AVS 45th International Symposium
    Surface Science Division Wednesday Sessions
       Session SS2-WeM

Paper SS2-WeM2
Adsorption, Absorption, and Abstraction of Hydrogen on Cu(111) Surfaces

Wednesday, November 4, 1998, 8:40 am, Room 309

Session: Gas-Surface Dynamics
Presenter: J. Küppers, Universität Bayreuth, Germany
Authors: Th. Kammler, Universität Bayreuth, Germany
J. Küppers, Universität Bayreuth, Germany
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The interaction of H(D) with Cu(111) surfaces and the abstraction of D adsorbed on Cu(111) by H was studied with thermal desorption spectroscopy and direct product rate measurements. H atoms were produced in a W tube heated at 2000 K. H atoms directed at clean Cu(111) surfaces at 85 lead to H adsorption with an initial sticking coefficient of 0.19. After completion of the adsorbed layer with a saturation coverage of 0.25, further H uptake leads to H absorption. Adsorbed H desorbs as H@sub 2@ via a second order process in the temperature range 250 to 400 K. Absorbed H desorbs as H@sub 2@ starting at 150 K and peaking around 200 K in a first order process. The interaction of H atoms with a saturated D layer, @THETA@=0.25, at 85 K leads to the formation of gas phase HD and D@sub 2@. The kinetics of HD formation as a function of H atom fluence follows a strict Eley-Rideal-type phenomenology: a HD rate jump at reaction start and a subsequent exponential decay of the HD rate. However, the D@sub 2@ product suggests that the reaction mechanism is not of an Eley-Rideal but of hot atom type. An analysis of the abstraction kinetics in terms of hot-atom mechanisms reveals that on Cu(111) the reaction probability of hot atoms is of the same magnitude as their sticking probability at empty surface sites. About 1% of the adsorbed D appears in the D@sub 2@ reaction channel. The abstraction cross section towards HD is 2.3 Å@super 2@. The HD yields and rate steps scale with the D coverage at reaction start, the D@sub 2@ yields and rate steps scale with the square of the D coverage. Absorbed D cannot be abstracted by gas phase H. The kinetics of HD formation is independent of the H flux.