A Time-Resolved Resonance Raman Study of Chlorine Dioxide Photochemistry in Water and Acetonitrile

Abstract

The photochemistry of chlorine dioxide (OClO) in water and acetonitrile is investigated using time-resolved resonance Raman spectroscopy. Stokes and anti-Stokes spectra are measured as a function of time following photoexcitation using degenerate pump and probe wavelengths of 390 nm. For aqueous OClO, the time-dependent Stokes intensities are found to be consistent with the re-formation of ground-state OClO by subpicosecond geminate recombination of the primary ClO and O photofragments. This represents the first unequivocal demonstration of primary-photoproduct geminate recombination in the condensed-phase photochemistry of OClO. Anti-Stokes intensity corresponding to the OClO symmetric stretch is observed demonstrating that, following geminate recombination, excess vibrational energy is deposited along this coordinate. Analysis of the anti-Stokes decay kinetics demonstrates that, in water, intermolecular vibrational relaxation occurs with a time constant of ∼9 ps. For OClO dissolved in acetonitrile, the Stokes scattering intensities are consistent with a significant reduction in the geminate-recombination quantum yield relative to water. Comparison of the OClO anti-Stokes decay kinetics in acetonitrile and water demonstrates that the rate of intermolecular vibrational relaxation is ∼4 times smaller in acetonitrile. Finally, in both solvents the appearance of symmetric-stretch anti-Stokes intensity is significantly delayed relative to geminate recombination. This delay is consistent with the initial deposition of excess vibrational energy along the asymmetric-stretch coordinate followed by intramolecular vibrational energy redistribution. The time scale for this redistribution is ∼5 ps in water and ∼20 ps in acetonitrile suggesting that intramolecular vibrational energy reorganization is solvent dependent.

Determination of differential quantum yields in solution by electron paramagnetic resonance spectroscopy

“…This property simplifies the problem notably, since the path lengths depend only on zas it may be seen in Eq. (12). The path lengths FG and GH are equal due to the equality between the angles FGO and HGO (y') in the triangles OGH and OFG (reflection law).…”

“…Although studies on the photolysis of OC10 have increased since it was determined as one of the main processes related to the destruction of the stratospheric ozone, most of the works on this topic refer to the primary processes [5][6][7][8][9][10][11][12][13][14][15]:…”

Mode-specific vibrational excitation and energy redistribution after ultrafast intramolecular electron transfer

“…However, despite the broad spectral width of the absorption near 300 nm of about 4700 cm Ϫ1 , different Raman excitation profiles for Stokes and anti-Stokes Raman scattering are to be expected. 16,18,33,34 It is more realistic to assume that the ratios of cross sections between the vibrations in the Stokes Raman spectrum are nearly equal to the ratios between the same vibrations in the anti-Stokes Raman spectrum. Consequently, we derived ''relative vibrational excess populations'' in dividing the anti-Stokes Raman intensities by the corresponding relative Raman cross sections which we take from the Stokes Raman intensities recorded at 300 nm ͑cf.…”

“…11 Vibrational excitation of Raman active modes of product states of several molecules being populated by photoinduced reactions have been monitored by timeresolved Stokes and anti-Stokes Raman spectroscopy, respectively. [15][16][17][18][19][20] In contrast to IR and Stokes Raman spectroscopy, anti-Stokes Raman measurements offer the advantage that the spectra originate exclusively from excited vibrational levels.…”

Femtosecond Pump−Probe Studies of Nitrosyl Chloride Photochemistry in Solution

“…[5][6][7] However, the photochemical production of the ClOO isomer is observed in low-temperature matrixes, [8][9][10] and photolysis of solution-phase OClO results in both ClO/O and ClOO production. [11][12][13][14][15][16][17][18][19][20][21] Another halooxide, dichlorine monoxide (ClOCl), demonstrates phase-dependent photochemical reactivity similar to OClO. In the gas phase, direct dissociation to form Cl and ClO is the dominant photochemical pathway, [22][23][24][25][26][27][28][29][30] whereas, photoexcitation in low-temperature matrixes and in solution results in the formation of the isomer, ClClO.…”