PHOTOCHEMISTRY OF ADSORBED MOLECULES .18. PHOTODISSOCIATION AND EXCHANGE-REACTION IN CH3BR MGO(001) AT 193 NM/

Methyl photofragments from the 193 nm photodissociation of CH3Br adsor bed on MgO(001) were studied by angularly resolved time-of-flight mass spectrometry. The translational energy distributions of the photofrag ments showed evidence of three pathways for the release of CH3. The pa thways were termed ''direct'' [DIR], ''indirect(1)'' [IND(1)], and ''i ndirect(2)'' [IND(2)]. The DIR methyl translational energy distributio n, P(E-T') 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(E-T') of the DIR pathway was consiste nt with CH3 escaping directly from the topmost molecular layer without collisions. For these DIR methyls, as previously demonstrated, the an gular distribution reflected the prior Br-C bond direction. In the hig her coverage range, 1.5-10 monolayers (ML) angular distributions, P(Th eta'), for the DIR methyls peaked at 22 degrees to the normal with FWH M of 20 degrees. At low coverage, 0.75 ML, the DIR peak shifted to sim ilar to 40 degrees with doubled FWHM. The IND(1) methyls, despite the loss of 1.2 eV in a strong inelastic encounter, exhibited the same ang ular distributions and FWHM as the DIR component: 22 degrees peak, 20 degrees FWHM at high coverage; 40 degrees peak, 40 degrees FWHM at low coverage. The mechanism attributed to IND(1), which accounts for the retention of direction of methyl with concurrent substantial energy lo ss, is one proposed in a prior theoretical study [Barclay et al., J. P hys. Chem. 97, 12541 (1993)]: an exchange reaction favored by the adso rbate geometry; in the present case CH3+BrCH3'-->CH3Br+CH3'. The secon d indirect channel, IND(2), exhibited broader translational energy dis tribution than DIR or IND(1) peaked at 0.6 eV lower energy than IND(1) , and a broad angular distribution (cos(2) Theta') peaked at the norma l, characteristic of strongly inelastic encounters in which memory of the initial CH3 recoil direction is lost. (C) 1997 American Institute of Physics.