State-selective studies of the associative desorption of hydrogen from Pd(100) and Cu(100)

Abstract

Vibrational-state populations and velocity distributions of H2, HD, and D2 desorbing from Pd(100) are measured with rotational state selectivity over a wide temperature range from Ts= 325 to 825 K. At all surface temperatures the vibrational populations, increasing exponentially with Ts, are found to be significantly higher than those expected for thermally equilibrated molecules. The slopes of Boltzmann plots are considerably lower than expected for a thermal excitation mechanism of the vibrational states at the corresponding gas-phase energies. They also show a non-trivial isotope effect. The velocity distributions from clean Pd(100) are found to be Maxwell–Boltzmann-like. The translational energies of H2 molecules are accommodated at the surface temperatures, whereas those of D2 are higher than kTs by ca. 10–30 meV. Both the vibrational excitation and the isotope effect in translation can be understood with a quantum-mechanical model calculation on a two-dimensional potential-energy surface. The vibrational-state selective angular distributions for desorption from Cu(100) display a different behaviour for ground and excited states. The distributions for the vibrationally excited states are broader than that of ground-state molecules. They also show an isotope effect.