A time-resolved resonance Raman study of chlorine dioxide photochemistry in water and acetonitrile
- Univ. of Washington, Seattle, WA (United States). Dept. of Chemistry
The photochemistry of chlorine dioxide (OClO) has attracted much interest due to its participation in the atmospheric chlorine reservoir as well as its potential role in stratospheric ozone depletion. Since the environmental impact of OClO arises from its ability to produce atomic chlorine, understanding this phase-dependent reactivity is essential if models capable of predicting the environmental impact of OClO in both homogeneous and heterogeneous settings are to be obtained. The photochemistry of chlorine dioxide (OClO) in water and acetonitrile is investigated using time-resolved resonance Raman spectroscopy. Stokes and anti-Stokes spectra are measured as a function of time following photoexcitation using degenerate pump and probe wavelengths of 390 nm. For aqueous OClO, the time-dependent Stokes intensities are found to be consistent with the re-formation of ground-state OClO by subpicosecond geminate recombination of the primary ClO and O photofragments. This represents the first unequivocal demonstration of primary-photoproduct geminate recombination in the condensed-phase photochemistry of OClO. Anti-Stokes intensity corresponding to the OClO symmetric stretch is observed demonstrating that, following geminate recombination, excess vibrational energy is deposited along this coordinate. Analysis of the anti-Stokes decay kinetics demonstrates that, in water, intermolecular vibrational relaxation occurs with a time constant of {approximately}9 ps. For OClO dissolved in acetonitrile, the Stokes scattering intensities are consistent with a significant reduction in the geminate-recombination quantum yield relative to water. Comparison of the OClO anti-Stokes decay kinetics in acetonitrile and water demonstrates that the rate of intermolecular vibrational relaxation is {approximately}4 times smaller in acetonitrile. Finally, in both solvents the appearance of symmetric-stretch anti-Stokes intensity is significantly delayed relative to geminate recombination. This delay is consistent with the initial deposition of excess vibrational energy along the asymmetric-stretch coordinate followed by intramolecular vibrational energy redistribution. The time scale for this redistribution is {approximately}5 ps in water and {approximately}20 ps in acetonitrile suggesting that intramolecular vibrational energy reorganization is solvent dependent.
- OSTI ID:
- 682160
- Journal Information:
- Journal of Physical Chemistry A: Molecules, Spectroscopy, Kinetics, Environment, amp General Theory, Vol. 103, Issue 28; Other Information: PBD: 15 Jul 1999
- Country of Publication:
- United States
- Language:
- English
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