Photochemistry of adsorbed molecules: Part XXI. CH3Br adsorbed on electron-impact- or hydrogen-atom-modified CaF2(111), at 193 nm

The photochemistry of CH3Br adsorbed on CaF2 has been studied using CaF2(111) modified by (a) low-energy electron impact and (b) dosing with hydrogen atoms. In each case the temperature-programmed desorption (TPD) behaviour and the photodynamics, i.e. the energy and angular distributions of the photoformed methyl radicals were compared with the same diagnostics applied to unmodified CaF2(111). Electron irradiation of the CaF2(111) surface resulted in an increased desorption temperature of the adsorbate, and the emergence of a broadened translational energy photodissociation channel, P(ET′), for CH3(g), associated in earlier work with photoinduced charge-transfer dissociation of the adsorbate. This suggested the formation of Ca metal clusters by process (a). By contrast, the hydrogen-atom-modified surface exhibited a decreased desorption temperature of the adsorbate and no broadening of P(ET′). The thermal hydrogen atoms may modify the surface by abstracting surface F, forming HF(g). Electron impact and hydrogen atom modification of the CaF2 changed the angular distributions for the ‘directly formed’ (fast) CH3(g) photofragments in a markedly different fashion, indicative of differing structure in the adlayer for CaF2(111) when modified by processes (a) or (b).