A Time-Resolved Resonance Raman Study of Chlorine Dioxide Photochemistry in Water and Acetonitrile

Abstract

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 ∼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 ∼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 ∼5 ps in water and ∼20 ps in acetonitrile suggesting that intramolecular vibrational energy reorganization is solvent dependent.