Time-resolved absorption studies of chlorine dioxide photochemistry in solution

The solution phase photochemical reaction dynamics of chlorine dioxide (OClO) are studied using femtosecond transient absorption (TRA) spectroscopy. In particular, the dependence of these dynamics on solvent and actinic wavelength are explored. The effect of solvent on photoproduct formation, geminate recombination and vibrational relaxation is investigated using time-resolved infrared absorption spectroscopy. These studies demonstrate that following photoexcitation, a subset of the primary photofragments recombine on the subpicosecond timescale to reform OClO on the ground-state potential energy surface. The excess energy available to OClO following reformation is initially deposited along the asymmetric stretch coordinate. The vibrational-relaxation rate is found to be solvent dependent. Comparisons to theoretical studies on the vibrational relaxation dynamic of OClO reveal that the frequency dependent coupling between solvent and solute must be accounted for in order to found to reproduce the solvent dependent vibrational relaxation rates. In addition, the efficiency of geminate recombination is reduced in solvents in which the intermolecular vibrational relaxation rates are reduced.; TRA studies are presented which demonstrate that the quantum yields for geminate recombination and atomic chlorine (Cl) production following OClO photolysis are dependent on actinic energy. In opposition to earlier assumptions, the decrease in geminate recombination is found to correlate with enhanced Cl production. The mechanism of Cl production is invariant to changes in actinic energy, with 80% of Cl occurring through a "prompt" (<10 ps) process and 20% being formed from the thermal decomposition of ClOO. Excited-state absorption and stimulated emission signals are found to decrease with increasing actinic energy, demonstrating that the decay rate of the optically-prepared excited state is also dependent on actinic wavelength. These studies provide substantial insight into the actinic-wavelength dependent photochemistry of OClO.