Photodissociation of ClO2 in aqueous solution at 400 nm results in the formation of ClO+O and Cl+O2 ClO and O geminately recombine to ClO2 in the electronic ground state (2B1), formed with an initial vibrational energy of ≈25 eV In this paper the vibrational relaxation of ClO2(2B1) in aqueous solution is studied by femtosecond transient absorption spectroscopy in the spectral range 234 to 1024 nm The measured transient absorption of the vibrationally relaxing ClO2 molecules is compared with the transient absorption calculated for relaxation in the asymmetric stretch as well as the symmetric stretch and bending modes The calculations of the absorption spectra pertaining to the asymmetric stretch are based on a harmonic potential derived from the experimentally determined fundamental vibrational energy, whereas that of the symmetrical vibrations are based on ab initio potentials An excellent agreement is obtained by assuming that the vibrational relaxation predominantly occurs in the asymmetric stretch with a 95 ps relaxation time A weak spectral feature in the ultraviolet part of the spectrum is assigned to vibrational relaxation in the symmetric stretch and bending modes, indicating a coupling between the asymmetric and symmetric modes

Vibrational relaxation of ClO2 in water

The photolysis of aqueous ClO2 has been studied with a new femtosecond transient absorption spectrometer, allowing absorbance changes as small as ΔA ≈ 1 × 10-4 to be recorded with a time resolution of 150 fs. ClO2 was photolyzed at 390 nm and the ultrafast formation and decay of photoproducts were monitored at 260, 390, and 780 nm. As expected from earlier studies, Cl atoms are formed with a quantum yield of Φ(Cl) = 0.1. However, the rate of formation is nearly 2 orders of magnitude higher than that reported. Moreover, Cl is the only photoproduct that survives 25 ps after the photolysis pulse. The main photolytic products, ClO + O, formed with a quantum yield of 0.9, disappear through fast geminate recombination, producing vibrational excited ClO2 in the electronic ground state. The vibrational relaxation of this species occurs with a time constant of 10 ps. The vanishing yield of cage escape for ClO + O, which contrasts with the reported result of the photolysis at 355 nm, indicates that the amount of ex...

Femtosecond Photolysis of ClO2 in Aqueous Solution

The condensed phase relaxation dynamics of electronically excited 1,1‘,3,3,3‘,3‘-hexamethylindotricarbocyanine iodide HITC (a cyanine dye) has been examined by transient bleach/stimulated emission experiments. These measurements were performed using tunable pump and probe laser pulses with ∼200 fs time resolution. The dynamics observed was assigned to vibrational relaxation in the S1 state of HITC. Solvation effects make a negligible contribution to these experiments because the dipole moment of HITC only changes by a small amount when the S1 ← S0 transition is excited. Experiments performed with variable wavelength pump and probe pulses show that vibrational relaxation is faster at high energies in the S1 state. At low energies in the S1 state the vibrational relaxation times depend on the solvent. The measured relaxation times at low energies are1.7 ps in acetonitrile, 3.2 ps in dimethyl sulfoxide, 2.4 ps in methanol, 3.5 ps in ethanol, 7.1 ps in 1-butanol, and 6.4 ps in ethylene glycol. These results s...

Ultrafast investigation of vibrational relaxation in the s1 electronic state of hitc

The vibrational relaxation of OClO in bulk water, acetonitrile, and ethanol is studied using classical and semiclassical molecular dynamics computer simulations. Nonequilibrium classical trajectory calculations provide insight into the early stages of vibrational energy relaxation of highly excited states. Equilibrium force autocorrelation functions are used to determine the relaxation rate for the v=1→v=0 transition. Good agreement with experiments is found. The calculations suggest that the hydrogen bonding in water, as reflected by the high density of librational modes, is the reason for the fast relaxation in this liquid compared with that in ethanol and acetonitrile.

Molecular dynamics study of the vibrational relaxation of OClO in bulk liquids

Low pressure flash thermolysis of different precursor molecules containing-ClO, -ClO3 or -OClO3 yield, when highly diluted in Ne or O2 and subsequent quenching of the products in a matrix at 5 or 15 K, ClOx (x = 1, 3, 4) radicals, respectively. If Ne or O2 gas is directed over solid ClO2 at -120 degrees C and the resulting gas mixtures are immediately deposited as a matrix, a high fraction of (OClO)2 is trapped. This enables recording of IR and UV spectra of weakly bonded (OClO)2 dimers and detailed studying of their photochemistry. For Ne or O2 matrix isolated ClO radicals the vibrational wavenumbers and electronic transitions are only slightly affected compared with the gas phase. In this study strong evidence is found for long lived ClO in the electronically excited 2 [symbol: see text] 1/2 state. A comprehensive IR study of Ne matrix isolated ClO3 (fundamentals at 1081, 905, 567, 476 cm-1) yield i) a reliable force field; ii) a OClO bond angle of alpha e = 113.8 +/- 1 degrees and iii) a ClO bond length of 148.5 +/- 2 pm in agreement with predicted data from quantum chemical calculations. The UV/Vis spectrum of ClO3 isolated in a Ne matrix (lambda max at 32,100 and 23,150 cm-1) agrees well with the photoelectron spectrum of ClO3- and theoretical predictions. The origin of the structured high energy absorption is at 22,696 cm-1 and three fundamentals (794, 498, 280 cm-1) are detected in the C2E state. By photolysis of ClO3 with visible light the complex ClO.O2 with ClO in the 2 [symbol: see text] 1/2 state is formed. In an extended spectroscopic study of the elusive ClO4 radical, isolated in a Ne or O2 matrix, three additional IR bands, a complete UV spectrum and a strong interaction with O2 are found. This leads to the conclusion that ClO4 exhibits C2v or Cs symmetry with a shallow potential minimum and forms with O2 the previously unknown peroxy radical O3ClO-O2. All these results are discussed in the context of recent developments in the chemistry and spectroscopy of the important and interesting ClOx (x = 1-4) family of radicals.

Chlorine oxide radicals ClOx (x = 1-4) studied by matrix isolation spectroscopy.

The ultrafast dynamics of chlorine dioxide (OClO) photochemistry in aqueous solution was studied by femtosecond transient absorption spectroscopy. Following the photoexcitation of OClO at 395 nm, the transient absorption dynamics were probed at 12 different wavelengths ranging from 350 to 700 nm. The transient absorption features observed in the visible wavelengths are assigned to correspond to the vibrationally hot photoisomer ClOO*. The spectral dynamics reveal the vibrational relaxation of this molecule in its ground electronic state. The total vibrational energy relaxation occurs within ∼9 ps. The dynamics of the formation of chlorine atom was examined by measuring the absorption dynamics in the 350−390 nm range. The time constant for Cl formation is calculated to be ∼200 ps. The data show that the dominant pathway for Cl formation is via the isomerized ClOO molecule. No fast component for Cl is detected, indicating that the ClOO molecule reaches thermal equilibrium before dissociating into Cl and O2.

Yong Joon Chang and

Papers

Ultrafast Dynamics of Chlorine Dioxide Photochemistry in Water Studied by Femtosecond Transient Absorption Spectroscopy