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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]

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 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]

The photochemistry of nitrosyl chloride (ClNO) dissolvedin cyclohexane is investigated using ultrafast time-resolved infrared(TRIR) spectroscopy. Following 266 nm photolysis, the photochemistryis measured by following changes in optical density at frequenciesspanning the NO stretch fundamental transition. A photoinduceddepletion in optical density is observed consistent with the depletionof ground-state ClNO. The depletion in optical density remains constantout to ∼50 ps demonstrating that ClNO photodissociation isnot followed by recombination of the Cl and NO photofragments. Inaddition, no evidence for the formation of the ClON photoisomer isobserved. These results stand in contrast to previous studies in acetonitrilewhere ClNO photolysis is followed by geminate recombination of Cland NO, and by the production of ClON. These differences in ClNO photochemistryare proposed to arise from the population of different excited-statescaused by solvent dependence of the ground-state potential energysurface minimum along the Cl–N stretch coordinate. Solvent-dependentvibrational relaxation and differences in strength of the solventcage are also proposed to contribute to the solvent-dependent photochemistry.Finally, these results are placed in the context of recent modelsof ClNO photochemistry and role of this compound in tropospheric ozoneproduction. [ABSTRACT FROM AUTHOR]

Investigates the photoproduct formation, vibrational and conformational relaxation kinetics in the photochemical ring-openings of 1,3-cyclohexadiene and alpha-phellandrene (alpha-PHE). Picosecond, time-resolved UV Stokes and anti-Stokes resonance Raman spectroscopy; Ground-state surfaces; Determination of the molecular temperature of cis-Hexatriene.