A time-resolved resonance Raman study of chlorine dioxide photochemistry in water and acetonitrile

The photochemistry of chlorine dioxide (OClO) in water and acetonitrile is investigated using time-resolved resonance Raman spectroscopy. Stokes and a nti-Stokes spectra are measured as a function of time following photoexcita tion using degenerate pump and probe wavelengths of 390 nm, For aqueous OCl O, the time-dependent Stokes intensities are found to be consistent with th e re-formation of ground-state OClO by subpicosecond geminate recombination of the primary ClO and O photofragments. This represents the first unequiv ocal demonstration of primary-photoproduct geminate recombination in the co ndensed-phase photochemistry of OClO. Anti-Stokes intensity corresponding t o the OClO symmetric stretch is observed demonstrating that, following gemi nate recombination. excess vibrational energy is deposited along this coord inate. Analysis of the anti-Stokes decay kinetics demonstrates that, in wat er, intermolecular vibrational relaxation occurs with a time constant of si milar to 9 ps. For OClO dissolved in acetonitrile, the Stokes scattering in tensities are consistent with a significant reduction in the geminate-recom bination quantum yield relative to water. Comparison of the OClO anti-Stoke s decay kinetics in acetonitrile and water demonstrates that the rate of in termolecular vibrational relaxation is similar to 4 times smaller in aceton itrile. Finally, in both solvents the appearance of symmetric-stretch anti- Stokes intensity is significantly delayed relative to geminate recombinatio n. This delay is consistent with the initial deposition of excess vibration al energy along the asymmetric-stretch coordinate followed by intramolecula r vibrational energy redistribution. The time scale for this redistribution is similar to 5 ps in water and similar to 20 ps in acetonitrile suggestin g that intramolecular vibrational energy reorganization is solvent dependen t.