THE RECOMBINATIVE DESORPTION OF D-2 FROM AG(111) - TEMPERATURE-PROGRAMMED DESORPTION AND LOW-ENERGY-ELECTRON DIFFRACTION

The interaction of deuterium atoms with Ag(111) has been investigated using angle-resolved temperature-programmed desorption (TPD) and low energy electron diffraction (LEED). Deuterium atoms, formed in a microwave discharge-powered atom beam source, adsorb on this surface at 110 K, resulting in a TPD feature near 170 K at very low coverage. With increasing exposure, this feature shifts up in temperature to 220 K and is joined at high exposures by two lower temperature peaks near 200 and 175 K. The low temperature desorption feature does not saturate but grows linearly with exposure and is attributed to deuterium atoms absorbed into the sub-surface, the absorption probability being a factor of 350 less than for D adsorption onto the dean surface. LEED shows a (2 x 2) pattern at 110 K for low coverages where only the highest temperature peak is observed in TPD. At coverages slightly above this a superposition of (2 x 2) and (3 x 3) patterns is observed at 110 K, the (2 x 2) spots diminishing in intensity as the coverage is increased further, eventually leaving only a (3 x 3) pattern at high coverages. The low temperature desorption peak shows a complicated kinetic behaviour. Desorption from the (2 x 2) phase is approximately 1/2-order with respect to D coverage, with an activation barrier of 26.8 +/- 0.6 kJ mol(-1). Angle-resolved measurements at these coverages reveal a D-2 distribution peaked along the surface normal with P(theta) = cos(8) theta, rather broader than for desorption from Cu(111). At low coverages (theta < 0.12) the desorption profiles show zero-order behaviour in their rising edge with more symmetric desorption peaks reappearing at the very lowest coverages.