Thermal Excitation of H2 and D2 at Clean Metal Surfaces. Noteworthy Differences between d‐ and s‐Metals

Ad- and desorption measurements of hydrogen (deuterium) on clean metal surfaces under high and ultrahigh vacuum conditions by means of the capillary flow method with pressure control only by inert gauges show a small fraction of desorbing molecules in an activated state. These molecules adsorb on glass and on carbon, respectively, or are able to reduce copper-I-oxid. This experiment indicates two different ways for activation of hydrogen molecules in collision with hot metal surfaces: (i) the well known dissociation of molecules dominant at TS > 1100 K and (ii) new, the excitation at hydrogen (deuterium) molecules at 800 < Ts < 1100 K with activation energies of E = 0.52 (0.37) eV. The results show a significant pressure dependence of the vibration accommodation coefficient. The s-metals Cu and Au are total ineffective in dissociation and in activation molecules, respectively. The energy for excitation (H2, D2) is in accordance with the term difference of the harmonic oscillator.