Of Chlorine Dioxide Research Articles

OClO as observed by TROPOMI: a comparison with meteorological parameters and polar stratospheric cloud observations

Abstract. Chlorine dioxide (OClO) is a by-product of the ozone-depleting halogen chemistry in the stratosphere. Although it is rapidly photolysed at low solar zenith angles (SZAs), it plays an important role as an indicator of the chlorine activation in polar regions during polar winter and spring at twilight conditions because of the nearly linear dependence of its formation on chlorine oxide (ClO). Here, we compare slant column densities (SCDs) of chlorine dioxide (OClO) retrieved by means of differential optical absorption spectroscopy (DOAS) from spectra measured by the TROPOspheric Monitoring Instrument (TROPOMI) with meteorological data for both Antarctic and Arctic regions for the first three winters in each of the hemispheres (November 2017–October 2020). TROPOMI, a UV–Vis–NIR–SWIR instrument on board of the Sentinel-5P satellite, monitors the Earth's atmosphere in a near-polar orbit at an unprecedented spatial resolution and signal-to-noise ratio and provides daily global coverage at the Equator and thus even more frequent observations at polar regions. The observed OClO SCDs are generally well correlated with the meteorological conditions in the polar winter stratosphere; for example, the chlorine activation signal appears as a sharp gradient in the time series of the OClO SCDs once the temperature drops to values well below the nitric acid trihydrate (NAT) existence temperature (TNAT). Also a relation of enhanced OClO values at lee sides of mountains can be observed at the beginning of the winters, indicating a possible effect of lee waves on chlorine activation. The dataset is also compared with CALIPSO Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) polar stratospheric cloud (PSC) observations. In general, OClO SCDs coincide well with CALIOP measurements for which PSCs are detected. Very high OClO levels are observed for the northern hemispheric winter 2019/20, with an extraordinarily long period with a stable polar vortex being even close to the values found for southern hemispheric winters. An extraordinary winter in the Southern Hemisphere was also observed in 2019, with a minor sudden stratospheric warming at the beginning of September. In this winter, similar OClO values were measured in comparison to the previous (usual) winter till that event but with a OClO deactivation that was 1–2 weeks earlier.

Retrieval algorithm for OClO from TROPOMI (TROPOspheric Monitoring Instrument) by differential optical absorption spectroscopy

Abstract. Here we present a new retrieval algorithm of the slant column densities (SCDs) of chlorine dioxide (OClO) by differential optical absorption spectroscopy (DOAS) from measurements performed by TROPOspheric Monitoring Instrument (TROPOMI) on board of Sentinel-5P satellite. To achieve a substantially improved accuracy, which is especially important for OClO observations, accounting for absorber and pseudo absorber structures in optical depth even of the order of 10−4 is important. Therefore, in comparison to existing retrievals, we include several additional fit parameters by accounting for spectral effects like the temperature dependency of the Ring effect and Ring absorption effects, a higher-order term for the OClO SCD dependency on wavelength and accounting for the BrO absorption. We investigate the performance of different retrieval settings by an error analysis with respect to random variations, large-scale systematic variations as a function of solar zenith angle and also more localized systematic variations by a novel application of an autocorrelation analysis. The retrieved TROPOMI OClO SCDs show a very good agreement with ground-based zenith sky measurements and are correlated well with preliminary data of the operational TROPOMI OClO retrieval algorithm currently being developed as part of ESA's Sentinel-5P+ Innovation project.

OClO and BrO observations in the volcanic plume of Mt. Etna – implications on the chemistry of chlorine and bromine species in volcanic plumes

