Femtosecond UV pump/near-IR probe studies of the solvent-dependent excited-state decay dynamics of chlorine dioxide

Abstract

The excited-state decay and geminate-recombination dynamics of chlorine dioxide (OClO) are investigated using UV pump/near-IR probe spectroscopy. Experiments are performed with 200-fs time resolution on OClO dissolved in water, cyclohexane, acetonitrile, and chloroform. In all solvents, a reduction in optical density is observed at early times and is attributed to stimulated emission from the optically prepared 2A2 surface. The emission decays on the subpicosecond time scale to reveal an increase in optical density corresponding to the production of vibrationally hot ground-state OClO formed by geminate recombination of the primary photoproducts. Kinetic analysis of these data reveals that the excited-state decay time constant increases from ∼200 fs in water and cyclohexane to ∼400 fs in acetonitrile and chloroform. The vibrational-relaxation dynamics of OClO are also found to be solvent dependent in agreement with earlier work. Pump-probe anisotropy experiments on aqueous OClO are presented. In these studies, an initial anisotropy of 0.40 ± 0.05 is observed consistent with emission from the optically prepared 2A2 state. However, the optical-density evolution for vibrationally hot OClO displays an anisotropy of 0.08 ± 0.03 suggesting that the mechanism of geminate recombination results in the retention of memory regarding the photoexcitation event. Potential recombination mechanisms consistent with this result are discussed.