Dynamics of D2 released from the dissociation of D2O on a zirconium surface

Hydrogen is efficiently released during water dissociation on zirconium (Zr), while even very rapid temperature programed heating of a hydrogen covered Zr surface predominantly leads to dissolution (similar to 99% dissolution). To help resolve these apparently contradictory observations, we have studied the dynamics of water (D2O) dissociation on a crystalline Zr surface by probing the rotational and vibrational energy distributions of the D-2 produced using resonant enhanced multiphoton ionization spectroscopy. The internal-state energy distribution of the D-2 product was found to be rotationally cold and vibrationally hot with respect to the temperature of the surface. The rotational distribution shows slight deviations from Boltzmann's law, with a mean rotational temperature of 426 K while the surface is at 800 K. The population of the nu(n)=1 vibration is at least four times higher than a 800 K temperature would allow, this corresponding to a vibrational temperature of 1100 K. Information on the translational energy of the D-2 product have also been obtained by time-of-flight spectroscopy and it is found to be nearly thermally equilibrated with the surface temperature. Similar results were obtained from studies of D-2 scattered from a clean Zr surface, and of D-2 released by a slow thermal desorption process which involves dissolved hydrogen as the source. The reconciliation of the present results with those for thermal desorption of preadsorbed hydrogen implies a role for both surface and subsurface adsorption sites on the Zr surface and clearly demonstrates that at high temperatures, the release of D-2 arises from the recombinative desorption of adsorbed hydrogen formed by the complete dissociation of D2O.