CORRUGATION IN THE NITROGEN-GRAPHITE POTENTIAL PROBED BY INELASTIC NEUTRON-SCATTERING

Inelastic neutron spectra of the commensurate 3 ×3 herringbone monolayer phase of nitrogen physisorbed on the graphite (002) surface at low temperature have been compared with lattice dynamics calculations of the polycrystalline-averaged one-phonon coherent neutron cross section. The observed zone-center energy gap of 0.4 THz in the acoustic-phonon branches is a factor of 2 larger than calculated from central atom-atom potentials. We conclude that current models of the corrugation in the adatom substrate potential greatly underestimate the lateral restoring forces in this relatively simple molecular film. © 1990 The American Physical Society.