Adsorption States of NO₂ over Water−Ice Films Formed on Au(111)

The adsorption states of NO 2 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 H 2 O, while after NO 2 adsorption on it an additional weak H 2 O desorption peak appears at 185 K. This higher-temperature peak is attributable to decomposition of NO 2 −H 2 O adducts. IRAS measurements revealed that NO 2 adsorbs on ice surfaces as N 2 O 4 with D 2 h symmetry and that neither N 2 O 4 isomers such as D-isomers nor NO x ( x = 1, 2, and 3) species are produced in the temperature range of 90−140 K. Interaction of the ice surfaces with NO 2 (N 2 O 4 ) as well as orientation of N 2 O 4 adsorbed on the ice surfaces are investigated as a function of temperature. Thermal decomposition of NO 2 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 NO 2 decomposition is proposed.