MULTILAYER ADSORPTION OF DEUTERIUM HYDRIDE ON GRAPHITE

We report ellipsometric-coverage vapor-pressure isotherm measurements for HD on graphite over the temperature range 8.5-16.7 K. As many as 1 7 discrete layer adsorption steps are resolved at temperatures near 16 .0 K, slightly below the bulk triple point (T-t = 16.60 K). Layer cond ensation critical temperatures are determined for layers two through n ine. The sequence of critical temperatures indicates that the rougheni ng temperature T-r of the close-packed facet of HD is certainly above 14.9 K and could be as high as T-t. With some assumptions; an estimate of T-r = 15.3 K is obtained. A conspicuous feature of the isotherms i s occurrence of hysteresis and splitting over limited temperature rang es in the fourth, fifth, and sixth layer adsorption/desorption steps. This apparently indicates a change in structure of the underlying film when the thickness equals or exceeds four layers.