Session D40 - Poster Session I.
POSTER session, Monday afternoon, March 12
Exhibit Hall, Washington State Convention Center
Helium atom scattering has been used to probe the surface structure and dynamical character of H_2, p-H_2, HD and D_2 molecules grown by isothermal adsorption at partial pressures in the 10^-7 - 10^-5 mbar range onto an in situ cleaved MgO(100) substrate held at temperatures between 7-12 K. The background pressure was in the mid 10^-11 mbar range. A first and second adsorbed layer of hydrogen were observed. In the first, the sequence c(2x2), c(4x2) and c(6x2), as mentioned in the literature, was measured while in the second layer, the structure was not so clear. The surface dynamics for the c(4x2) showed a single mode at 9-10 meV in the [100] direction, but in the [110] direction two Einstein modes at about 10.5 and 8.5 meV were observed and a third mode (with dispersion) which ranged from 2 to 6 meV was also measured. The c(6x2) surface dynamics was similar to c(4x2). The second layer had a single Einstein surface mode very near 5 meV. The dispersive mode is believed associated with parallel oscillations along the long edge of the cell and the dispersionless modes from parallel and perpendicular oscillations along the short length of the elementary cell.