Isabelle Lampre

Solvation dynamics of electron produced by two-photon ionization of liquid polyols. III. Glycerol.

The solvation dynamics of excess electrons in glycerol have been measured by the pump-probe femtosecond laser technique at 333 K. The electrons are produced by two-photon absorption at 263 nm. The change in the induced absorbance is followed up to 450 ps in the spectral range from 440 to 720 nm. The transient signals of electron solvation have been analyzed by two kinetic models: a stepwise mechanism and a continuous relaxation model, using a Bayesian data analysis method. The results are compared with those previously published for ethylene glycol (J. Phys. Chem. A 2006, 110, 175) and for propanediols (J. Phys. Chem. A 2007, 111, 4902). From the comparison, it is pointed out that solvation dynamics in glycerol is very fast despite its high viscosity. This is interpreted as the existence of efficient traps for the electrons in glycerol with low potential energy. The small shift of the absorption band of the excess electron indicates that the potential of these traps is very close to that corresponding to the fully solvated electron.

Oxidation of Bromide Ions by Hydroxyl Radicals: Spectral Characterization of the Intermediate BrOH•–

The reaction of (•)OH with Br(-) has been reinvestigated by picosecond pulse radiolysis combined with streak camera absorption detection and the obtained spectro-kinetics data have been globally analyzed using Bayesian data analysis. For the first time, the absorption spectrum of the intermediate species BrOH(•-) has been determined. This species absorbs in the same spectral domain as Br(2)(•-): the band maximum is roughly at the same wavelength (λ(max) = 352 nm instead of 354 nm) but the extinction coefficient is smaller (ε(max) = 7800 ± 400 dm(3) mol(-1) cm(-1) compared with 9600 ± 300 dm(3) mol(-1) cm(-1)) and the band is broader (88 nm versus 76 nm). Quantum chemical calculations have also been performed and corroborate the experimental results. In contrast to Br(2)(•-), the existence of several water-BrOH(•-) configurations leading to different transition energies may account for the broadening of the absorption spectrum in addition to the higher number of degrees of freedom.

Temperature effect on the absorption spectrum of the hydrated electron paired with a lithium cation in deuterated water

The absorption spectra of the hydrated electron in 1.0 to 4.0 M LiCl or LiClO4 deuterated water solutions were measured by pulse radiolysis techniques from room temperature to 300 degrees C at a constant pressure of 25 MPa. The results show that when the temperature is increased and the density is decreased, the absorption spectrum of the electron in the presence of a lithium cation is shifted to lower energies. Quantum classical molecular dynamics (QCMD) simulations of an excess electron in bulk water and in the presence of a lithium cation have been performed to compare with the experimental results. According to the QCMD simulations, the change in the shape of the spectrum is due to one of the three p-like excited states of the solvated electron destabilized by core repulsion. The study of s --> p transition energies for the three p-excited states reveals that for temperatures higher than room temperature, there is a broadening of each individual s --> p absorption band due to a less structured water solvation shell.

Pulse Radiolysis Studies on the Temperature-Dependent Spectrum and the Time-Dependent Yield of Solvated Electron in Propane-1,2,3-triol

With a revisit of the absorption coefficient of the solvated electron in propane-1,2,3-triol, the temperature-dependent behavior of the absorption spectrum of solvated electron was studied from room temperature to 573 K by pulse radiolysis techniques. The change in the absorption spectrum of solvated electron in propane-1,2,3-triol observed by cooling down from a high temperature to 333 K is compared with that occurring during the electron solvation process at 333 K. The effect of the specific molecular structure of propane-1,2,3-triol compared to other alcohols is discussed.

Nanosecond kinetics of hydrated electrons upon water photolysis by high intensity femtosecond UV pulses

We report a nanosecond laser study of the transient absorption of hydrated electrons generated by multiphoton ionisation of liquid water upon excitation at 266 and 400 nm by femtosecond pulses with power densities higher than 1 TW/cm2. For both wavelengths, as the pump power density increases, the signal amplitude increases and the decay becomes faster proving that more electrons are produced. However, we show that in the nanosecond time range, under pump power densities higher than 1 TW/cm2, the distribution of the hydrated electrons is not uniform along the optical pathway of the pump beam in the water sample.

DIMERISATION PROCESSES OF TRIARYL PYRYLIUM SALTS

Abstract The present paper deals with dialkylamino 2,4,6-triaryl pyrylium tetrafluoroborates studied in solution at room temperature. It is intended to clarify the dimerisation process described previously. The photophysical properties of the didodecylamino derivative, the synthesis of which is reported for the first time, are practically identical to those of the methyldodecylamino analogue, but its solubility in non-polar solvents is higher by two orders of magnitude allowing a better investigation of the dimers. The absorption spectra of both compounds, recorded as a function of concentration in solvents of low dielectric constant, exhibit isosbestic points. Different equilibria, involving the cationic chromophore and its counterion, are discussed. Two-dimensional 1 H-NMR experiments performed by off-resonance ROESY (Rotating frame nuclear Overhauser Effect SpectroscopY) confirm dimerisation. Electrical conductivity measurements show that in the monomer-dimer equilibrium related to the isosbestic point, the “monomer” corresponds to an ion-pair and the “dimer” is composed of two ion-pairs.

TRIARYLPYRYLIUM SALTS : DYNAMICS OF THE MONOMER-DIMER EQUILIBRIUM VIA A TRIPLET ABSORPTION STUDY

Abstract Dimers formed by triarylpyrylium tetrafluoroborates in toluene and polystyrene were studied at room temperature by nanosecond flash photolysis. The time evolution of the differential absorption spectra observed for toluene solutions was interpreted in terms of a monomer–dimer equilibrium in the ground state; the association (∼10 9 l mol−1 s −1 ) and dissociation (∼10 4 s −1 ) rate constants were derived from the concentration dependence of the transient signals.

TICT and triplet states of triarylpyrylium cations

Abstract Triarylpyrylium cations bearing an electron donating group are studied in solution by two pump-probe techniques with picosecond and nanosecond resolution. The transient spectra recorded on the picosecond time-scale as a function of solvent viscosity are in agreement with the formation of a TICT (twisted intramolecular charge transfer) state, predicted by quantum chemistry calculations. The ground state relaxation from the twisted conformation is observed. It is shown that the triplet state is formed only in solid matrices where the non-radiative decay of the excited singlet to the ground state is slowed down.

Interaction of terawatt laser pulses with neat water

Femtosecond time-resolved experiments in neat water under terawatt ultraviolet laser pump power density are reported. For the first time, the formation of a decimolar concentration of hydrated electron is demonstrated. The contribution of non-geminate recombination is discussed.

Time-dependent radiolytic yields at room temperature and temperature-dependent absorption spectra of the solvated electrons in polyols

Abstract The molar extinction coefficients at the absorption maximum of the solvated electron spectrum have been evaluated to be 900, 970, and 1000 mol−1·m2 for 1,2-ethanediol (12ED), 1,2-propanediol (12PD), and 1,3-propanediol (13PD), respectively. These values are two-third or three-fourth of the value usually reported in the published report. Picosecond pulse radiolysis studies have aided in depicting the radiolytic yield of the solvated electron in these solvents as a function of time from picosecond to microsecond. The radiolytic yield in these viscous solvents is found to be strongly different from that of the water solution. The temperature dependent absorption spectra of the solvated electron in 12ED, 12PD, and 13PD have been also investigated. In all the three solvents, the optical spectra shift to the red with increasing temperature. While the shape of the spectra does not change in 13PD, a widening on the blue side of the absorption band is observed in 12ED and 12PD at elevated temperatures.