy-loss intensities of matrix isolated ${\mathrm{O}}_{2}$ exhibit strong variations as a function of the incident energy (i.e., excitation functions), in contrast to its gas-phase counterpart. Except for a relative change in intensity, the features in the excitation functions remain essentially at the same energy for different scattering angles and follow the inverse of the CB DOS. We show that under resonant scattering condition this effect arises more specifically from changes in the resonance lifetime due to variations in the CB DOS of the host medium. Using a simple Boltzmann-type multiple-scattering analysis, the vibrationally scattered electron intensities from matrix-isolated species are further discussed in terms of transport phenomena subjected to the CB DOS.