Chain-length dependence of the dissociation dynamics of oriented molecularadsorbates: n-alkyl bromides on GaAs(110)

Brominated hydrocarbons adsorbed on semiconductor surfaces serve as ideal m odel systems for investigating the photoinduced chemistry of oriented molec ules in the condensed phase. Under UV irradiation these adsorbates dissocia te via attachment of photoexcited substrate electrons giving rise to energe tic alkyl and surface-bound bromine fragments. In this report we describe t he effect on the fragmentation dynamics due to systematic variation of the complexity (alkyl chain length) of the adsorbate. Increasing the length of the alkyl chain leads to distinct changes in the alkyl fragment angular dis tributions. For methyl bromide, the angular distribution is dominated by a focused beam of directly ejected hyperthermal methyl radicals at 44 degrees tin the [0 (1) over bar] direction from the surface normal. while a simila r direct beam is observed for ethyl and propyl bromide, inelastic scatterin g of these fragments is found to result in increased importance of a slower diffuse cos(n) theta desorption. In addition, significant retention of alk yl fragments is detected by postirradiation thermal desorption measurements for these longer-chain homologues. Increasing the number of degrees of fre edom of the adsorbate is also observed to dramatically alter the energetics of the ejection of the photofragments from the surface. As the number of c arbons in the fragment is increased from one to three, the average energy o f the directly ejected radicals decreases from 1.48 to 1.1 to 0.69 eV (UV i ncident at lambda = 193 nm). Variations in the energy and angular distribut ions are discussed in terms of initial adsorbate orientation, energy partit ioning into rovibrational modes, and influence of radical-surface interacti ons.