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

Invited Paper SS2+VT-WeA1
The Role of Two-dimensional Compressibility in Physisorption, Competitive Adsorption and Dynamic Displacement

Wednesday, October 4, 2000, 2:00 pm, Room 209

Session: Adsorption and Desorption Phenomena I
Presenter: G.A. Kimmel, Pacific Northwest National Laboratory
Authors: G.A. Kimmel, Pacific Northwest National Laboratory
Z. Dohnálek, Pacific Northwest National Laboratory
R.S. Smith, Pacific Northwest National Laboratory
B.D. Kay, Pacific Northwest National Laboratory
Correspondent: Click to Email
We have investigated the physisorption of gases (Ar, N@sub 2@, CH@sub 4@, etc) on Pt(111) with modulated beam techniques, beam reflection measurements and temperature programmed desorption (TPD). The kinetics of adsorption and desorption for systems where the coverage is larger than ~0.8 monolayer (ML) is dominated by the two-dimensional compression of the adsorbate layer. In particular, at coverages near 1 ML the repulsive interaction between adsorbates increases causing an approximately linear decrease in binding energy as the coverage increases. The formation of the first monolayer is complete when the binding energy of the adsorbates in the compressed first layer is equivalent to the binding energy of an adsorbate in the second layer (i.e. when the chemical potential of the first and second layers are equal). Evidence for this compression arises in the TPD of physisorbed gases from Pt(111) which reveal a non-zero, nearly constant desorption in the temperature range between the desorption peaks of the "first" and "second" monolayers. Modulated beam experiments also support this interpretation. In that case, the Pt(111) surface is pre-covered with ~1 ML of gas and then exposed to gas pulses of varying duration (~0.05 s - 1.0 s) and the desorbed/reflected flux is monitored. Another example where the two dimensional compression of the adsorbate layer is important occurs when the "competitive" adsorption of two gases is examined. In these experiments, an adsorbate can be displaced/desorbed when the surface is exposed to a gas which has a higher binding energy to the substrate. We will present a simple model for the binding energy as a function of coverage which explains the observed adsorption/desorption kinetics for a variety of systems.