Vibrational mode-specific photochemical reaction dynamics of chlorine dioxide in solution

摘要

We study the reaction dynamics of OCIO in cyclohexane, acetonitrile, and water by femtosecond pump-probe spectroscopy. In all solvents we observe a quantum beat in a 403 nm one-color pump-probe experiment with 55 fs temporal resolution, that decays with a 1.3-1.5 ps time constant. From this we conclude that, in contrast to previous'reports, not all OCIO molecules dissociate after excitation with 403 nm light. In both cyclohexane and water we observe in the 403 nm experiment an increase in stimulated emission between 0.5 and 2 ps that appears to be connected to the quantum beat decay. We explain these results as the consequence of vibrational relaxation of the bending mode ofOCIO.Relaxation from (vl'I,O) to (v1,O,O) leads to population ofa state with a two times higher transition dipole moment, which accounts for the increased stimulated emission. Further proof that not all OCIO molecules dissociate immediately after excitation is found in the identification of a stimulated emission contribution in femtosecond 400 nm pump/800 nm probe experiments, which also decays with about a 1.5 ps time constant. Femtosecond 400 nm pump/267 Dm probe measurements indicate that a fraction of the OCIO molecules dissociate very rapidly, with dissociation times of .;;;60, 80, and 140 fs, in acetonitrile, water, and cyclohexane, respectively. An anisotropy decay is re~olved at 267 nm of the formed CIO in water and cyclohexane, with anisotropy decay times of0.17 and 0..27ps, respectively. In all solvents a fraction of the CIO+O fragments recombine, with time constants of 1.2 and 4.1 ps in water, 6.0ps in acetonitrile, and 8.9 ps in cyclohexane.. In acetonitrile a secondary dissociation pathway is identified with a 2.1 ps time constant. This pathway might also be responsible for the biexponentiality of the recombination process in water. In particular, in acetonitrile and cyclohexane the data indicate cage escape of a significant amount of fragments.