The structures and vibrations of H2 monolayers on NaCl, MgO and LiF: similarities and differences

A new low-temperature target manipulator (TS approx. = 7 K) has made it possible to extend diffractive and inelastic time-of-flight He atom scattering to the investigation of the structures and external vibrations of the monolayers H2/NaCl, H2/MgO and H2/LiF. On NaCl, H2 forms a (1 x 1) monolayer, whereas in the other systems a series of ordered increasingly compressed structures develops with increasing coverage. On MgO these structures are assigned to c(2 x 2), c(4 x 2) and c(6 x 2) phases. On LiF the most stable structure is, in analogy to H2/MgO, proposed to be close to c(8 x 2) with a 5% misfit with respect to the LiF substrate. For p-H2/NaCl, single and overtone vibrational excitations are observed up to the second overtone as well as combinations of a parallel nearly dispersionless mode, at 7.0 meV, and a new dispersive branch with 8.5 meV at the zone boundary. On MgO and LiF three collective vibrations are found. On MgO two non-dispersive perpendicularly polarized modes at 8.5 and 10.5 meV and a dispersive parallel mode at a lower energy are identified. On LiF only one non-dispersive mode at 10.5 meV and two dispersive unassigned modes at lower energies are found. The results for all three systems are compared, and they lead to some general conclusions about dynamical processes on cryogenic films of H2 on ionic crystals