AVS 46th International Symposium
    Surface Science Division Wednesday Sessions
       Session SS2-WeA

Paper SS2-WeA7
Molecular Beam Studies of the Extrinsic Precursor State: Trapping of Alkanes on Alkane-Covered Pt(111)

Wednesday, October 27, 1999, 4:00 pm, Room 607

Session: Gas-Surface Dynamics
Presenter: A.F. Carlsson, Stanford University
Authors: A.F. Carlsson, Stanford University
R.J. Madix, Stanford University
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Adsorbed species can significantly affect probabilities of adsorption of incident molecules. The classic studies of CO adsorption on metals lead to the concept of the extrinsic precursor state, and the analysis of Kisliuk of this process is well known. However, there few systematic studies of the dynamics of this process and the way in which properties of the adsorbed state affect adsorption. Using a liquid helium cryostat to achieve surface temperatures upward from 25 K and molecular beam methods, we have studied the adsorption probabilities of methane and ethane on Pt(111) surfaces presaturated by the adsorption of another alkane from a low pressure ambient. Firstly, methane adsorbs on the clean surface with near-normal energy scaling. On all covered surfaces, except that saturated by ethane, methane traps with probabilities ranging from 0.8 to 0.9, dependent on incident angle and energy. Generally, the trapping probability depends weakly on incident angle and energy. The weak dependence on incident angle is attributed to surface corrugation produced by the adsorbed alkanes. In the case of methane trapping on an ethane-covered surface a threshold to adsorption is observed. Below 16 kJ/mol the trapping probability remains very small, but above this threshold the trapping probability increases to 0.9. This "activated" molecular adsorption is quite dramatic. Ethane, however, shows distinctly different effects. The differences are attributed to size effects, not binding energy differences. These and other observations will be discussed.