In this work, we present an experimental study of the energy dependence of the photolysis of aqueous HOCl and the subsequent recombination of the OH and Cl photofragments. Using femtosecond transient absorption spectroscopy, we follow the time-dependent concentration of the fragments. The excess energy in the photolysis is given to the fragments as kinetic energy, and tuning the wavelength enables a study of the energy-dependent thermalization and geminate recombination of the fragments in the liquid environment. The recombination yield and rate are governed by the initial fragment separation, and we deduce an approximately linear dependence of the fragment separation versus the kinetic energy with a slope of 0.13 nm/eV. Performing a simple MD simulation of the system gives qualitative agreement with the observations, although the calculated slope is only 0.04 nm/eV.

The production of ClOO 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.

Vibrational relaxation of aqueous CS2 formed by geminate recombination of CS+S after photolysis of CS2(aq) at 200 nm is studied by subpicosecond transient absorption spectroscopy. Comparison of the transient absorption measured from 200 to 313 nm with calculated absorption transients pertaining to vibrationally relaxing CS2 shows that relaxation to the vibrational ground state of CS2 predominantly occurs in the asymmetric stretch on a 5 ps time scale.

This paper reports an experimental study of the photolysis of aqueous HOCl using femtosecond pulses at 266 nm. The formation of photoproducts is monitored by transient absorption spectroscopy from 230 to 400 nm. The HOCl molecules dissociate with unity quantum yield to form OH+Cl faster than 1 ps, and as a result of the potential along the HO–Cl reaction coordinate, all excess energy is given to the fragments as translational energy. After dissociation, and solvent cage escape, the majority of the Cl and OH fragments recombine after diffusion on a time scale of 50 ps. The diffusion dynamics is studied using a simple model for diffusive recombination and a more extensive molecular dynamics simulation. A minor fraction of the Cl atoms (∼10%) reacts with HOCl in a diffusion limited reaction to form Cl2+OH.

Summary form only given. Using THz-time domain spectroscopy, we have studied the dynamics of pure water in the far-infrared spectral range, where the diffusive and inertial rotational motion of the water molecules is observed. With femtosecond transient absorption spectroscopy we have studied the relaxation (vibration, solvation, thermalization, and translation) of different solutes in liquid water. The solutes include, the hydrated electron, ClO/sub 2/, ClOH, Cl, and CS/sub 2/. Performing the femtosecond experiments at different temperatures gives insight into the connection between the dynamics observed in the far-infrared and the relaxation dynamics of the different solutes. In addition we also discuss the validity of employing macroscopic parameters (diffusion coefficients and temperature) in the description of single molecule dynamics on an ultrashort time scale.

The ultrafast solvation and recombination dynamics of the hydrated electron generated by two-photon ionization of water at 4.65 eV is studied by transient absorption spectroscopy as a function of temperature in the range from 277 K to 355 K. The part of the spectral blue shift which is observed in the absorption spectrum of the hydrated electron after 1 ps is purely continuous and is accurately described by the well known analytical expression for the temperature dependent absorption spectrum of the ground state hydrated electron. This indicates that thermal relaxation or more likely solvation of the hydrated electron predominantly causes the blue shift. The survival probability of the hydrated electron shows a strong temperature dependence, which is satisfactory explained by the temperature dependent mobility and reaction rates of the species involved in the recombination. This implies that the average initial separation between the hydrated electron and the ionization site of 〈r0〉=1.0±0.1 nm does not depend significantly on the bulk water temperature.

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.

Summary form only given. Ultrafast spectroscopy on small molecules in solution offers a unique possibility for studying solvent-solute interaction, since the spectroscopic properties of the species involved often are well determined. In this context, photodissociation of tri-atomic molecules stands out as being particularly interesting, since the dissociation of these species may follow several different solvent dependent reaction paths, while still being simple enough to allow for high level calculations of the reaction dynamics. Photodissociation and subsequent recombination have been studied in a number of triatomic molecules in solution including HgI/sub 2/, I/sub 3//sup -/, O/sub 3//sup -/, and ClO/sub 2/. In these systems the photodissociation either leads to a vibrationally excited diatomic fragment which relaxes prior to the recombination, or a fast geminate recombination of the photofragments followed by vibrational relaxation. Apart from the important information about the primary quantum yields of these processes, ultrafast spectroscopy of these molecules have already revealed detailed information about solvent-solute interaction.

