AVS 49th International Symposium
    Surface Science Monday Sessions
       Session SS1-MoM

Paper SS1-MoM6
Adsorption and Adhesion Energies of Pb on (1x1)-MoC/Mo(001) by Single-crystal Adsorption Calorimetry

Monday, November 4, 2002, 10:00 am, Room C-108

Session: Adsorption and Chirality
Presenter: M.H. Smedh, University of Washington
Authors: M.H. Smedh, University of Washington
S.F. Diaz, University of Washington
C.T. Campbell, University of Washington
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The heat of adsorption, sticking probability and film growth mode of Pb adsorption was studied on a thin, ordered MoC film at 300 K. A several layers thick film of MoC displaying a (1x1) LEED pattern was grown by dissociating ethylene on hot Mo(100), following Frühberger.@footnote 1@ The Pb atoms were deposited in a chopped molecular beam, with 0.1 s pulses containing 0.015 ML Pb every 2 s. The Pb beam flux was determined using a quartz crystal microbalance. The heat of Pb adsorption was measured by single-crystal adsorption microcalorimetry, based on King's approach,@footnote 2@ with a different heat detection scheme. The adsorption of a Pb pulse causes a transient heat input and temperature rise, detected by a pyroelectric polymer ribbon in contact with the backside of the sample.@footnote 3@ An initial heat of adsorption of 245 ± 5 kJ/mol was found. It stayed constant up to ~0.5 ML, then dropped smoothly up to 1 ML to ~195 kJ/mol, within a few percent of the heat of sublimation for Pb, where it remained constant up to ~ 8 ML. The implications of these results with respect to Pb-Pb repulsions and Pb mobility in the adlayer will be discussed. From the integral heat of adsorption at 8 ML, a Pb/MoC adhesion energy of ~180 µJ/cm@super 2@ was obtained. This will be compared to results obtained from the contact angle of molten Pb drops. The sticking probability was found to be 0.96 initially, increasing linearly up to ~1 ML to 0.994 ± 0.003, where it remained for higher coverages. The growth mode of Pb on (1x1)-MoC/Mo(100) was further investigated by comparing the behavior of the Pb and Mo AES intensities as a function of the Pb coverage to different growth models. @FootnoteText@ @footnote 1@B. Frühberger and J.G. Chen, Surf. Sci. 342 (1995) 38.@footnote 2@C.E. Borroni-Bird and D.A. King, Rev. Sci. Instrum. 62 (1991) 2177@footnote 3@J.T. Stuckless, N.A. Frei, and C.T. Campbell, Rev. Sci. Instrum. 69 (1998) 2427.