Geminate recombination and vibrational relaxation dynamics of aqueous chlorine dioxide: A time-resolved resonance Raman study

Abstract

The photochemical dynamics of aqueous chlorine dioxide (OClO) are investigated using time-resolved resonance Raman spectroscopy. Stokes and anti-Stokes spectra are measured as a function of time following photoexcitation of OClO using degenerate pump and probe wavelengths at 390 nm. The temporal evolution of OClO Stokes intensity is found to be consistent with the reformation of ground-state OClO by subpicosecond geminate recombination of the primary ClO and O photofragments. Anti-Stokes intensity is observed for transitions corresponding to the symmetric stretch of OClO demonstrating that upon geminate recombination, excess vibrational energy is deposited along this coordinate. Dissipation of this energy to the surrounding solvent occurs with a time constant of ∼9 ps. Finally, a delay in the appearance of OClO anti-Stokes intensity relative to geminate recombination is observed demonstrating that the excess vibrational energy available to OClO is initially deposited along the resonance Raman inactive asymmetric stretch coordinate with the exchange of energy between this coordinate and the symmetric stretch occurring with a time-constant of ∼5 ps.

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

“…The laser system employed here is identical to that used in our earlier time-resolved work. − An argon-ion laser (Spectra Physics 2065-07) operating all-lines was used to pump a home-built Ti:sapphire oscillator that produced 30 fs pulses (full-width at half-maximum) centered at 780 nm with a repetition rate of 91 MHz. The oscillator output was temporally elongated using an optical stretcher and delivered to a Ti:sapphire regenerative amplifier (Clark-MXR CPA-1000-PS) equipped with independently tunable single- and double-plate birefringent filters to constrain the amplifier bandwidth.…”

“…Recent studies of OClO solution-phase photochemistry have attempted to identify the specific solvent−solute interactions that give rise to phase-dependent reactivity of this compound. ,,,− Recently, picosecond and femtosecond pump−probe techniques have been used to monitor the kinetics of photoproduct formation. ,,− These studies have provided the first information concerning the dynamics that occur following internal conversion to the ground state. Universal agreement exists concerning the spectral evolution observed in pump−probe studies of aqueous OClO; however, the structural interpretation of this evolution is currently at issue.…”

“…In this manuscript, we present a time-resolved resonance Raman study of OClO dissolved in water and acetonitrile. A preliminary report of this investigation concerning aqueous OClO has appeared . Time-dependent Stokes and anti-Stokes scattering intensities are measured following the photoexcitation of OClO.…”

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.…”

Theoretical studies and model calculations of femtosecond stimulated Raman spectroscopy

“…[17,18,19,20,21,22,23,24] Alternatively, a full vibrational spectrum over a 3000 cm −1 window can be obtained by using spontaneous Raman spectroscopy, but pulse duration longer than 0.7 ps is necessary to achieve acceptable spectral resolution. [25,26,27] Thus, achieving both high time and high spectral resolution is impossible in traditional spontaneous Raman spectroscopy using a single pulse. On the other hand, with FSRS one can achieve < 100 fs time resolution as well as capture a high-resolution vibrational spectrum over a wide spectral window.…”