The photolysis of aqueous CS2 has been studied using subpicosecond transient absorption spectroscopy. CS2 was photolyzed at 6.2 eV and the fast formation and decay of the photoproducts were monitored from 6.2 to 3.96 eV. Upon excitation, aqueous CS2 dissociates into CS+S. However, 93%±2% of the fragments geminately recombine on the electronic ground state potential of CS2 within a few picosecond leaving only 7%±2% of the CS+S fragments separated 100 ps after the excitation. In the gas phase, most of the dissociation occurs on a triplet-state potential surface, and the high recombination yield observed in aqueous solution therefore indicates a strong, solvent-assisted coupling between this state and the singlet ground state of CS2. The vibrationally excited CS2 molecule formed by the recombination transfers its high excess energy to the surrounding water molecules in two processes with time constants 8.4±1 and 33±7 ps. The rotational reorientation time of ground state CS2 is 6±1 ps, suggesting a surprisingly strong interaction between the neutral, nonpolar CS2 and the surrounding water molecules.

The photolysis of aqueous OClO, a key reaction in the understanding of atmospheric photochemistry, was studied using femtosecond spectroscopy. We report experimental results that resolve the current dispute over the chlorine dioxide photolysis and provide a detailed and complete example of chemical reaction dynamics in liquids. Two channels are active in the photolysis: (i) the formation of Cl atoms with a quantum yield of 0.07 +/- 0.03 and (ii) the formation of ClO+O, that geminately recombine as a result of caging and forms vibrationally excited OClO, which subsequently relaxes to the vibrational ground state on a 10 ps time scale. By continuously tuning the photolysis wavelengths from 400 to 300 nm, we observe a gradual increase in the cage escape yield, as the kinetic energy of the oxygen atom becomes sufficient to break the solvent cage.

The photodissociation and photoionization of liquid water following two-photon absorption at 266 nm is studied in the spectral range from 213 to 1108 nm with subpicosecond time resolution. Probing in the UV enables the first direct simultaneous observation of the photoproducts eaq−, Haq, and OHaq. This makes it possible to follow the geminate recombination kinetics between the photoproducts and to determine the relative yields of the dissociation and ionization channels. The concentration of hydrated electrons deduced from the visible and near-infrared transient absorption measurements decays by 40%±2% within the first 90 ps due to recombination with OHaq and H3O+. Analyzing our measurements of the hydrated electron concentration using the independent reaction time approximation results in the relative yields of 82%±3% and 18%±3% for recombination with OHaq and with H3O+, respectively. This is in excellent agreement with the relative yield of 82%±10% for recombination with OHaq determined directly from our ultraviolet transient absorption measurements. The contribution of hydrated electrons from direct ionization is insignificant when liquid water is excited below 9.32 eV and the ionization is likely to occur via dissociation and proton transfer. The transient ultraviolet absorption data shows that if ionization exclusively occurs via dissociation, 65% of the produced H(hot) atoms react with the surrounding solvent molecules to produce hydrated electrons. If proton transfer, on the other hand, is the only process responsible for the ionization, our measurements show that the ratio between dissociation and ionization is 55%. Geminate recombination of OHaq and Haq fragments following the photodissociation at 9.32 eV is not observed indicating that the translation energy of at least one of the fragments is sufficient to penetrate the water solvent cage. Finally, we have measured the two-photon absorption spectrum of liquid water from 110 to 160 nm, and the spectrum is in good agreement with our ab initio gas-phase calculations of the two-photon absorption cross sections for the transitions involved.

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 ≈2.5 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 9.5 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.

The intersystem crossing, internal conversion, and vibrational relaxation of p-nitroanaline (PNA) in water and 1,4-dioxane have been studied using ultrafast transient absorption spectroscopy. Following the photoexcitation of PNA at 400 nm, the transient absorption dynamics were probed from 340 to 960 nm. The measurements were performed on a common, absolute absorption scale, permitting an accurate determination of the temporal evolution of the absorption spectrum. The data reveal that relaxation on the excited singlet state surface, followed by internal conversion to the ground state and intersystem crossing to the triplet state, is extremely rapid (< 0.3 ps) in both solvents. The observed intersystem crossing efficiency is Phi(isc) approximate to 0.4 in dioxane and Phi(isc) approximate to 0.03 in water, indicating that the coupling between the excited singlet and tripler states depends strongly on the solvent polarity. With the estimated quantum yield for intersystem crossing in water and dioxane, we find a time constant for intersystem crossing of less than or equal to 10 ps in water and less than or equal to 0.8 ps in dioxane, The transient absorption features observed in the visible region are assigned to vibrationally excited PNA in the electronic ground state and three triplet-triplet absorption bands.

The photolysis of aqueous C1O2 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 from 230 nm to 78 nm, using either harmonics of the 780 nm pulses or a white light continuum. The main photolytic product, ClO + O, is formed with a quantum yield of 0.9, but disappears through a fast geminate recombination, producing vibrational excited C1O2 in the electronic ground state. The hot ClO, subsequently cools down in 10 ps. In addition to ClO + O, Cl + O2 is formed, with a quantum yield of 0.1 within the first 10 ps of the photolysis pulse, thus indicating the absence of the expected long-lived ClOO species. Using a simple model including the vibrational cooling of C1O2 and the absorbance of aqueous Cl we are able to account for the experimental observations.