Time-resolved resonance Raman and femtosecond pump-probe study of chlorine dioxide (OClO) photochemistry in solution

The solution-phase photochemical reaction dynamics of chlorine dioxide (OClO) are studied using femtosecond pump-probe (FPP) and time-resolved resonance Raman (TRRR) spectroscopy. Specifically, the photoproduct formation, geminate recombination, and vibrational relaxation dynamics of OClO are studied in a variety of solvents to ascertain those aspects of the solvent responsible for defining the solution-phase reactivity of OClO. The studies presented here demonstrate that in condensed environments, ClO and O are the dominant photoproducts. Due to the solvent-caging effect, these photoproducts undergo geminate recombination resulting in the formation of vibrationally-excited OClO. Both the geminate recombination quantum yield and the OClO vibrational relaxation rate are solvent dependent. The geminate recombination quantum yield is largest in polar-protic solvents, demonstrating the importance in intermolecular hydrogen bonding in defining the rigidity of the solvent cage. In addition, the OClO vibrational relaxation rate is largest in polar-protic solvents, suggesting that solvent-solute hydrogen bonding provides an effective conduit for energy deposition into the solvent. The studies presented here also demonstrate that Cl and O2 are also produced following OClO photoexcitation. In addition to prompt Cl production, two-color, TRRR studies demonstrate that the thermal decomposition of ClOO formed through OClO photoisomerization results in Cl production on a much longer (∼200 ps) timescale. In summary, the FPP and TRRR studies presented here provide a detailed look into the solution-phase photochemical reaction dynamics of OClO.