Phonons in the Commensurate Monolayer ofD2on Graphite

Abstract
The in-plane phonons of the commensurate monolayer of deuterium adsorbed on graphite have been measured. They show evidence for dispersion rather than completely localized mode character. In addition to the phonon gap a transverse and a longitudinal zone-boundary phonon could be measured. The coupling constants between the adsorbed molecules themselves and between molecules and the substrate have been deduced. It is found that the in-plane coupling to the substrate is nearly 10 times stronger than the coupling to neighboring molecules.