Abstract. Spatial and temporal profiles of chlorine dioxide (OClO), bromine monoxide (BrO) and sulfur dioxide (SO2) of the volcanic plume at Mt. Etna, Italy, were investigated in September 2012 using Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS). OClO was detected in 119 individual measurements covering plume ages up to 6 min. BrO could be detected in 452 spectra up to 23 min downwind. The retrieved slant column densities (SCDs) reached maximum values of 2.0 × 1014 molecules cm-2 (OClO) and 1.1 × 1015 molecules cm-2 (BrO). Mean mixing ratios of BrO and OClO were estimated assuming a circular plume cross section. Furthermore, ClO mixing ratios were derived directly from the BrO and OClO-SCDs. Average abundances of BrO = 1.35 ppb, OClO = 300 ppt and ClO = 139 ppt were found in the young plume (plume age τ < 4 min) with peak values of 2.7 ppb (BrO), 600 ppt (OClO) and 235 ppt (ClO) respectively. The chemical evolution of BrO and OClO in the plume was investigated in great detail by analysing the OClO/SO2 and BrO/SO2 ratios as a function of plume age τ. A marked increase of both ratios was observed in the young plume (τ < 142 s) and a levelling off at larger plume ages showing mean SO2 ratios of 3.17 × 10-5 (OClO/SO2) and 1.65 × 10-4 (BrO/SO2). OClO was less abundant in the plume compared to BrO with a mean OClO/BrO ratio of 0.16 at plume ages exceeding 3 min. A measurement performed in the early morning at low solar radiances revealed BrO/SO2 and OClO/SO2 ratios increasing with time. This observation substantiates the importance of photochemistry regarding the formation of BrO and OClO in volcanic plumes. These findings support the current understanding of the underlying chemistry, namely, that BrO is formed in an autocatalytic, heterogeneous reaction mechanism (in literature often referred to as "bromine explosion") and that OClO is formed in the reaction of OClO with BrO. These new findings, especially the very detailed observation of the BrO and OClO formation in the young plume, were used to infer the prevailing Cl-atom concentrations in the plume. Relatively small values ranging from [Cl] = 2.5 × 106 cm-3 (assuming 80 ppb background O3) to [Cl] = 2.0 × 108 cm-3 (at 1 ppb O3) were calculated at plume ages of about 2 min. Based on these Cl abundances, a potential – chlorine-induced – depletion of tropospheric methane (CH4) in the plume was investigated. CH4 lifetimes between 14 h (at 1 ppb O3) and 47 days (at 80 ppb O3) were derived. While these lifetimes are considerably shorter than the atmospheric lifetime of CH4, the impact of gaseous chlorine on the CH4 budget in the plume environment should nevertheless be relatively small due to plume dispersion (decreasing Cl concentrations) and ongoing mixing of the plume with the surrounding atmosphere (replenishing O3 and CH4). In addition, all spectra were analysed for signatures of IO, OIO and BrO. None of these species could be detected. Upper limits for IO/SO2, OIO/SO2 and OBrO/SO2 are 1.8 × 10-6, 2.0 × 10-5 and 1.1 × 10-5 respectively.

Core-level excitation and fragmentation of chlorine dioxide

Inner-shell excitation and fragmentation of chlorine dioxide (OClO) in the Cl 2p- and O 1s-excitation regime is reported. The electronic structure of the element-selectively excited radical is studied by X-ray absorption and total cation yields. A comparison of both approaches allows us to estimate the absolute photoionization cross-section and the ionization yield near the Cl 2p- and O 1s-absorption edges. The latter quantity is characteristically enhanced in core-ionization continua. We observe below both core-absorption edges intense core-to-valence-transitions. These are assigned in comparison with related work on core-excited sulfur dioxide. These results give clear evidence that the highest molecular orbital of OClO is half-filled. High-resolution spectra recorded in the Cl 2p-regime show evidence for Rydberg transitions. The extrapolation of the term values of the low-lying Rydberg states allows us to derive the Cl 2p-ionization energy of OClO. Fragmentation of core-excited OClO is reported. Photoelectron–photoion-coincidence (PEPICO) spectra are recorded, indicating that singly and doubly charged fragments are formed. Fission of the doubly and multiply charged OClO leads to singly charged fragments. These are measured by photoion–photoion-coincidence (PIPICO) spectra, where characteristic changes in intensity of the fission channels in the Cl 2p- and O 1s-continuum are observed.

