Adsorption and desorption of hydrogen by evaporated molybdenum films at low temperatures

Abstract

The adsorption and desorption of hydrogen on evaporated molybdenum films has been studied in the temperature range below 250°K. Sticking probabilities have been measured using a flow technique. The initial value (0.7) is higher than found in flash filament experiments due to the multiple collisions with the surface. At 78°K the adsorbed layer builds up on the outermost regions of the film and comes into pseudo-equilibrium with the gas phase whilst the less exposed parts are still bare. Above a coverage of 8 × 10¹⁴ molecules per geometric cm² of film surface, cessation of the flow of hydrogen results in a slow decay of pressure P such that 1/P is linear with time. This is shown to be due to a slow redistribution of adsorbed hydrogen, via the gas phase, between the concentrated outer layers and the interior surface of the film. The desorption of hydrogen from films has been studied on warming from 78 to 300°K. Two, and in some cases three, pressure peaks have been observed, but the spectrum is complicated by surface diffusion and rearrangements in the temperature range 170 and 200°K. It is concluded that there is good evidence for only one low-temperature state of adsorption of hydrogen on molybdenum; this is largely desorbed below 150°K.