State selective investigation of recombinative D2 and HD desorption from Pd(100) and Au/Pd(100)

The recombinative desorption of D2 and HD from a Pd(100) surface is studied with rotational state selectivity by resonantly enhanced (vuv + uv) two photon ionisation spectroscopy. For the rotational degree of freedom a strong cooling effect is observed. The first excited vibrational level (v'' = 1) is much more populated than expected for molecules in thermal equilibrium with the surface. This population increases strongly with the surface temperature. The activation energy for (v'' - 1) desorption deduced from a Boltzmann plot is less than the vibrational energy of free D2 and HD molecules. (author)