Observational evidence of rapid chlorine activation by mountain waves above northern Scandinavia

The Global Ozone Monitoring Experiment (GOME) routinely maps atmospheric column densities of chlorine dioxide (OClO), an important indicator of chlorine activation. In previous studies it was shown that the level of OClO during polar winters is anticorrelated to synoptic‐scale temperatures inside the polar vortex. Here we present observations of exceptionally fast chlorine activation by mesoscale polar stratospheric clouds during the mountain wave event above northern Scandinavia between 20 and 22 January 1997. The observed slant column densities of OClO in the area of the mountain wave activity increase from 0.6 * 1014 molec/cm2 to 1.3 * 1014 molec/cm2. Combining the GOME observations with radiative transfer calculations, the corresponding vertical column densities are derived, and by applying pressure and temperature profiles from mesoscale simulations, the mixing ratios for OClO and ClO at the different altitude levels are estimated. The vertical column densities of OClO indicate a level of chlorine activation that is about two times higher compared to the days before the mountain wave event, and the deduced mixing ratios for ClO imply that chlorine was almost completely activated over a wide altitude range. The area showing enhanced OClO values increased by approximately 750.000 km2 which is more than 3% of the polar vortex area (≈23.0 * 106 km2). These observations are in good agreement with previous model studies, where comparable increases and areas of chlorine activation were calculated. The case of 21 January 1997 and similar GOME OClO observations in other Arctic winters confirm the importance of stratospheric mountain waves for the activation of chlorine species in the northern hemisphere.

The Phase‐dependent Photochemical Reaction Dynamics of Halooxides and Nitrosyl Halides¶

AbstractRecent progress in understanding the phase‐dependent reactivity of halooxides and nitrosyl halides is outlined. Halooxide reactivity is represented by the photochemistry of chlorine dioxide (OCIO) and dichlorine monoxide (CIOCI). The gas phase photochemical dynamics of OCIO are contrasted with the dynamics in condensed environments. The role of excited‐state symmetry in defining the reaction dynamics and the observation of photoisomerization resulting in the production of CIOO are discussed. The current understanding of the excited‐state reaction dynamics of CIOCI and evidence for photoisomerization of this species resulting in the production of CICIO are outlined. Finally, the photochemical reaction dynamics of the nitrosyl halide CINO are presented. The main difference between the gas and condensed phase reaction dynamics of this species is that whereas photodissociation to form CI and NO dominates the gas phase reaction dynamics, photoisomerization resulting in CION production occurs to an appreciable extent in condensed environments. The observation of photoisomerization for OCIO, CIOCI and CINO suggests that this process is a general feature of the condensed phase reaction dynamics for smaller halooxides and nitrosyl halides. Finally, future areas for study in both halooxide and nitrosyl halide photoreactivity are outlined.

Reduced mechanism for the 366 nm chlorine dioxide photodecomposition in N 2-saturated aqueous solutions

A basic reaction scheme for the integral process of chlorine dioxide (OClO) photolysis at 366 nm in N 2-saturated aqueous solution is proposed. The mechanism is supported by the initial quantum yield Φ° 366 nm determined experimentally from OClO decay rates measured by electron spin resonance (ESR), chemical analysis of the stable products in solution and numerical simulation of the OClO profiles. In the concentration range 0.5 mM ≤ [OClO] ≤ 20 mM, Φ° 366 nm = 0.52 with a 95% confidence interval of 0.50–0.55. Product concentrations determined after complete photobleaching were proportional to OClO initial concentration: [H +] = 0.856 [OClO] 0; [HClO] = (0.141 ± 0.010) [OClO] 0; [ Cl O 3 − ] = ( 0.551 ± 0.014 ) [OClO] 0 and [Cl −] = (0.251 ± 0.015) [OClO] 0. In the range of homogeneous light absorption, i.e. [OClO] 0 ≤ 1.5 mM, OClO profiles and concentrations of the stable products in solution are well reproduced from numerical integration of the proposed mechanism. The reaction scheme requires the participation of dichlorine pentoxide (Cl 2O 5), which till now remains no isolated. However, reactions involving formation and hydrolysis of Cl 2O 5 are critical to reproduce the experimental profiles and explain the identity of the final products.

The phase-dependent photochemical reaction dynamics of halooxides and nitrosyl halides.

