The adsorption states of NO2 over amorphous and crystalline water-ice films formed on an Au(111) surface have been studied in an ultrahigh vacuum system by the temperature programmed desorption (TPD) technique and IR absorption-reflection spectroscopy (IRAS). The ice films are prepared by deposition of gas phase water on the Au substrate at <100 K for amorphous ice and at 140 K for crystalline ice. The surface of amorphous ice is characterized by the high density of free OH, while that of crystalline ice is characterized by grain boundaries and the lack of free OH. TPD for pure ice shows only one desorption peak of H2O, while after NO2 adsorption on it an additional weak H2O desorption peak appears at 185 K. This higher-temperature peak is attributable to decomposition of NO2-H2O adducts. IRAS measurements revealed that NO2 adsorbs on ice surfaces as N2O4 with D2h symmetry and that neither N2O4 isomers such as D-isomers nor NOx (x ) 1, 2, and 3) species are produced in the temperature range of 90-140 K. Interaction of the ice surfaces with NO2 (N2O4) as well as orientation of N2O4 adsorbed on the ice surfaces are investigated as a function of temperature. Thermal decomposition of NO2 adsorbed on the water-ice formed on an Au surface is reconfirmed, which has been reported by Wang and Koel (J. Phys. Chem. A 1998, 102, 8573). A possible mechanism for the NO2 decomposition is proposed.

The photochemistry of cyclohexane on Cu(111) and its excitation mechanism have been studied by temperature-programmed desorption, ultraviolet and X-ray photoelectron spectroscopy. Cyclohexane weakly adsorbed on Cu(111) has been known to show a broadened and redshifted CH stretching band, i.e., CH vibrational mode softening. Although no dehydrogenation takes place thermally on this surface and by the irradiation of photons at 5.0 eV, adsorbed cyclohexane is dissociated to cyclohexyl and hydrogen by the irradiation of photons at 6.4 eV. This is a marked contrast to cyclohexane in the gas phase where the onset of absorption is located at 7 eV. When the surface irradiated by 6.4-eV photons is further annealed, cyclohexyl is dehydrogenated to form cylcohexene that desorbs at 230 K. The systematic measurements of photochemical cross sections at 6.4 eV with linearly polarized light as a function of incident angle indicate that the electronic transition from the highest occupied band of cyclohexane to a partially occupied hybridized band near the Fermi level is responsible for the photochemistry. The hybridized band is formed by the interactions between the electronic states of cyclohexane and the metal substrate. The role of the hybridized band in the photochemistry and the CH vibrational mode softening is discussed.

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The photochemistry of N 2 O 3 adsorbed on a Au(111) surface has been studied at <100 K. N 2 O 3 is prepared by a reaction of NO 2 chemisorbed on a Au surface with gas-phase NO. The adsorption states of N 2 O 3 are characterized by IR reflection-absorption spectroscopy. Adsorbed N 2 O 3 is found to undergo rapid photodissociation to form chemisorbed NO 2 and gas-phase NO under UV and visible illumination. The photodissociation cross section is determined to be (3.0 ( 1.8) × 10 -17 cm 2 at 350 nm, which is much greater than the maximum absorption cross section of gas-phase N 2 O 3 and one of the highest values ever observed for photochemical reactions on a metal surface. The spectral profile of the NO yield is significantly shifted to longer wavelengths as compared to the absorption spectrum of gas-phase N 2 O 3 . The significant red shift of N 2 O 3 absorption in aromatic solvents suggests that electron transfer from the surface to N 2 O 3 enhances the absorption cross section of adsorbed N 2 O 3 at longer wavelengths.