HYDROGEN AND HELIUM FILMS AS MODEL SYSTEMS OF WETTING

Optical experiments on the wetting properties of liquid He-4 and molec ular hydrogen are reviewed. Hydrogen films on noble metal surfaces ser ve as model systems for studying triple point wetting, a continuous tr ansition between wetting and non-wetting. By means of optically excite d surface plasmons, the adsorbed film thickness for temperatures aroun d, and far below, the bulk melting temperature is measured, and the ph ysical mechanisms responsible for the transition are elucidated. Possi ble applications for other experiments in pure and applied research ar e discussed. Thin films and droplets of liquid helium are studied on c esium surfaces, on which there is a first order wetting transition. Ou r studies concentrate on dynamical observations via surface plasmon mi croscopy, which provide insight into the morphology of liquid helium d roplets spreading at different temperatures. Features corresponding to pinning forces, the prewetting line, and the Kosterlitz-Thouless tran sition are clearly observed.