Binding States of Hydrogen on Tungsten

The binding states, adsorbate densities, and desorption kinetics of hydrogen on the (110), (111), and (100) crystal planes of tungsten are examined. There are two atomic states on the (110) plane and four atomic states on the (111) plane. Saturation hydrogen densities are approximately equal in the major states on both planes and correspond to one hydrogen atom for each surface tungsten atom on the (110) plane and approximately four hydrogen atoms for each surface tungsten atom on the (111) plane. While the desorption activation energies of the most tightly bound states on all three planes are nearly identical, there are weakly bound ${\mathrm{(E}}_d\text{\hspace{0.17em}=\hspace{0.17em}14}\mathrm{and}22{\mathrm{kcal\; mole}}^{\text{−1}})$ atomic states on the (111) plane which are entirely lacking on the other planes. These results show that there are considerable variations in hydrogen adsorption on these planes. Arguments are advanced that the surface atom configurations are nearly those of the bulk crystal, and hydrogen surface structures are proposed to account for the observed binding states. Some qualitative aspects of the adsorption bonds are also considered.