Chlorine dioxide photochemistry in solution: time-resolved resonance Raman and femtosecond pump-probe studies

Abstract

The solution-phase reaction dynamics of chlorine dioxide (OClO) are studied using time-resolved resonance Raman (TRRR) and femtosecond pump-probe (FPP) spectroscopy. OClO photochemistry is investigated in a variety of solvents in order to ascertain the influence of environment on the photoreactivity of this compound. Specifically, the effect of solvent on dynamics associated with photoproduct formation, primary photoproduct geminate recombination, vibrational relaxation, and internal conversion is investigated. Following OClO photoexcitation, subpicosecond geminate recombination of the primary ClO and O photofragments resulting in the reformation of ground-state OClO is observed. The quantum yield for this process is found to be solvent dependent. Newly formed OClO is produced vibrationally hot, with theoretical studies demonstrating that the excess vibrational energy is initially deposited along the asymmetric-stretch coordinate exclusively. Subsequent intramolecular vibrational reorganization results in the redistribution of this energy to all vibrational coordinates. The dynamics conclude with vibrational energy dissipation to the solvent, with the kinetics of dissipation demonstrating solvent dependence.FPP studies are presented which establish the lifetime of the optically-prepared excited state, and provide information regarding the first stages of geminate recombination. The excited-state state lifetime is found to be solvent dependent, and this behavior is attributed to solvent dependence of the excited state energetics thereby affecting the rate of internal conversion. Time-resolved absorption anisotropy studies are presented which reveal a residual anisotropy corresponding to vibrationally-excited ground-state OClO. This observation suggests that memory of the photoexcitation event is retained through the recombination process. Finally, TRRR studies are presented which establish that ClOO production occurs following OClO photoexcitation. A combination of experimental and theoretical studies is presented which reveals that ground-state ClOO is produced with minimal excess vibrational energy. These studies also provide insight as to the mechanism of Cl production. Specifically, Cl production is a bifurcated process with 80% of Cl being produced on a fast (∼6 ps) timescale, and the remaining 20% being formed from thermal decomposition of ground-state ClOO on the subnanosecond timescale.