FEMTOSECOND PUMP-PROBE STUDIES OF CHLORINE DIOXIDE PHOTOCHEMISTRY IN WATER AND ACETONITRILE

The reaction dynamics of chlorine dioxide (OClO) dissolved in water an d acetonitrile are investigated using femtosecond pump-probe spectrosc opy. The change in optical density following photoexcitation of OClO a t 400 nm is monitored at 12 wavelengths ranging from 267 to 900 nm. Th e dynamics observed at 267 and 400 nm demonstrate that the geminate re combination quantum yield of the primary ClO and O photofragments to r eform ground-state OClO is reduced by a factor of six in acetonitrile relative to water. Calculations are presented that model the contribut ion of vibrationally excited OClO formed by geminate recombination to the pump-probe dynamics. Comparison of the experimental and computatio nal results demonstrates that a portion of the dynamics can be attribu ted to vibrationally excited OClO. However, the optical-density change s observed between 700 and 900 nm are similar in magnitude for both so lvents, suggesting that another species not produced by geminate recom bination is responsible for these dynamics. The appearance and relaxat ion kinetics in acetonitrile are significantly slower than in water de monstrating the solvent dependence of photoproduct formation and groun d-state vibrational relaxation. Reasons for this dependence including Coulombic solvent-solute interactions and intermolecular hydrogen bond ing are discussed. (C) 1998 Elsevier Science B.V. All rights reserved.