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.

The photochemistry of chlorine dioxide (OClO) is investigated by two-color time-resolved resonance Raman spectroscopy. Pump and probe wavelengths of 390 and 260 nm, respectively, are used to monitor photoproduct formation following aqueous OClO photoexcitation. Depletion and subsequent recovery of the OClO scattering intensities is observed consistent with subpicosecond reformation of OClO via geminate recombination of the primary photoproducts. Intensity is observed at 1442 cm−1 consistent with ClOO formation that appears and decays with time constants of 27.9±4.5 ps and 398±50 ps, respectively. The results presented here represent the first direct evidence for ClOO formation following the photoexcitation of aqueous OClO.

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