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The actinic or photolysis-wavelength dependence of aqueous chlorine dioxide (OClO) photochemistry is investigated using femtosecond pump-probe spectroscopy. Following photoexcitation at 310, 335, and 410 nm the photoinduced evolution in optical density is measured from the UV to the near IR. Analysis of the optical-density evolution illustrates that the quantum yield for atomic chlorine production ([uppercase_phi_synonym]Cl) increases with actinic energy, with [uppercase_phi_synonym]Cl=0.16±0.02 for 410 nm excitation and increasing to 0.25±0.01 and 0.54±0.10 for 335 and 310 nm excitations, respectively. Consistent with previous studies, the production of Cl occurs through two channels, with one channel corresponding to prompt (<5 ps) Cl formation and the other corresponding to the thermal decomposition of ClOO formed by OClO photoisomerization. The partitioning between Cl production channels is dependent on actinic energy, with prompt Cl production enhanced with an increase in actinic energy. Limited evidence is found for enhanced ClO production with an increase in actinic energy. Stimulated emission and excited-state absorption features associated with OClO populating the optically prepared 2A2 surface decrease with an increase in actinic energy suggesting that the excited-state decay dynamics are also actinic energy dependent. The studies presented here provide detailed information on the actinic-wavelength dependence of OClO photochemistry in aqueous solution. [ABSTRACT FROM AUTHOR]

The results of our earlier work [C. Brooksby, O. V. Prezhdo, and P. J. Reid, J. Chem. Phys. 119, 9111 (2003)] rationalizing the surprisingly weak solvent dependence of the dynamics following photoexcitation of chlorine dioxide in water, chloroform, and cyclohexane are thoroughly tested. Comparisons are made between equilibrium and nonequilibrium solvent response, equilibrium response in the ground and excited electronic states, as well as the cumulant and direct evaluation of the optical response function. In general, the linear response and cumulant approximations are found to hold, although minor deviations are found with all solvents. The ground state, linear response, and cumulant data show best agreement with experiment, most likely due to the better tested ground-state force field and the robust behavior of the linear response and cumulant approximations. The main conclusion of our earlier work explaining the weak solvent dependence by the domination of the van der Waals interaction component remains intact within the more advanced treatments. However, the molecular origin of this surprising experimental observation is different in water and chloroform compared to cyclohexane. [ABSTRACT FROM AUTHOR]

We report a series of time-resolved infrared absorption studies on chlorine dioxide (OClO) dissolved in H2O, D2O, and acetonitrile. Following the photoexcitation at 401 nm, the evolution in optical density for frequencies corresponding to asymmetric stretch of OClO is measured with a time resolution of 120±50 fs. The experimentally determined optical-density evolution is compared with theoretical models of OClO vibrational relaxation derived from collisional models as well as classical molecular-dynamics (MD) studies. The vibrational relaxation rates in D2O are reduced by a factor of 3 relative to H2O consistent with the predictions of MD. This difference reflects modification of the frequency-dependent solvent-solute coupling accompanying isotopic substitution of the solvent. Also, the geminate-recombination quantum yield for the primary photofragments resulting in the reformation of ground-state OClO is reduced in D2O relative to H2O. It is proposed that this reduction reflects enhancement of the dissociation rate accompanying vibrational excitation along the asymmetric-stretch coordinate. In contrast to H2O and D2O, the vibrational-relaxation dynamics in acetonitrile are not well described by the theoretical models. Reproduction of the optical-density evolution in acetonitrile requires significant modification of the frequency-dependent solvent-solute coupling derived from MD. It is proposed that this modification reflects vibrational-energy transfer from the asymmetric stretch of OClO to the methyl rock of acetonitrile. In total, the results presented here provide a detailed description of the solvent-dependent geminate-recombination and vibrational-relaxation dynamics of OClO in solution. [ABSTRACT FROM AUTHOR]

Ultrafast time-resolved infrared absorption studies of aqueous chlorine dioxide (OClO) photochemistry are reported. Following photoexcitation at 401 nm, the evolution in optical density at frequencies between 1000 to 1100 cm-1 is monitored to investigate vibrational energy deposition and relaxation along the asymmetric-stretch coordinate following the reformation of ground-state OClO via geminate recombination of the primary photofragments. The measured kinetics are compared to two proposed models for the vibrational-relaxation dynamics along the asymmetric-stretch coordinate. This comparison demonstrates that the perturbation model derived from molecular dynamics studies is capable of qualitatively reproducing the observed kinetics, where the collisional model employed in previous UV-pump, visible probe experiments demonstrates poor agreement with experiment. The ability of the perturbation model to reproduce the optical-density evolution observed in these studies demonstrates that for aqueous OClO, frequency dependence of the solvent–solute coupling is important in defining the level-dependent vibrational relaxation rates along the asymmetric-stretch coordinate. The absence of optical-density evolution corresponding to the population of higher vibrational levels (n>8) along the asymmetric-stretch coordinate suggests that following geminate recombination, energy is initially deposited into a local Cl–O stretch, with the relaxation of vibrational energy from this coordinate providing for delayed vibrational excitation of the asymmetric- and symmetric-stretch coordinates relative to geminate recombination, as previously observed. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

