AVS 45th International Symposium
    Surface Science Division Monday Sessions
       Session SS2-MoM

Paper SS2-MoM10
Negative and Positive Adsorbate-Induced Reflectance Changes: Formic Acid on Cu(100)

Monday, November 2, 1998, 11:20 am, Room 309

Session: Molecular Adsorbates on Metals
Presenter: C.-L. Hsu, Tufts University
Authors: C.-L. Hsu, Tufts University
E.F. McCullen, Tufts University
R.G. Tobin, Tufts University
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Adsorption on metal surfaces typically reduces the broadband reflectance of the surface, an effect that is usually attributed to diffuse scattering of the conduction electrons by the adsorbate. The resistance of the film typically increases. On the other hand, a dielectric film typically increases the broadband reflectance of a metal. Both effects are observed for the adsorption of formic acid (HCOOH) on epitaxial Cu(100) films at 125 K. Both the dc resistance and the infrared reflectance at 1500 cm@super -1@, away from any vibrational resonances, were measured during dosing. Upon initial exposure to formic acid, the film's resistance increases by 1% and its reflectance decreases by 0.5%. With continued exposure, leading to the growth of a multilayer formic acid film, the reflectance begins to increase and ultimately reaches a level more than 10% greater than the clean-surface reflectance. The resistance, however, scarcely changes after adsorption of the first monolayer. While the results are qualitatively consistent with expectations, there are discrepancies. The resistance and reflectance changes in the monolayer region are not proportional, and the increase in reflectance with thickness is more rapid than expected.