AVS 46th International Symposium
    Surface Science Division Thursday Sessions
       Session SS2-ThA

Paper SS2-ThA8
Adsorption Driven Displacement of N@sub 2@ from Pt(111)

Thursday, October 28, 1999, 4:20 pm, Room 607

Session: Adsorption at Surfaces
Presenter: G.A. Kimmel, Pacific Northwest National Laboratory
Authors: G.A. Kimmel, Pacific Northwest National Laboratory
K.P. Stevenson, Pacific Northwest National Laboratory
B.D. Kay, Pacific Northwest National Laboratory
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Pt(111) with beam reflection measurements and temperature programmed desorption (TPD). The interaction of weakly adsorbed species on surfaces is of fundamental interest since these systems provide a benchmark for more complicated systems. Our experiments involve the preparation of monolayer or sub-monolayer coverages of N@sub 2@ on Pt(111) followed by the adsorption of another species (e.g. CH@sub 4@, Kr and H@sub 2@O). Both species are monitored throughout the experiments, allowing for quantitative measurements of the coverages and sticking coefficients versus time. In all cases, N@sub 2@ is displaced from direct contact with the Pt(111) by the adsorbates which have higher binding energies. The fate of the displaced N@sub 2@ molecules is governed by the surface temperature and its interaction energy with the co-adsorbate. At higher temperatures, the N@sub 2@ desorbs as soon as it is displaced from the first layer. As the temperature is lowered, the N@sub 2@ desorbs when it is displaced from successively higher layers. This behavior results from the successively lower binding energy of N@sub 2@ to increasingly thick co-adsorbate layers. At "high" temperatures (~38 K), the N@sub 2@ desorption rate is approximately constant during the adsorption of the first monolayer of CH@sub 4@, falling abruptly to zero with its completion. This suggests that the mobility of the adsorbed CH@sub 4@ is high allowing it to rapidly find and displace N@sub 2@. The CH@sub 4@ sticking coefficient increases linearly with time as the surface layer is converted from N@sub 2@ to CH@sub 4@. TPD subsequent to the CH@sub 4@ exposure shows that the entire N@sub 2@ monolayer was displaced and desorbed during the CH@sub 4@ adsorption. At lower temperatures, lower diffusion rates lead to departures from zero-order desorption kinetics. The displacement of N@sub 2@ by H@sub 2@O is qualitatively similar, but quantitative comparisons are complicated by details of the N@sub 2@/H@sub 2@O interaction.