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

The role of the conduction-band density of states (CB DOS) for electron sca ttering in the condensed phase is investigated using electron-energy-loss s pectroscopy of a molecular target isolated in a host medium. As an experime ntal model we study O-2 embedded in an Ar matrix, since O-2 in the gas phas e offers smooth resonant vibrational cross sections spanning most of the en ergy range where the CB DOS of solid Ar is known. The vibrational energy-lo ss intensities of matrix isolated O-2 exhibit strong variations as a functi on of the incident energy (i.e., excitation functions), in contrast to its gas-phase counterpart. Except for a relative change in intensity, the featu res in the excitation functions remain essentially at the same energy for d ifferent scattering angles and follow the inverse of the CB DOS. We show th at 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 spe cies an further discussed in terms of transport phenomena subjected to the CB DOS.