Hydrogen on W(100): electron stimulated description and a structural model

Prompted by the peculiar shape of the curve of electron-induced H+ current versus coverage for hydrogen on W(100) which has been observed before, the authors have restudied this system by correlated ESD, thermal desorption, and LEED measurements. The maxima in ESD signal and intensity of the half-order LEED spots, and the appearance of a second desorption peak are roughly correlated at 1/4 coverage. Using this observation and the exact shape of the ESD curve, a structural model is derived in which two different, but uniform states (i and f) exist, the first, i, being exclusively occupied up to 1/4 monolayer, and the second, f, at saturation. A gradual conversion from i to f takes place at 1/4 < theta < 1 which is caused by the interaction between adsorbed H atoms. From this model a differential equation for the occupation numbers of i and f states, respectively, is derived and solved numerically; it yields good agreement with the experimental ESD curve. Discussing the possible physical realization of the two states, it can be made probable that they must exist on two different sites and have fourfold (i) and twofold (f) symmetry, so that the conversion is accompanied by a shift of the binding site of the adatom. (Auth.)