ADSORPTION DYNAMICS FOR THE SYSTEM HYDROGEN PALLADIUM AND ITS RELATION TO THE SURFACE ELECTRONIC-STRUCTURE/

We have determined differential sticking coefficients for a monoenerge tic nozzle beam of hydrogen on Pd(111) and Pd(110). In particular the energy dependence and the angular variation of the initial sticking co efficient were measured. The results indicate that adsorption of hydro gen on palladium occurs in parallel processes through a direct path wi th an activation barrier of perhaps 50 meV or less and a precursor pat h. There is relatively little difference in the adsorption properties of the (111) and the (110) plane. The appearance of a molecular precur sor on the (111) plane can be related to the electronic structure of p alladium, in particular to the absence of occupied Shockley surface st ates, as compared to Ni(111) and Pt(111). Pre-adsorbed potassium on a (110) plane acts as an inhibitor to adsorption. Different inhibiting m echanisms are observed for the direct adsorption path and the precurso r path. At high potassium coverage the precursor path is completely su ppressed.