A molecular beam study of the interaction between hydrogen and the Pt(111) surface

Abstract

Hydrogen adsorption on and desorption from a Pt(111) surface is investigated in the temperature range 570 < T s < 1200 K by studying the exchange reaction H 2 + D 2 → 2 HD. The HD molecules are found to desorb with a cos 5ϑ f distribution. The mean energy of the desorbing molecules is E d = 4kT s . Slight deviations from this behaviour were observed for large desorption angles (ϑ f > 45°). The results can be described by a desorption function which is proportional to the square of the perpendicular energy: S d = S 1 E ⊥ 2. According to the principle of detailed balance the sticking probability S a should show a cos 4ϑ i dependence in that case. However a marked deviation from this behaviour is observed which shows that a second interaction mechanism is involved which is not (or hardly) dependent on E ⊥: S a = S 1 E ⊥ 2 + S 2. It is found that S 2 increases strongly with temperature whereas S 1 decreases by about 35% when heating the surface from 670 to 1070 K. The present results are in good agreement with adsorption experiments performed at low temperature (160 K) which will be presented elsewhere. Both interaction mechanisms are discussed in terms of atomistic models. The temperature dependence of S 1 seems to be in conflict with the activation barrier model, which has been proposed previously, suggesting that another process is responsible for the observed behaviour. The second mechanism ( S 2) cannot be explained by surface defects. We attribute this mechanism to adsorption in a molecular precursor state which becomes more efficient with increasing surface temperature.