Recent progress in understanding the phase-dependent reactivity of halooxides and nitrosyl halides is outlined. Halooxide reactivity is represented by the photochemistry of chlorine dioxide (OClO) and dichlorine monoxide (ClOCl). The gas phase photochemical dynamics of OClO are contrasted with the dynamics in condensed environments. The role of excited-state symmetry in defining the reaction dynamics and the observation of photoisomerization resulting in the production of ClOO are discussed. The current understanding of the excited-state reaction dynamics of ClOCl and evidence for photoisomerization of this species resulting in the production of ClClO are outlined. Finally, the photochemical reaction dynamics of the nitrosyl halide ClNO are presented. The main difference between the gas and condensed phase reaction dynamics of this species is that whereas photodissociation to form Cl and NO dominates the gas phase reaction dynamics, photoisomerization resulting in ClON production occurs to an appreciable extent in condensed environments. The observation of photoisomerization for OClO, ClOCl and ClNO suggests that this process is a general feature of the condensed phase reaction dynamics for smaller halooxides and nitrosyl halides. Finally, future areas for study in both halooxide and nitrosyl halide photoreactivity are outlined.

The Phase-dependent Photochemical Reaction Dynamics of Halooxides and Nitrosyl Halides¶

Recent progress in understanding the phase-dependent reactivity of halooxides and nitrosyl halides is outlined. Halooxide reactivity is represented by the photochemistry of chlorine dioxide (OClO) and dichlorine monoxide (ClOCl). The gas phase photochemical dynamics of OClO are contrasted with the dynamics in condensed environments. The role of excited-state symmetry in defining the reaction dynamics and the observation of photoisomerization resulting in the production of ClOO are discussed. The current understanding of the excited-state reaction dynamics of ClOCl and evidence for photoisomerization of this species resulting in the production of ClClO are outlined. Finally, the photochemical reaction dynamics of the nitrosyl halide ClNO are presented. The main difference between the gas and condensed phase reaction dynamics of this species is that whereas photodissociation to form Cl and NO dominates the gas phase reaction dynamics, photoisomerization resulting in ClON production occurs to an appreciable extent in condensed environments. The observation of photoisomerization for OClO, ClOCl and ClNO suggests that this process is a general feature of the condensed phase reaction dynamics for smaller halooxides and nitrosyl halides. Finally, future areas for study in both halooxide and nitrosyl halide photoreactivity are outlined.

The Phase-dependent Photochemical Reaction Dynamics of Halooxides and Nitrosyl Halides¶

Recent progress in understanding the phase-dependent reactivity of halooxides and nitrosyl halides is outlined. Halooxide reactivity is represented by the photochemistry of chlorine dioxide (OClO) and dichlorine monoxide (ClOCl). The gas phase photochemical dynamics of OClO are contrasted with the dynamics in condensed environments. The role of excited-state symmetry in defining the reaction dynamics and the observation of photoisomerization resulting in the production of ClOO are discussed. The current understanding of the excited-state reaction dynamics of ClOCl and evidence for photoisomerization of this species resulting in the production of ClClO are outlined. Finally, the photochemical reaction dynamics of the nitrosyl halide ClNO are presented. The main difference between the gas and condensed phase reaction dynamics of this species is that whereas photodissociation to form Cl and NO dominates the gas phase reaction dynamics, photoisomerization resulting in ClON production occurs to an appreciable extent in condensed environments. The observation of photoisomerization for OClO, ClOCl and ClNO suggests that this process is a general feature of the condensed phase reaction dynamics for smaller halooxides and nitrosyl halides. Finally, future areas for study in both halooxide and nitrosyl halide photoreactivity are outlined.

Molecular dynamics study of the weakly solvent dependent relaxation dynamics following chlorine dioxide photoexcitation

The solvation dynamics following photoexcitation of chlorine dioxide (OClO) in different solvents are investigated by classical molecular dynamics. Following previous work on the aqueous response to OClO photoexcitation [J. Chem. Phys. 118, 4563 (2003)], the present study considers the response of chloroform and cyclohexane; these three liquids present unique solvent environments that differ significantly in both polarity and structure. The study is designed to ascertain the origin of the solvent-invariant homogeneous linewidth associated with OClO photoexcitation and to confirm, at the molecular level, whether the relaxation dynamics are similar across dissimilar solvents due to chance or a common relaxation origin. The results obtained here are used to predict the time scale of solvent-induced optical dephasing, and excellent agreement with experiment is observed for all solvents. Analysis demonstrates that the solvation dynamics of OClO are dominated by short-ranged mechanical solute–solvent interactions regardless of the identity and electrostatic properties of the solvent. Low-frequency translational motions dominate the coupling spectrum, and virtually no contribution to energy gap relaxation is achieved through intramolecular solvent motions. The invariant homogeneous linewidth is attributed to the similarity in the primary response of all solvents to OClO photoexcitation.

