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 T_s= 325 to 825 K. At all surface temperatures the vibrational populations, increasing exponentially with T_s, 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 kT_s 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.