AVS 45th International Symposium
    Surface Science Division Wednesday Sessions
       Session SS2-WeM

Paper SS2-WeM11
The Role of Surface Reconstruction in the Photofragmentation Dynamics of Oriented Methyl Bromide Adsorbed on GaAs

Wednesday, November 4, 1998, 11:40 am, Room 309

Session: Gas-Surface Dynamics
Presenter: K.A. Khan, Columbia University
Authors: K.A. Khan, Columbia University
N. Camillone III, Columbia University
R.M. Osgood Jr., Columbia University
J.A. Yarmoff, University of California, Riverside
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Our previous work has detailed the UV-initiated dissociation of CH@sub 3@Br physisorbed on GaAs(110), both by numerous experimental techniques and theoretical calculations. The substrate-electron-mediated dissociation yields energetic (1.4eV) methyl fragments whose dynamics are governed by surface-alignment forces acting on the adsorbate. The anisotropic angular distribution of the methyl fragments reflects the surface corrugation of the 1x1 reconstruction of GaAs(110). In this paper we demonstrate that significant changes are observed in the fragment dynamics upon modifying the surface reconstruction via well-controlled surface preparation protocols. Specifically, to further explore and understand the role played by the surface morphology in fragmentation dynamics, we have compared the photodissociation of methyl bromide on two different surface reconstructions of the (100) face of GaAs. In contrast to the (110) surface, mass and angle-resolved time-of-flight measurements show the ejection of methyl fragments from the (100) surface to be symmetrical about the surface normal. Photoinduced fragmentation of monolayers of methyl bromide on c(8x2)-reconstructed Ga-rich and c(2x8)-reconstructed As-rich GaAs(100) surfaces also reveals a striking difference between these two surface structures. On the Ga-terminated surface we observe desorption of intact CH@sub 3@Br as well as methyl fragments formed by photoinduced dissociation. Only methyl fragments are detected leaving the (110) surface and the As-terminated (100) surface. As in the (110) case, the mechanism for C-Br bond cleavage in the (100) cases is photo-initiated electron attachment from the surface to the molecule. However, the energetics of the methyl fragments are quite different on the three surfaces suggesting site-specific dissociation. Therefore, the energetics of the methyl fragments will be discussed in terms of the surface texture and stoichiometry of the three surfaces.