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

We study the reaction dynamics of OClO in cyclohexane, acetonitrile, and wa ter by femtosecond pump-probe spectroscopy. In all solvents we observe a qu antum 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 concl ude that, in contrast to previous reports, not all OClO molecules dissociat e after excitation with 403 nm light. In both cyclohexane and water we obse rve 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 explai n these results as the consequence of vibrational relaxation of the bending mode of OClO. Relaxation from (nu (1),1,0) to (nu (1),0,0) leads to popula tion of a state with a two times higher transition dipole moment, which acc ounts for the increased stimulated emission. Further proof that not all OCl O molecules dissociate immediately after excitation is found in the identif ication of a stimulated emission contribution in femtosecond 400 nm pump/80 0 nm probe experiments, which also decays with about a 1.5 ps time constant . Femtosecond 400 nm pump/267 nm probe measurements indicate that a fractio n of the OClO molecules dissociate very rapidly, with dissociation times of less than or equal to 60, 80, and 140 fs, in acetonitrile, water, and cycl ohexane, respectively. An anisotropy decay is resolved at 267 nm of the for med ClO in water and cyclohexane, with anisotropy decay times of 0.17 and 0 .27 ps, respectively. In all solvents a fraction of the ClO+O fragments rec ombine, with time constants of 1.2 and 4.1 ps in water, 6.0 ps in acetonitr ile, and 8.9 ps in cyclohexane. In acetonitrile a secondary dissociation pa thway 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 esc ape of a significant amount of fragments. (C) 2001 American Institute of Ph ysics.