IRAS and TDS study on the photolytic decarbonylation of iron pentacarbonyl adsorbed on a SiO2 film with a buried metal layer

IR reflection-absorption spectroscopy (IRAS) is successfully applied to the observation of surface species formed during the photolytic decarbonylation of Fe(CO)5 adsorbed on a SiO2 film with a buried Al layer, and thermal desorption spectroscopy (TDS) as well. TDS shows that Fe(CO)5 is molecularly adsorbed on the SiO2 surface at temperatures lower than 150 K. Irradiation of adsorbed Fe(CO)5 with synchrotron orbital radiation (SOR) light (>150 nm) leads to CO photo-evolution due to photo-decarbonylation without the photo-desorption of molecular Fe(CO)5. The average COFe ratio in photo-product obtained from CO evolution during irradiation and subsequent TDS is close to 4, suggestive of formation of Fe(CO)4 intermediate, while the photo-product exhibits a broad IRAS band and a broad TDS peak with three maxima, indicative of formation of oligomer species. The photo-product is, however, unlikely to be Fe3(CO)12, a typical product of the photolysis of Fe(CO)5 adsorbed on a SiO2 powder, since its IRAS band disappears upon heating the substrate up to 200 K even though the product remains unchanged in composition. The disappearance of the band is interpreted in terms of the surface selection rule of IRAS as a change in the geometry of the product.