Photoinduced dissociation of n-alkyl bromides on gallium arsenide(110) and gallium arsenide(100): Electron and fragment dynamics

In this study we investigate the UV-initiated electron transfer and dissociation fragment dynamics of selected n-alkyl bromides physisorbed on single crystals of GaAs. By systematically varying different chemical and structural parameters of the adsorbate/substrate system we explore a number of fundamental questions regarding the basic physics and chemistry of photochemical processes on surfaces. Monolayers of methyl, ethyl and propyl bromide were deposited on the (110), Ga-terminated (100) and As-terminated (100) surfaces of GaAs without thermal decomposition at 80 K. Substrate-mediated electron transfer to the molecule, induced by exposure to UV light at 193, 248 and 351 nm, causes C-Br bond cleavage. The electron transfer dynamics of this mechanism are examined as a function of wavelength and molecular complexity of the adsorbate to better understand the flow of energy and charge across the adsorbate/substrate interface. The photodynamics of the alkyl fragments are studied using mass-, energy- and angle-resolved time-of-flight measurements. The alkyl fragment dynamics are observed to be sensitive to the length of the hydrocarbon chain, the wavelength of the dissociating light and the structure and chemical composition of the surface. The photodynamics are discussed in terms of adsorbate orientation on the surface, energy partitioning, and radical-surface interactions. Differences in the methyl fragment dynamics from the GaAs(100) surfaces suggest site-specific adsorption of the molecule. Pre- and postirradiation thermal desorption studies were employed to investigate the photoreactions of the alkyl radicals and bromine fragments on all three surfaces. On the Ga-terminated GaAs(100) surface we have demonstrated that the reaction between photogenerated bromine and surface Ga atoms can be used to selectively modify the structure and stoichiometry of the surface.