IRAS and TDS study on the photolytic decarbonylation of iron pentacarbonyl adsorbed on a SiO 2 film with a buried metal layer

Abstract

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 SiO 2 film with a buried Al layer, and thermal desorption spectroscopy (TDS) as well. TDS shows that Fe(CO) 5 is molecularly adsorbed on the SiO 2 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 CO Fe 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 Fe 3(CO) 12, a typical product of the photolysis of Fe(CO) 5 adsorbed on a SiO 2 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.