Femtosecond pump–probe studies of chlorine dioxide photochemistry in water and acetonitrile Original Research Article

The reaction dynamics of chlorine dioxide (OClO) dissolved in water and acetonitrile are investigated using femtosecond pump–probe spectroscopy. The change in optical density following photoexcitation of OClO at 400 nm is monitored at 12 wavelengths ranging from 267 to 900 nm. The dynamics observed at 267 and 400 nm demonstrate that the geminate recombination quantum yield of the primary ClO and O photofragments to reform ground-state OClO is reduced by a factor of six in acetonitrile relative to water. Calculations are presented that model the contribution of vibrationally excited OClO formed by geminate recombination to the pump–probe dynamics. Comparison of the experimental and computational results demonstrates that a portion of the dynamics can be attributed to vibrationally excited OClO. However, the optical-density changes observed between 700 and 900 nm are similar in magnitude for both solvents, suggesting that another species not produced by geminate recombination is responsible for these dynamics. The appearance and relaxation kinetics in acetonitrile are significantly slower than in water demonstrating the solvent dependence of photoproduct formation and ground-state vibrational relaxation. Reasons for this dependence including Coulombic solvent–solute interactions and intermolecular hydrogen bonding are discussed.