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 e nergy electron diffraction (LEED). Deuterium atoms, formed in a microw ave 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 in creasing exposure, this feature shifts up in temperature to 220 K and is joined at high exposures by two lower temperature peaks near 200 an d 175 K. The low temperature desorption feature does not saturate but grows linearly with exposure and is attributed to deuterium atoms abso rbed into the sub-surface, the absorption probability being a factor o f 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 tempe rature peak is observed in TPD. At coverages slightly above this a sup erposition 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 fur ther, 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 re spect 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 dist ribution peaked along the surface normal with P(theta) = cos(8) theta, rather broader than for desorption from Cu(111). At low coverages (th eta < 0.12) the desorption profiles show zero-order behaviour in their rising edge with more symmetric desorption peaks reappearing at the v ery lowest coverages.