Structure and lattice dynamics of o-H2 and p-D2 monolayers on graphite

Abstract

Using the H 2-H 2 potential from ab initio calculations and the isotropic H 2-graphite potential from selective adsorption measurements, we have calculated the stability and lattice dynamics of o-H 2 and p-D2 monolayers on graphite. An anisotropic H 2-graphite potential was modelled such that either the herringbone or the pinwheel ordering of the adsorbed layers is stable, at very low temperature. For both these orientationally ordered overlayers we have computed the phonon and libron band structure by the time-dependent Hartree method. We find small coupling between the translational phonons and the librons, which can be well taken into account by renormalisation of the appropriate coupling constants. In the pinwheel structure, observed for o-H 2 monolayers by NMR, the differences between the axial and the wheel molecules are considerably smaller than in the corresponding structure for N-, layers on graphite.

Helium atom scattering studies of the structure and vibrations ofH2physisorbed on MgO(001) single crystals

“…Gottlieb and Bruch 65,66 as well as Novaco 68 and Janssen et al 70,71 have simulated the structures and dynamics, respectively. In unpublished work Bruch investigated the H 2 /MgO dynamics by extrapolating the H 2 /graphite dynamics via scaling the H 2 /graphite potential to agree with the experimentally determined H 2 /MgO binding energies ͑20-25 meV per molecule͒.…”

“…20 Theoretical calculations regarding the structure and energies have been performed by Bruch and co-workers 65-67 as well as calculations on the dynamics by Novaco, 68,69 and will be referred to later on in the discussion of the present results. Studies on the related problem of hydrogen isotope layers have been performed by Janssen et al 70,71 Up to now, the phonons and external vibrations of hydrogen layers on insulator surfaces have not been studied as far as we are aware. Here we report on phonon dispersion relations for a monolayer quantum solid on a single crystal substrate as compared to the powder or exfoliated substrates in the neutron scattering studies.…”