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

Abstract

The broadband UV (230-420 nm) photoinduced chemistry of CH 3 Br adsorbed on Cu(2ML)/Ru(001) in the coverage range of 1-50 ML was studied by monitoring the desorption products (∆p-TPD mode) in combination with post irradiation work function change measurements before and during surface heating (∆ -ΤPD mode). ∆ 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 µ 0 ) 0.48 D. Dissociative electron attachment (DEA) driven CH 3 Br dissociation produced CH 3 and CH 2 fragments within the parent molecules multilayer matrix. At the multilayer coverage range, ∆ 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 CH 3 Br dielectric film.Comparison of the ∆ -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.

The effect of C–OH functionality on the surface chemistry of biomass-derived molecules: ethanol chemistry on Rh(100)

“…At 220 K, the work function value is 0.15 eV larger than that of the clean surface, due to the presence of a mixture of decomposition products, i. Calibration experiments with the Rh(100) surface covered by various amounts of surface carbon from ethylene decomposition reveals that 0.5 ML of surface carbon gives 0.25 eV rise to the work function. Furthermore, the work function changes linearly with surface carbon coverage, which is consistent with the Helmholtz equation (Df = 4peNm 0 , where N is the density of surface carbon on the surface, m 0 is the dipole moment, and e is the unit charge 45 ). Hence, the final work function of +0.035 eV corresponds to 0.07 ML surface carbon, which is consistent with the value obtained from TPO results, as shown in Table 2.…”

Application of work function measurements in the study of surface catalyzed reactions on Rh(1 0 0)

“…The ∆Φ due to surface carbon shows a linear increase with surface coverage, consistent with the Helmholtz equation [23][24][25], indicating that there is no significant change in the chemical nature of the ad-layer such as a different bonding site or the formation of carbide. Since H ad occupies fourfold hollow sites at all coverages [11,26,27] and subsurface adsorption of hydrogen is not feasible due to the low stability of bulk rhodium hydride [28,29] we also expect a linear correlation between ∆Φ on the hydrogen surface coverage.…”

Steric Effect in Electron−Molecule Interaction

“…The identical cross section obtained by the real-time QMS detection and the work function change measurements is explained assuming that photoelectron-mediated desorption is the dominant process in the MD configuration (0.35 ML O/Ru). Dominance of the electron-mediated dissociative channel in the MD configuration of the β 2 molecules would have resulted in smaller integrated differential work function change near 160 K. This is due to the negative work function change contribution of the two dissociation fragments, methyl and bromine, , if they separately adsorb on the surface. If fragments (e.g.…”