Absorption of D(g) atoms in ultrathin Mg films and Pd-catalyzed decomposition of MgD2

We have investigated the interaction of the gas-phase D atoms with ultrathin Mg films thermally evaporated on SiO2 by temperature programmed desorption. The gas-phase D atoms impinging upon a Mg (100 Å) film directly penetrate into the bulk with a constant probability of 0.22 until the top 16 Å is deuterated, whereafter the MgD2(s) growth quickly slows down. Upon heating the partially deuterated Mg film, Mg atoms diffuse out to the surface to desorb at the same temperature as a pure Mg film. On the other hand, MgD2(s) decomposes to result in simultaneous desorption of D2 and Mg at 590 K with an activation energy of 133 ± 5 kJ/mol. A Pd layer of 20 Å thickness deposited on top of MgD2(s) acts as a catalyst to greatly reduce the decomposition temperature from 590 to 350 K. We interpret the catalytic effect of Pd in terms of reverse spill-over, in which the surface of Pd clusters in contact with MgD2(s) provides an intermediate adsorption sites with a low activation energy for the recombinative desorption of D atoms.