AVS 47th International Symposium
    Surface Science Wednesday Sessions
       Session SS2+VT-WeA

Paper SS2+VT-WeA4
Optical Effects of Monolayer and Multilayer Adsorption: Formic Acid and Methanol on Cu(100)

Wednesday, October 4, 2000, 3:00 pm, Room 209

Session: Adsorption and Desorption Phenomena I
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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We have studied the adsorption of formic acid (HCOOH) and methanol on epitaxial Cu(100) films at 125 K in both the monolayer and multilayer regimes, using infrared reflectance and dc resistance measurements and visible-light ellipsometry. The results are analyzed using an electron scattering model for reflectance changes due to chemisorbed monolayers, and a macroscopic three-layer model for physisorbed multilayers. For methanol, which is only physisorbed on Cu, the infrared reflectance increases linearly with film thickness (determined ellipsometrically) in agreement with the three-layer model. For formic acid the first monolayer is chemisorbed and scattering of conduction electrons from the adsorbates leads to an initial decrease in reflectance, together with an increase in the electrical resistance of the Cu film. At higher formic acid exposures physisorbed multilayers form, leading to a reflectance increase similar to that observed for methanol. This behavior is qualitatively consistent with expectations, but there are some surprising features. As formic acid exposure increases, the initial reflectance drop continues even after the resistance change is largely complete, even though both effects are believed to result from the same process. The subsequent rapid rise in reflectance suggests that the sticking coefficient is much lower for the first layer than for subsequent layers. Possible explanations for these effects will be discussed.