Adsorption states of NO2 over water-ice films formed on Au(111)

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 syst em by the temperature programmed desorption (TPD) technique and IR absorpti on-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 b y the high density of free OH, while that of crystalline ice is characteriz ed 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 reveal ed that NO2 adsorbs on ice surfaces as N2O4 With D-2h symmetry and that nei ther N2O4 isomers such as D-isomers nor NOx (x = 1, 2, and 3) species are p roduced in the temperature range of 90-140 K. Interaction of the ice surfac es with NO2 (N2O4) as well as orientation of N2O4 adsorbed on the ice surfa ces are investigated as a function of temperature. Thermal decomposition of NO2 adsorbed on the water-ice formed on an Au surface is reconfirmed, whic h has been reported by Wang and Keel (J. Phys. Chem. A 1998, 102, 8573). A possible mechanism for the NO decomposition is proposed.