Diffusion of solid molecular hydrogen and chemical potential changes in submonolayer helium flow

Molecular hydrogen and helium form quantum systems when cooled to their condensed phases. The thin films that these two systems can form and the interactions of these films with the surfaces onto which they are absorbed is a field which has received particular attention in recent years. I report and discuss two studies of the migration of these systems over different strong binding substrates. The hydrogen study is concerned with the processes by which hydrogen molecules migrate over their own solid. Several groups have measured the movement of hydrogen molecules when their solid was annealed. However, until recently, no direct measurements of the migration of these molecules have been made and hence no direct determinations of the processes involved could be made. A recent study has shown that the migration of hydrogen molecules is apparently linearly dependent on time, leading to the conclusion that the migration of the hydrogen molecules is not diffusive. This thesis reports new measurements on the motion of H_2 on adsorbed solid layers of H_2 using a hole-burning technique. These confirm the findings of recent experiments but not the conclusion drawn from them. On the contrary, a model is developed for the recovery of the hydrogen which shows that H_2 moves diffusively with a thermally activated diffusion process and determinations have been made of the diffusion constant, D_0 = 1.34 x 10"-"3 #+-# 2.5 x 10"-"5 m"2s"-"1, and the activation energy, E = 47 #+-# 11 K. "4He flow measurements have been used to detect weakly bound He films on alkali metal substrates. The flow of similar submonolayer films on strong binding substrates at constant temperatures has not yet been investigated so a novel experiment has been devised to measure the flow rate induced by a chemical potential gradient. A mobile submonolayer above 2 solid layers of helium was formed on a glass substrate. Chemical potential gradients are formed by evaporating atoms from one end of the flow path. Then the chemical potential difference, as a function of flow rate, was obtained from film thicknesses at each end of the flow path from a sampling heater. This shows that for small flow rates, in the normal phase, there is a linear relationship between the chemical potential and flow rate but the flow rate saturates at higher chemical potential gradients. From the linear regime the impedance of the substrate to the flow of the submonolayer film was determined. (author)SIGLEAvailable from British Library Document Supply Centre-DSC:DXN037342 / BLDSC - British Library Document Supply CentreGBUnited Kingdo