Photochemistry of adsorbed molecules. XVIII. Photodissociation and exchange reaction in CH3Br/MgO(001) at 193 nm

Abstract

Methyl photofragments from the 193 nm photodissociation of CH3Br adsorbed on MgO(001) were studied by angularly resolved time-of-flight mass spectrometry. The translational energy distributions of the photofragments showed evidence of three pathways for the release of CH3. The pathways were termed “direct” [DIR], “indirect(1)” [IND(1)], and “indirect(2)” [IND(2)]. The DIR methyl translational energy distribution, P(ET′) peaked at 2.3 eV, with a full width at half-maximum (FWHM) of 0.65 eV, corresponding roughly to that reported for gas phase CH3 (peak=2.5 eV, FWHM=0.5 eV). The P(ET′) of the DIR pathway was consistent with CH3 escaping directly from the topmost molecular layer without collisions. For these DIR methyls, as previously demonstrated, the angular distribution reflected the prior Br–C bond direction. In the higher coverage range, 1.5–10 monolayers (ML) angular distributions, P(Θ), for the DIR methyls peaked at 22° to the normal with FWHM of 20°. At low coverage, 0.75 ML, the DIR peak shifted to ∼40° with doubled FWHM. The IND(1) methyls, despite the loss of 1.2 eV in a strong inelastic encounter, exhibited the same angular distributions and FWHM as the DIR component: 22° peak, 20° FWHM at high coverage; 40° peak, 40° FWHM at low coverage. The mechanism attributed to IND(1), which accounts for the retention of direction of methyl with concurrent substantial energy loss, is one proposed in a prior theoretical study [Barclay et al., J. Phys. Chem. 97, 12541 (1993)]: an exchange reaction favored by the adsorbate geometry; in the present case CH3+BrCH3′→CH3Br+CH3′. The second indirect channel, IND(2), exhibited broader translational energy distribution than DIR or IND(1) peaked at 0.6 eV lower energy than IND(1), and a broad angular distribution (cos2 Θ) peaked at the normal, characteristic of strongly inelastic encounters in which memory of the initial CH3 recoil direction is lost.