On the actinic wavelength dependence of OClO photochemistry in solution

The actinic (i.e., photolysis) wavelength dependence of chlorine dioxide (OClO) photochemistry in solution is investigated. Femtosecond pump–probe studies performed with 355-nm photoexcitation are presented for OClO dissolved in water and acetonitrile. Relative to earlier studies employing 400-nm photoexcitation, a decrease in primary-photoproduct geminate-recombination efficiency is observed in both solvents. This decrease in recombination efficiency is especially dramatic in acetonitrile where the recombination quantum yield is found to be only 0.08±0.04. The solvent and actinic-wavelength dependence of geminate recombination is discussed with respect to the recent theoretical work of Benjamin and co-workers [J. Chem. Phys. 116, 8930 (2002)]. Substantial increases in the optical density at 267 nm are also observed that cannot be explained by considering the increase in ClO concentration accompanying a reduction in geminate-recombination efficiency exclusively. Instead, these increases are consistent with an increase in the quantum yield for ClOO and/or Cl production suggesting that the partitioning between OClO photoproduct channels is actinic-wavelength dependent.

Investigating the phase-dependent photochemical reaction dynamics of chlorine dioxide using resonance Raman spectroscopy

Recent progress in understanding the phase-dependent reactivity demonstrated by halooxides is outlined. Specifically, resonance Raman intensity analysis (RRIA) and time-resolved resonance Raman (TRRR) studies of chlorine dioxide (OClO) photochemistry in solution are presented. Using RRIA, it has been determined that the excited-state structural evolution that occurs along the asymmetric-stretch coordinate in the gas phase is restricted in solution. The absence of evolution along this coordinate results in the preservation of groundstate symmetry in the excited state. The role of symmetry in defining the reaction coordinate and the solvent-solute interactions responsible for modification of the excited-state potential energy surface are discussed. TRRR studies are presented which demonstrate that geminate recombination of the primary photoproducts resulting in the reformation of ground-state OClO is a central feature of OClO photochemistry in solution. These studies also demonstrate that a fraction of photoexcited OClO undergoes photoisomerization to form ClOO, with the ground-state thermal decomposition of this species resulting in Cl production on the subnanosecond timescale. Finally, time-resolved anti-Stokes experiments are presented which demonstrate that the OClO vibrational-relaxation dynamics are solvent dependent. The current picture of OClO photochemistry derived from these studies is discussed, and future directions for study are outlined.

Investigating the Phase-Dependent Reactivity of Chlorine Dioxide Using Resonance Raman Spectroscopy

Recent progress in understanding the phase-dependent reactivity of chlorine dioxide (OClO) is outlined. Specifically, resonance Raman intensity analysis (RRIA) and time-resolved resonance Raman (TRRR) studies of OClO photochemistry in solution are presented. The RRIA studies demonstrate that the optically prepared excited-state undergoes significant modification in solution relative to the gas phase. Specifically, the substantial evolution that occurs along the asymmetric-stretch coordinate in the gas phase is restricted in solution. The absence of substantial evolution along the asymmetric-stretch coordinate results in the preservation of ground-state symmetry in the excited state. The role of symmetry in defining the reaction coordinate and the solvent−solute interactions responsible for modification of the excited-state potential energy surface are discussed. TRRR studies performed with pump and probe wavelengths at 390 nm are presented which demonstrate that geminate recombination of the primary photo...

Femtosecond UV Pump/Near-IR Probe Studies of the Solvent-Dependent Excited-State Decay Dynamics of Chlorine Dioxide

The excited-state decay and geminate-recombination dynamics of chlorine dioxide (OClO) are investigated using UV pump/near-IR probe spectroscopy. Experiments are performed with 200-fs time resolution on OClO dissolved in water, cyclohexane, acetonitrile, and chloroform. In all solvents, a reduction in optical density is observed at early times and is attributed to stimulated emission from the optically prepared 2A2 surface. The emission decays on the subpicosecond time scale to reveal an increase in optical density corresponding to the production of vibrationally hot ground-state OClO formed by geminate recombination of the primary photoproducts. Kinetic analysis of these data reveals that the excited-state decay time constant increases from ∼200 fs in water and cyclohexane to ∼400 fs in acetonitrile and chloroform. The vibrational-relaxation dynamics of OClO are also found to be solvent dependent in agreement with earlier work. Pump−probe anisotropy experiments on aqueous OClO are presented. In these stu...

