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

The excited-state decay and geminate-recombination dynamics of chlorine dio xide (OClO) are investigated using UV pump/near-IR probe spectroscopy. Expe riments are performed with 200-fs time resolution on OClO dissolved in wate r, cyclohexane, acetonitrile, and chloroform. In all solvents, a reduction in optical density is observed at early times and is attributed to stimulat ed emission from the Optically prepared (2)A(2) surface. The emission decay s on the subpicosecond time scale to reveal an increase in optical density corresponding to the production of vibrationally hot ground-state OClO form ed by geminate recombination of the primary photoproducts. Kinetic analysis of these data reveals that the excited-state decay time constant increases from similar to 200 fs in water and cyclohexane to similar to 400 fs in ac etonitrile and chloroform. The vibrational-relaxation dynamics of OClO are also found to be solvent dependent in agreement with earlier work. Pump-pro be 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 (2)A(2) state. However, the optical-density ev olution for vibrationally hot OClO displays an anisotropy of 0.08 +/- 0.03 suggesting that the mechanism of geminate recombination results in the rete ntion of memory regarding the photoexcitation event. Potential recombinatio n mechanisms consistent with this result are discussed.