AVS 45th International Symposium
    Surface Science Division Tuesday Sessions
       Session SS3-TuA

Paper SS3-TuA8
Photoinitiated Electron Transfer to Selected Alkyl Bromides Physisorbed on GaAs: The Effects of Alkyl Chain Length on Dissociation Cross-Sections and Fragment Dynamics

Tuesday, November 3, 1998, 4:20 pm, Room 314/315

Session: Photon- and Electron-Induced Chemistry
Presenter: R.M. Osgood Jr., Columbia University
Authors: R.M. Osgood Jr., Columbia University
K.A. Khan, Columbia University
N. Camillone III, Columbia University
J.E. Moryl, Columbia University
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Brominated hydrocarbons adsorbed on semiconductor surfaces serve as ideal model systems for investigating the photoinduced chemistry of oriented molecules in the condensed phase. These adsorbates dissociate via attachment of photoexcited substrate electrons giving rise to energetic alkyl and surface-bound bromine fragments. We will describe the effect on the dissociation cross-section and fragmentation dynamics due to systematic variation of the complexity (alkyl chain length) of the adsorbate. The enhancement in the dissociation cross-section with increased chain-length will be discussed in the context of substrate-mediated dissociative-electron attachment (DEA). Comparison of these observations with those made in the gas-phase DEA of similar haloalkanes allows us to consider the role of the surface in this dissociation process. Increasing the length of the alkyl chain also leads to distinct changes in the alkyl fragment angular distributions. For methyl bromide, the angular distribution is dominated by a focused beam of hyperthermal methyl radicals at 45° (in the [-1,0] direction) from the surface normal. For ethyl and propyl bromide, inelastic scattering of the fragments results in increased importance of a slower diffuse cos@super n@ @theta@ desorption. In addition, significant retention of alkyl fragments is detected by post-irradiation thermal desorption measurements. Increasing the number of degrees of freedom of the adsorbate dramatically alters the energetics of the ejection of the photofragments from the surface. Velocity distributions of the fragments clearly demonstrate that the energy partitioned into translational motion is strongly reduced when the number of rovibrational degrees of freedom is increased. Quantitative variations in the energy and angular distributions are discussed in terms of changes in energy partitioning with alkyl chain length.