VIBRATIONAL-RELAXATION OF CLO2 IN WATER

Photodissociation of ClO2 in aqueous solution at 400 nm results in the formation of ClO+O and Cl+O-2. ClO and O geminately recombine to ClO2 in the electronic ground state (B-2(1)), formed with an initial vibra tional energy of approximate to 2.5 eV. In this paper the vibrational relaxation of ClO2(B-2(1)) in aqueous solution is studied by femtoseco nd transient absorption spectroscopy in the spectral range 234 to 1024 nm. The measured transient absorption of the vibrationally relaxing C lO2 molecules is compared with the transient absorption calculated for relaxation in the asymmetric stretch as well as the symmetric stretch and bending modes. The calculations of the absorption spectra pertain ing to the asymmetric stretch are based on a harmonic potential derive d from the experimentally determined fundamental vibrational energy, w hereas that of the symmetrical vibrations are based on ab initio poten tials. An excellent agreement is obtained by assuming that the vibrati onal relaxation predominantly occurs in the asymmetric stretch with a 9.5 ps relaxation time. A weak spectral feature in the ultraviolet par t of the spectrum is assigned to vibrational relaxation in the symmetr ic stretch and bending modes, indicating a coupling between the asymme tric and symmetric modes. (C) 1998 American Institute of Physics. [S00 21-9606(98)01120-9]