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

Abstract

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.

Formation mechanisms of oxygen atoms in the O(PJ3) state from the 157nm photoirradiation of amorphous water ice at 90K

“…This exposure resulted in the formation of 500 monolayers ͑ML͒ of H 2 O on the substrate if we adopt the reported experimental conversion factor of 1 ML coverage deposition by 3 L exposure. 23 Unfocused 157 nm laser radiation with a full width at half maximum duration of 10 ns was incident at an angle of about 80°to the surface normal on the ice surface at a fluence Ͻ0.1 mJ cm −2 pulse −1 . O͑ 3 P J=2,1,0 ͒ atom products were subsequently ionized at a distance of 4 mm ͑2 mm for OH͒ from the substrate surface by the ͑2+1͒ REMPI transition via the O͑ 3 P J − 3 P J ͒ transition at 225.6-226.4 nm, 24 and collected with a small mass spectrometer aligned perpendicular to the ice surface.…”

The heterogeneous hydrolysis of NO2 in laboratory systems and in outdoor and indoor atmospheres: An integrated mechanism

“…2 is uptake of gaseous N 2 O 4 on the surface and its isomerization to surface asymmetric ONONO 2 . This isomerization is known to occur in solid matrices at low temperatures or high pressures, on ice and in solution, 101,[135][136][137][138][139][140][141][142][143][144][145][146][147][148][149][150] (although one study 151 observed only the symmetric form of N 2 O 4 on ice films). Koel and coworkers [148][149][150]152 proposed that this isomerization occurs via the free O-H groups on amorphous ice, and it is possible that a similar process occurs in the thin films studied here.…”

“…167 In the case of the NO 2 studies, Cheung et al 132 found no evidence for a reactive NO 2 complex at the interface. There is one report 151 …”

Role of OH radicals in the formation of oxygen molecules following vacuum ultraviolet photodissociation of amorphous solid water

“…ASW was prepared with the backfilling deposition of water vapor for 60 min onto an optically flat sapphire disk substrate, sputter coated with a thin polycrystalline film of Au͑111͒ cooled by liquid nitrogen at 90 K. The exposure was typically 1500 L ͑1 L=1ϫ 10 −6 Torr s͒. This exposure resulted in the formation of roughly 500 ML ͑monolayer͒ of H 2 O on the substrate if we adopt the reported experimental conversion factor of 1 ML deposition by 3 L exposure, 26 which is sufficiently thick to neglect the possible influence of reactions at the ASW/Au substrate interface and of any photoelectrons from the Au substrate. 4,5,27 For the concentrated H 2 O 2 photolysis experiments, a commercially available H 2 O 2 solution ͑30%͒ was concentrated in a glass container by vacuum distillation, and the H 2 O 2 / H 2 O vapor was deposited on ASW.…”