Photoinduced Fragmentation of Multilayer CH3Br on Cu/Ru(001)

Abstract

The broadband UV (230−420 nm) photoinduced chemistry of CH3Br adsorbed on Cu(2ML)/Ru(001) in the coverage range of 1−50 ML was studied by monitoring the desorption products (Δp-TPD mode) in combination with post irradiation work function change measurements before and during surface heating (Δφ-ΤPD mode). Δφ measurements enabled us to follow multilayer restructure and desorption of parent molecules and photochemical reaction products in the temperature range of 80−700 K. Methyl radicals accumulated on the surface are the precursor for the thermal formation of methane and ethylene at 450 K. Dehydrogenation of the methyl group is the rate-limiting step of the surface reaction resulting in the formation of these molecules. Based on work function change measurements, an estimate of the adsorbed methyl dipole moment is μ0 = 0.48 D. Dissociative electron attachment (DEA) driven CH3Br dissociation produced CH3 and CH2 fragments within the parent molecules multilayer matrix. At the multilayer coverage range, Δφ increases by up to 1.1 eV after 10 min UV irradiation. Model calculations qualitatively describe the post irradiation work function changes induced by the embedded photofragments (mostly Br- ions) inside the CH3Br dielectric film. Comparison of the Δφ-TPD spectra on Cu(2ML)/Ru(001) to those on clean Ru(001) indicates that the nature of the molecule−surface interaction and the structure of the first few layers strongly influence the resulting photochemistry at layer thickness up to at least 20 ML.