Surface species formed during UV photolysis of ozone adsorbed on water ice films at 80 K. A combined RA-FTIR and DFT study
The reflection–absorption FTIR spectra of the ozone–water films deposited on a cold metallic surface at 80 K before and after irradiation at 250 and 320 nm were studied to model the ozone photolysis and superoxidant adsorption taking place on the ice particles in the Earth's atmosphere. A new sharp intense band at 2860 cm−1 and a low-intense band at 1450 cm−1 were observed after irradiation at both wavelengths together with remarkable reshaping of the ice Ih bands in the regions 3100–3600 cm−1 and 500–1000 cm−1. The annealing of the reaction mixture has small effect on the intensities and positions of the new bands. The adsorption of formed species was modelled by DFT cluster calculations. The species O(1D), O(3P), O2(a 1Δg), O2(X 3Σg), HO˙, HOO˙, H2O2 were considered as possible intermediates and products. Among them, HO˙, H2O2, and HOO˙, form hydrogen bound complexes with estimated binding energies of −17.6, −28.5, and −37.2 kJ mol−1, correspondingly. The observed IR band of 2860 cm−1 can not be assigned to the OH vibrations of H2O2 shifted due to the adsorption and it is assigned to the ν2 + ν6 combination band of H2O2. Apart from H2O2, two other species can absorb in the region 2700–2900 cm−1: the adsorption complex of HOO˙ on the ice surface and O(1D) forming the complex with the ice of oxywater structure O⋯OH2 stabilized by additional hydrogen bonds.