Photochemistry of adsorbed molecules: Part XX: Photodissociation and photoreaction in CH 3Br/CaF 2(111) at 193 nm

Abstract

Methyl photofragments from the 193 nm photodissociation of CH 3Br adsorbed on CaF 2(111) were studied by angularly resolved time-of-flight mass spectrometry. The translational energy distributions, P( E T′), of the CH 3 photofragments showed evidence of four pathways, depending on the coverage. The pathways were termed ‘direct’ (DIR), ‘indirect(1′)’ [IND(1′)], ‘indirect(1)’ [IND(1)] and ‘indirect(2)’ [IND(2)] in order of decreasing peak E T′. The DIR methyl P( E T′) peaked at 2.4 eV, with a full-width at half-maximum (FWHM) of 0.6 eV, corresponding closely to that reported for gas-phase CH 3 (peak=2.5 eV, FWHM=0.5 eV). The P( E T′) of this DIR pathway was consistent with CH 3 escaping directly without collisions. For these DIR methyls the angular distribution, P( θ′), reflected the prior BrC bond direction at the various coverages. The DIR P( θ′) indicated a change in the alignment of BrCH 3 with increasing coverage, from one in which the Br–C axis was initially roughly parallel to the surface plane (0.25 ML), to one with the Br–C axis along the surface normal (0.5 and 2 ML), and finally to one with the Br–C axis 23° from the surface normal (7 ML). The IND(1′) and IND(1) methyls, despite an energy loss of 0.7 and 1.3 eV, respectively, in a strong inelastic encounter, both exhibited similar energy and angular distributions to the DIR component. We propose that the IND(1) pathway is due to the occurrence of an ‘intralayer’ exchange reaction ( C H 3+BrCH 3′→CH 3Br+C H 3′ ) in one layer, and in the case of the IND(1′) pathway the same process occurring in ‘interlayer’ collisions, since IND(1′) methyls were only observed at ∼2 ML coverage. This methyl-exchange reaction has been invoked previously to explain a similar inelastic pathway observed for photorecoiling CH 3 coming from CH 3Br on LiF(001), NaCl(001) and MgO(001). The IND(2) pathway was characterized by a broader P( E T′) than the other three pathways, and a broad P( θ′) (cos n θ ′, n=1.7–3.8), peaking at the surface normal. These attributes of P( θ′) for IND(2) are characteristic of strongly inelastic encounters in which memory of the initial BrC bond direction is lost.