CHARGE SHIFTING IN THE ULTRAFAST PHOTOREACTIONS OF CLO- IN WATER

The reaction dynamics of ClO- in water following femtosecond ultraviol et photolysis is investigated by measuring time-resolved absorption an d anisotropy. Ab initio calculations show that Light absorption induce s charge shifting from the O- atom to the Ci atom. Molecular dynamics simulations predict that the charge shift is followed by the destructi on of the solvent structure around the O atom and its formation around newly formed negative charge on the Cl atom. An ultrafast (similar to 60 fs) transient absorption change is observed and likely corresponds to the inertial part of the destruction of the solvent structure arou nd the newly formed neutral O atom of the excited state OCl-. The earl y time anisotropy of -0.13 +/- 0.05 decays on the 230 fs time scale an d is attributed to the dissociation along a new reaction path toward C l + O- that is seen independently through the evaluation of the spectr um of Cl. The remaining anisotropy decays within 6 ps due to rotationa l diffusion of the ion. Probe wavelength dependence of the longer time dynamics (1.3-7 ps) is proposed to be the vibrational relaxation of t he vibrationally hot ground state of ClO-, the generation of which can be described as electron transfer from the excited state OCl- to the ground state ClO-. (C) 1997 American Institute of Physics.