| Summary: | The 'Wagener' flow technique is developed to give accurate measurements of the sticking probability of hydrogen on evaporated metal films. Initial values are invariably high (>0.1) and are independent of flow rate and temperature. On sintered films the following initial sticking probabilities are obtained: molybdenum (o.75), nickel (0.38), titanium (0.29), tantalum (0.45) and palladium (0.99). Values obtained using unsintered films are generally somewhat higher due to the rougher surface enhancing the probability of multiple collisions. At 78 and 90°K the adsorbed layer builds up on the 'outer' surface of the film and comes into pseudo— equilibrium with the gas phase. The kinetics of the resulting slow pressure decays on interrupting gas flow are characterized, and a gas phase process is proposed for the redistribution at low uptakes. At higher uptakes a similar model is applicable to molybdenum and nickel, the other metals absorb hydrogen and the redistribution is different in form. The rate determining step in absorption may be at or near the surface (palladium) or within the bulk (titanium and tantalum). At 195 and 300°K the adsorbed layer is highly mobile and is near to an equilibrium distribution even during gas flow. Titanium absorbs hydrogen in this temperature range with an almost constant sticking probability until the hydride composition Till is approached whereas tantalum does so at 300oK only when there is a relatively high pseudo-equilibrium pressure (10-3 torr) above the film. Palladium does not absorb hydrogen appreciably at 300°K within the pressure range studied. Isotherms are constructed for the saturated layers and are found to approximate to the Temkin form. `Desorption spectra' are used to examine the energetics and population density of the adsorbed state. It is concluded that many of the adsorbed 'phases' which have been reported are merely due to surface rearrangements of the adsorbate and are not reflections of changes in population density with binding energy.
|
|---|
