Photodissociation of methyl bromide adsorbed on LiF(001), NaCl(001), and MgO(001)

The adsorption and 193-nm photodynamics of CH$\sb3$Br adsorbed on LiF(001), NaCl(001), and MgO(001) have been studied by temperature programmed desorption (TPD) and angularly resolved time-of-flight mass spectrometry (TOF-MS), from submonolayer to multilayer coverages. Both the translational energy distributions, $\rm P(E\sb{T}\sp\prime),$ and angular distributions, $\rm P(\Theta\sp\prime),$ of the CH$\sb3$ photofragments showed evidence of two pathways for CH$\sb3$ formation; direct (DIR) and indirect (IND). The DIR CH$\sb3$ was characterised by a gas phase-like $\rm P(E\sb{T}\sp\prime)$ (van Veen et al., Chem. Phys. 92, 59 (1985)), with a high mean translational energy $(\rm\langle E\sb{T}\sp\prime\rangle = 2.3{-}2.4$ eV) and a narrow width (FWHM = 0.6 eV), indicating that DIR CH$\sb3$ left the surface without collisions. The peak in the DIR $\rm P(\Theta\sp\prime)$ gave the angle at which the Br-C axes of the molecules were aligned with respect to the surface normal prior to photolysis. For CH$\sb3$Br/LiF(001) the peak of the DIR $\rm P(\Theta\sp\prime)$ showed that the Br-C axes were tilted approx. $20\sp\circ$ from the surface normal. In the case of LiF(001) the DIR pathway predominated at high coverage (DIR/IND = 5.0 at 13.3 ML), but was a minor channel at low coverage. This was attributed to a gradual reordering of the adsorbate, in accord with results of polarised Fourier transform infrared spectroscopy (FTIR). The DIR $\rm P(\Theta\sp\prime)$ of CH$\sb3$Br/NaCl(001) was peaked at $38\sp\circ$ irrespective of coverage, while for MgO(001) it was peaked at ${\sim}40\sp\circ$ at submonolayer coverage and $22\sp\circ$ at coverages $>$1 ML, indicating a change in orientation of CH$\sb3$Br/MgO(001). The DIR was a minor channel on both NaCl and MgO at all coverages (DIR/IND $\le$ 0.2), suggesting that CH$\sb3$Br(ad) primarily lay down on these substrates with the Br-C axis near the surface plane, an inference also in accord with FTIR results. The IND methyl photofragments had markedly lower $\rm\langle E\sb{T}\sp\prime\rangle$ than DIR, indicating that they had suffered collisions in leaving the surface. For LiF(001), only a single peak at 1.0 eV was observed, while NaCl(001) and MgO(001) showed evidence of two IND channels, centred at 1.1 and 0.6 eV; these were termed IND(1) and IND(2). A striking finding was that the IND $\rm P(E\sb{T}\sp\prime)$ were of similar width to DIR, despite the strong collisions, suggesting constrained dynamics due to surface alignment. The two IND channels of MgO could be separated. The change in $\rm P(\Theta\sp\prime)$ of the IND(1) channel of MgO with coverage mirrored the changes in the DIR $\rm P(\Theta\sp\prime),$ indicating that the Br-C bond direction was preserved in the IND(1) dynamics as in DIR recoil. Similarly, on LiF the IND $\rm P(\Theta\sp\prime)$ for multilayer coverages showed a modest rise near $20\sp\circ,$ the peak in the DIR $\rm P(\Theta\sp\prime),$ indicating that some of the IND methyls retained the initial bond direction. Participation of the exchange reaction $\rm CH\sb3 + BrCH\sb3\sp\prime(ad) \to CH\sb3Br + CH\sb3\sp\prime$ was proposed to explain retention of the bond direction in the IND CH$\sb3\sp\prime$ for the single indirect channel of LiF and for IND(1) on MgO.