The photodissociation dynamics of chlorine molecules adsorbed on amorphous and crystalline water ice films was investigated at 351 nm. The ice films were prepared on a gold polycrystalline substrate at 80−140 K. Time-of-flight spectra of the photofragment chlorine atoms, measured with the resonance-enhanced multiphoton ionization technique, were simulated with a composite of two translational energy distributions:  a Gaussian distribution and a flux-weighted Maxwell−Boltzmann distribution. For both amorphous and crystalline ice films, the Gaussian distribution is characterized by the average energy 〈E t〉 = 0.38 ± 0.02 eV, while the Maxwell−Boltzmann one by 〈E t〉 = 0.12 ± 0.01 eV. The Gaussian distribution is attributable to the chlorine atoms produced from the direct photodissociation of Cl2, while the Maxwell−Boltzmann characterizes those having undergone strong relaxation processes. The observed translational energy distributions for amorphous and crystalline ice films were similar to each other, but the relative contribution of the two energy distributions as well as the photodissociation yield of Cl atoms depend on the states of the ice films. Free OH groups and surface morphology of an ice film surface have a strong influence on the photodissociation quantum yield. The adsorbate−water interaction for Cl2 and an ice surface is discussed on the basis of the measurements of time-of-flight, infrared absorption, and temperature-programmed desorption spectra.

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.

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Relative rate techniques were used to measure k(OH + HFE-7500) = (2.6+/-0.6) x 10(-14), k(Cl + HFE-7500) = (2.3+/-0.7) x 10(-12), k[Cl + n-C3F7CF(OC(O)H)CF(CF3)2] = (9.7+/-1.4) x 10(-15), and k[Cl + n-C3F7CF(OC(O)CH3)CF(CF3)2] < 6 x 10(-17) cm3 molecule(-1) s(-1) at 295 K [HFE-7500 = n-C3F7-CF(OC2H5)CF(CF3)2]. From the value of k(OH + HFE-7500) an estimate of 2.2 years for the atmospheric lifetime of HFE-7500 is obtained. Two competing loss mechanisms for n-C3F7-CF(OCHO.CH3)CF(CF3)2 radicals were identified in 700 Torr of N2/O2 diluent at 295 K; reaction with O2 and decomposition via C-C bond scission with kO2/k(decomp) = 0.013+/-0.006 Torr(-1). The Cl atom initiated oxidation of HFE-7500 in N2/O2 diluent gives n-C3F7CF(OC(O)CH3)CF(CF3)2 as the major product and n-C3F7CF(OC(O)H)CF(CF3)2 as a minor product. The atmospheric oxidation of HFE-7500 gives n-C3F7-CF(OC(O)CH3)CF(CF3)2 and n-C3F7CF(OC(O)H)CF(CF3)2 as oxidation products. The results are discussed with respect to the atmospheric chemistry and environmental impact of HFE-7500.