Understanding the phase-dependent reactivity of chlorine dioxide using resonance Raman spectroscopy.

Progress in understanding the phase-dependent reactivity of chlorine dioxide (OClO) is outlined. Resonance Raman intensity analysis studies of gaseous and solution-phase OClO are presented which demonstrate that the optically prepared excited state undergoes significant modification in solution. In addition, time-resolved resonance Raman studies are presented which demonstrate that geminate recombination of the primary photoproducts, resulting in the re-formation of ground-state OClO, dominates the photochemical reaction dynamics in solution. The current picture of aqueous OClO photochemistry derived from these studies is discussed, and future directions of investigation are outlined.

Intermolecular hydrogen bonding in chlorine dioxide photochemistry: A time-resolved resonance Raman study

The geminate-recombination and vibrational-relaxation dynamics of chlorine dioxide (OClO) dissolved in ethanol and 2,2,2-trifluoroethanol (TFE) are investigated using time-resolved resonance Raman spectroscopy. Stokes spectra are measured as a function of time following photoexcitation using degenerate pump and probe wavelengths of 398 nm. For OClO dissolved in ethanol, subpicosecond geminate recombination occurs resulting in the reformation of ground-state OClO with a quantum yield of 0.5±0.1. Following recombination, intermolecular-vibrational relaxation of OClO occurs with a time constant of 31±10 ps. For OClO dissolved in TFE, recombination occurs with a time constant of 1.8±0.8 ps and a quantum yield of only 0.3±0.1. The intermolecular-vibrational-relaxation time constant of OClO in TFE is 79±27 ps. The reduced geminate-recombination quantum yield, delayed recombination, and slower vibrational relaxation for OClO in TFE is interpreted in terms of greater self-association of the solvent. Degenerate pump–probe experiments are also presented that demonstrate decay of the Cl-solvent charge-transfer complex on the ∼1-ns time scale in ethanol and TFE. This time is significantly longer than the abstraction times observed for other systems demonstrating that Cl hydrogen abstraction from alcohols occurs in the presence of a significant energy barrier.

Quantum Yield for ClOO Formation following Photolysis of Aqueous OClO

The photochemistry of chlorine dioxide (OClO) in aqueous solution was investigated by femtosecond transient absorption spectroscopy. Following the photoexcitation of OClO at 400 nm, the transient absorption dynamics were probed in the spectral range from 400 to 220 nm. As expected from earlier studies, the main photolytic products ClO + O, formed with a quantum yield of ∼90%, disappear through fast geminate recombination producing OClO in the electronic ground state. The total quantum yield for chlorine atom production is (ΦCl) ∼10%, with the chlorine atom production occurring through two competing processes. The dominant channel for chlorine atom production involves the formation of a short-lived intermediate on a ∼6 ps time scale with a quantum yield of 8 ± 2%. The remaining 2 ± 1% is formed through the formation and decomposition of ClOO. The lifetime of ClOO was found to be ∼0.32 ns, in very good agreement with the result of a recent time-resolved resonance Raman study. Finally, the UV absorption spectrum for aqueous ClOO is reported and compared with previously reported spectra obtained in condensed media.

The formation of ClOO following the photoexcitation of aqueous OClO studied by two-color, time-resolved resonance Raman spectroscopy

The photochemistry of chlorine dioxide (OClO) is investigated by two-color time-resolved resonance Raman spectroscopy. Pump and probe wavelengths of 390 and 260 nm, respectively, are used to monitor photoproduct formation following aqueous OClO photoexcitation. Depletion and subsequent recovery of the OClO scattering intensities is observed consistent with subpicosecond reformation of OClO via geminate recombination of the primary photoproducts. Intensity is observed at 1442 cm−1 consistent with ClOO formation that appears and decays with time constants of 27.9±4.5 ps and 398±50 ps, respectively. The results presented here represent the first direct evidence for ClOO formation following the photoexcitation of aqueous OClO.

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

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.