The photochemistry of CH4 adsorbed on Pt(1 1 1) studied by high resolution fast XPS

The photochemistry induced at 40 K by synchrotron radiation at 400 eV in CH4 molecules adsorbed on Pt(1 1 1) was studied by high resolution fast XPS. The photoreactions were monitored by acquiring the C1s core level spectrum as a function of time for methane adlayers with coverage ranging from submonolayer to multilayer regime. We have observed molecular desorption of the multilayer and effective dissociation in the layer in contact with the metal. Differently from UV photons and low energy e-beam induced dissociation of CH4 adsorbed on Pt(1 1 1) – which only produced adsorbed CH3 and H fragments – the C1s core level measured on methane photodissociated at 400 eV shows that other species are formed, which start to thermally dissociate at about 100 K, producing CH and possibly H. Probable candidates are CH2, formed by multiple C–H bond cleavage together with CH3 groups during the initial methane photodissociation, or a C–C containing intermediate, resulting from methyl surface chemistry.