Photochemistry of oriented molecules coadsorbed on solid surfaces: The formation of CO2+O from photodissociation of O2 coadsorbed with CO on Pt(111)

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Abstract
Measurement of a photoinduced reaction involving two types of molecular species coadsorbed with well-defined configurations on a solid surface is reported. The photoinduced reaction, occurring on Pt(111) at 100 K, is O2+CO+hν→O+{O→CO}→O+CO2. A mechanism involving photochemically produced hot O atoms (with high translational energy, and possibly electronically excited) is proposed, in which the initial step involves selective photodissociation of O2 coadsorbed with CO. The O atom collides with a neighboring CO and forms CO2 which desorbs immediately from the surface. The nature of the adsorbed species was probed before and after irradiation by thermal desorption spectroscopy (TDS) and high resolution electron energy loss spectroscopy (EELS). It was found that the wavelength dependence of the CO2 production followed that for O2 photodissociation. At 338 nm the cross section for CO2 production is 3.3±0.5×10−20 cm2 and decreases to 2×10−21 cm2 at 443 nm. CO2 was not observed in EEL spectra following quenching of thermoinduced or photoinduced reaction of either molecular or atomic oxygen with CO on Pt(111). CO2 was observed with EELS following direct CO2 adsorption on Pt(111) at 85 K. The results from these studies are believed to be generally relevant to understanding the dynamics of elementary chemical reactions.