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. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The solvation dynamics following photoexcitation of aqueous chlorine dioxide (OClO) are investigated by classical molecular dynamics. A total of four OClO/water models are employed, including standard and customized OClO models together with SPC/F and TIP3P water. The simulation results are used to predict the time scale of solvent-induced optical dephasing, and excellent agreement with experiment is observed. The simulations demonstrate that the solvation dynamics of aqueous OClO are dominated by the short-ranged mechanical solute-solvent interactions. When compared to the aqueous solvation of spherical solutes, the solvation dynamics of OClO exhibit a novel effect. The role of the faster translational and librational water motions is suppressed in the dielectric component of the response, but is emphasized in the mechanical response, in stark contrast to the results for other spherical systems. The faster translational motions appear in the mechanical response due to the noncentrosymmetric nature of the OclO-water interaction, enhanced by OclO-water hydrogen bonding. [ABSTRACT FROM AUTHOR]

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 C1OO and/or Cl production suggesting that the partitioning between OClO photoproduct channels is actinic-wavelength dependent. [ABSTRACT FROM AUTHOR]

The production of CIlO following OClO photolysis in water and fluorotrichloromethane (freon-11) is investigated using time-resolved resonance Raman (TRRR) spectroscopy. Stokes spectra are obtained as a function of time following OClO photoexcitation using pump and probe wavelengths of 390 and 260 nm, respectively. Scattering assignable to ClOO is observed, and appears with a time constant of 27.9 ± 4.5 ps in water and 172 ± 30 ps in freon-11. The ClOO intensity decays with a time constant of ∼398 ± 50 ps in water and 864 ± 200 ps in freon-11. Although the production and decay kinetics are solvent dependent, the quantum yield for ClOO production is similar between water and freon-11. Femtosecond pump-probe studies designed to monitor the evolution in optical density at 390 and 260 nm following OClO photoexcitation are also presented. These studies demonstrate that geminate recombination of the primary photoproducts is less efficient in freon-11 relative to water. This result taken in combination with the solvent invariance of the ClOO-production quantum yield indicates that ClOO is not formed via geminate recombination. Instead, the results presented here suggest that OClO photoisomerization results in the production of ClOO. Finally, the vibrational energy content of ClOO upon internal conversion to the ground state is studied through comparison of the ClOO Raman and absorption cross sections to those predicted using computational methods. These studies suggest that ground-state ClOO is produced with minimal excess vibrational energy. The results presented here provide new insight into the mechanism of ClOO formation following OClO photoexcitation. © 2001 American Institute of Physics. [DOI: 10.1063/1.1418733]. [ABSTRACT FROM AUTHOR]

Absolute resonance Raman cross sections for chlorine dioxide (OClO) dissolved in chloroform are obtained at several excitation wavelengths spanning the photochemically relevant [sup 2]B[sub 1]-[sup 2]A[sub 2] optical transition. The absolute scattering cross sections of OClO are determined by reference to the 666 cm[sup -1] transition of chloroform whose absolute scattering cross sections are reported here. The time-dependent theory for Raman and absorption are used to develop a mode-specific description of the [sup 2]A[sub 2] excited state surface. This description demonstrates that photoexcitation of OClO leads to significant structural evolution along the symmetric stretch and bend coordinates, with only limited evolution occurring along the asymmetric stretch. This description is similar to that determined for OClO dissolved in cyclohexane and water demonstrating that the excited-state structural evolution of OClO is similar in these solvents. Analysis of the OClO absolute scattering cross sections establishes that the homogeneous linewidth is 95±15 cm[sup -1] in chloroform, essentially identical to the linewidths in cyclohexane and water. To establish the origin of this linewidth, the fluorescence cross section for OClO dissolved in cyclohexane is measured and found to be consistent with an excited-state lifetime of ∼200 fs. Comparison of this lifetime to the homogeneous linewidth establishes that the homogeneous broadening is dominated by solvent-induced pure dephasing. It is proposed that the apparent solvent independence of the homogeneous linewidth reflects the mechanical response of the solvent to the photoinitiated change in solute geometry. In support of this hypothesis, the homogeneous linewidth is reproduced using the viscoelastic continuum model of nonpolar solvation. Finally, it is argued that the restricted evolution along the asymmetric-stretch coordinate is due to dielectric solvent-solute interactions consistent with the incr... [ABSTRACT FROM AUTHOR]

Analyzes the resonance Raman depolarization ratios of chlorine dioxide dissolved in cyclohexane. Measurement of the depolarization ratio of the symmetric stretch fundamental transition at several excitation wavelengths spanning the lowest-energy electronic transition; Maximum value of the depolarization ratio.

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. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]