AVS 47th International Symposium
    Surface Science Wednesday Sessions
       Session SS3-WeM

Paper SS3-WeM6
Speed Distribution of Ethane Molecular Beam Reflected after Temporary Trapping on a LiF(001) Surface

Wednesday, October 4, 2000, 10:00 am, Room 210

Session: Gas/Surface Dynamics
Presenter: T. Tomii, University of Tsukuba, Japan
Authors: T. Tomii, University of Tsukuba, Japan
T. Kondo, University of Tsukuba, Japan
S. Yagyu, University of Tsukuba, Japan
S. Yamamoto, University of Tsukuba, Japan
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We have measured at various scattering angles the time-of-flight of ethane molecules scattered from a LiF(001) surface of room temperature. With a high resolution cross-correlation time-of-flight technique we have successfully separated out the component of ethane molecules scattered after experiencing a temporary trapping on the sample surface. While the major peak in the time-of-flight spectrum, which consists of rotationally excited ethane molecules after single collision, can be fairly well fitted to a shifted Maxwell-Boltzmann speed distribution function, it was found that the temporarily trapped component can be reproduced by a single Maxwellian speed distribution function with a characteristic temperature lying slightly higher than the target surface temperature. This temperature tends to increase from around 300 to 600 K with the translational energy of the incident ethane beam in the range between 260 and 700 meV. The mean translational energy of the temporarily trapped molecules is almost uniform in the scattering angle, exhibiting a strong contrast to that of the directly scattered which qualitatively follows the washboard model predictions with the appropriate parameters. Despite the strong anisotropy of the ethane molecule, the directly scattered component shows a little surface rainbow features in the angular mean translational energy distribution, revealing the corrugation of the gas-surface repulsive potential. Temporarily trapped molecules are believed to be the ones which at first couple of bounces have stored their incident translational energy in the form of rotation, either cartwheel or helicopter mode, and tangential motion on a corrugated LiF(001) surface, resulting in a Maxwell-like speed distribution as a whole.