EXCITED-STATE REACTION DYNAMICS OF CHLORINE DIOXIDE IN WATER FROM ABSOLUTE RESONANCE RAMAN INTENSITIES

Resonance Raman spectra of aqueous chlorine dioxide (OClO) are measure d for several excitation wavelengths spanning the photochemically rele vant B-2(1)-(2)A(2) optical transition. A mode-specific description of the optically prepared, (2)A(2) potential energy surface is derived b y simultaneous analysis of the absolute resonance Raman and absorption cross sections. The resonance Raman spectra are dominated by transiti ons involving the symmetric stretch and bend coordinates demonstrating that excited-state structural evolution occurs predominately along th ese degrees of freedom. Scattering intensity is not observed for trans itions involving the asymmetric stretch, demonstrating that excited-st ate structural evolution along this coordinate is modest. The limited evolution along the asymmetric stretch coordinate results in the prese rvation of C-2 nu symmetry on the (2)A(2) surface. It is proposed that this preservation of symmetry is responsible for the increase in Cl p hotoproduct quantum yield in solution relative to the gas phase. Analy sis of the absolute scattering cross sections also demonstrates that t he homogeneous line width for the B-2(1)-(2)A(2) optical transition in water is essentially identical to that in cyclohexane; however, the e xtent of inhomogeneous broadening increases dramatically in aqueous so lution. Comparison of the spectroscopic properties of OClO to the prop erties of isoelectronic O-3(-) is made to elucidate the origin of the solvent response to OClO photoexcitation. It is suggested that solvent -solute dipole-dipole coupling and intermolecular hydrogen bonding rep resent the largest components of the solvent coordinate.