The reaction rate between atmospheric hydrogen chloride (HCl) and chlorine nitrate (ClONO(2)) is greatly enhanced in the presence of ice particles; HCl dissolves readily into ice, and the collisional reaction probability for ClONO(2) on the surface of ice with HCl in the mole fraction range from approximately 0.003 to 0.010 is in the range from approximately 0.05 to 0.1 for temperatures near 200 K. Chlorine (Cl(2)) is released into the gas phase on a time scale of at most a few milliseconds, whereas nitric acid (HNO(3)), the other product, remains in the condensed phase. This reaction could play an important role in explaining the observed depletion of ozone over Antarctica; it releases photolytically active chlorine from its most abundant reservoir species, and it promotes the formation of HNO(3) and thus removes nitrogen dioxide (NO(2)) from the gas phase. Hence it establishes the necessary conditions for the efficient catalytic destruction of ozone by halogenated free radicals. In the absence of HCl, ClONO(2) also reacts irreversibly with ice with a collision efficiency of approximately 0.02 at 200 K; the product hypochlorous acid (HOCI) is released to the gas phase on a time scale of minutes.

electronic lifetime T~~ at a temperature slightly higher than 21 1 K. At even lower temperatures, the greatest part of the molecules reorient very slowly or they cannot reorient, thus, 7, becomes so high that all the emission is observed from the X state and is well described by a single exponential function. These results are in good agreement with those deduced from the study of the spectral shifts at the same dilution.1° The variation of T~ with temperatures agrees well with the considerations made and the results obtained by other authors6*" in different compounds.In conclusion, a solutesolvent reorientation during fluorescence gives a coherent interpretation of the decay time curves and an estimation of the reorientation times at different temperatures. Acknowledgment. The authors are very grateful to Drs. A.Molecular complexes of H20, H202, CO, and C02 in solid O2 have been prepared by a conventional matrix deposition method using an appropriate gas mixture. FTIR spectra and vibrational assignments of matrix-isolated H20 dimer and trimer, H20C02, H 2 0 C 0 , H20~H202, and H202.nC0 in solid O2 are presented. Hydrogen bonds of the type. 0-H-0 involving H,O, species and C.-H-0 involving H,O,-nCO species are found except for H20.C02. In the H20"202 complex, H202 is an electron acceptor (acid) and H 2 0 is an electron donor (base). The frequency shift of the 0-H stretch in the electron acceptor molecule of the complex increases in order of H 2 0 C 0 , H202.C0, (H20),, and H20.H202.

The infrared absorptivities of nitric acid ices formed from vapors containing water and nitric acid were investigated by means of a Fourier-transform infrared spectrometer operated at a wavelength resolution of 1.0 cm-1 (triangular apodization). The solid films were prepared on the surface of a gold-coated, polished copper plate at a temperature of 153 K to 198 K. After collecting the infrared spectra, the substrates were vaporized and condensed into a U-tube for quantitative analyses of H2O and HN03 using a quadrupole mass spectrometer. Amorphous forms of nitric acid ices and four crystalline hydrates (nitric acid monohydrate (NAM), nitric acid dihydrate (NAD), and nitric acid trihydrate (both a-NAT and /3-NAT forms)) were identified and their infrared absorptivities measured. The infrared spectra were interpreted in terms of the known spectra of oxonium and nitrate ions. Possible implications for remote sensing and the analysis of polar stratospheric clouds using these infrared spectra are discussed.

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The kinetics of the reaction OH + NH3 have been studied by means of the flash photolysis/laser-induced fluorescence technique.The rate of this reaction was investigated in the range 273-433 K under more extensive conditions (68 < P/ < 504, 0.29 < [NH3]/1015 molecules cm"3 <36.1) than previously. The results from experiments with the Xe lamp and with the KrF laser for photolysis agree well within the experimental uncertainties, indicating the absence of interference due to excess NH2 which was produced by photolysis with the Xe lamp. A fit of rate coefficients to the Arrhenius equation yields k = (3.29 ± 1.02) X 10"12 exp[-(922 ± 100)/7] cm3 molecule"1 s"1, with k = (1.47 ± 0.07) X 10"13 cm3 molecule"1 s"1 at 297 K; the uncertainties represent one standard error. The rate constant of the interfering reaction, OH + NH2 -* products, was also estimated to be less than 7 X 10"12 cm3 molecule"1 s"1.