Photochemistry of iron pentacarbonyl adsorbed on Au(111)

The photodecomposition of Fe(CO)(5) adsorbed on the Au(111) surface at similar to 90 K has been studied using IR reflection absorption spectroscopy (IRAS), thermal desorption spectroscopy (TDS), and X-ray photoelectron spectroscopy(XPS). IRAS and TDS show that Fe(CO)(5) is molecularly adsorbed on the surface and desorbed completely around 180 K. A totally symmetric vibrational band appears at 2114 cm(-1) in IRAS, indicating that structural deformation of Fe(CO)(5) or charge transfer between the substrate and the adsorbate occurs to some extent upon adsorption. Irradiation of the adsorbed Fe(CO)5 leads to decarbonylation with a high quantum yield, especially at around the absorption threshold wavelengths of Fe(CO)(5) (300-360 nm). The enhanced photodecomposition at the absorption threshold is found limited to the close vicinity of the surface. At shorter wavelengths than 260 nm, however, the photolysis yield increases with increasing distance from the surface, suggesting significant quenching of photoexcited states near the surface. The average CO/Fe ratio in a photoproduct is close to 4 for monolayer (ML) coverage, suggesting intermediate species such as Fe(CO)(4), and decreases to similar to 3 as the coverage exceeds 3 ML. Since XPS shows formation of Fe-surface bonding, further photodecarbonylation may be inhibited by fast relaxation through the bonding. The photoproduct exhibits a sharp, single C-O stretching band at 2081 cm(-1) in IRAS, indicating formation of the carbonyl ligands with vertical vibrational modes in the same symmetry. Its structure is discussed in detail.