Abstract: Abstract: 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.
Abstract: Models of Titan ionospheric chemistry have shown that ion densities depend strongly on the neutral composition. The turbulent diffusion transport conditions, as modeled by eddy coefficients, can spectacularly affect the uncertainty on predicted neutral densities. In order to evaluate the error budget on ion densities predicted by photochemical models, we perform uncertainty propagation of neutral densities by Monte Carlo sampling and assess their sensitivity to two turbulent diffusion profiles, corresponding to the extreme profiles at high altitudes described in the literature. A strong sensitivity of the ion density uncertainties to transport is observed, generally more important than to ion-molecule reaction parameters themselves. This highlights the necessity to constrain eddy diffusion profiles for Titan ionosphere, which should progressively be done thanks to the present and future measurements of the orbiter Cassini.
Abstract: Validation of complex chemical models relies increasingly on uncertainty propagation and sensitivity analysis with Monte Carlo sampling methods. The utility and accuracy of this approach depend on the proper definition of probability density functions for the uncertain parameters of the model. Taking into account the existing correlations between input parameters is essential to a reliable uncertainty budget for the model outputs. We address here the problem of branching ratios between product channels of a reaction, which are correlated by the unit value of their sum. We compare the uncertainties on predicted time-dependent and equilibrium species concentrations due to input samples, either uncorrelated or explicitly correlated by a Dirichlet distribution. The method is applied to the case of Titan ionospheric chemistry, with the aim of estimating the effect of branching ratio correlations on the uncertainty balance of equilibrium densities in a complex model.
Abstract: ABSTRACT Urinary cytology is a noninvasive and unconstraining technique for urothelial cancer diagnosis but lacks sensitivity for detecting low-grade lesions. In this study, the fluorescence properties of classical Papanicolaou-stained urothelial cytological slides from patients or from cell lines were monitored to investigate metabolic changes in normal and tumoral cells. Time- and spectrally-resolved fluorescence imaging was performed at the single cell level to assess the spectral and temporal properties as well as the spatial distribution of the fluorescence emitted by urothelial cells. The results reveal quite different fluorescence distributions between tumoral urothelial cells, characterized by a perimembrane fluorescence localization, and the normal cells which exhibit an intracellular fluorescence. This is not caused by differences in the fluorescence emission of the endogenous fluorophores NAD(P)H, flavoproteins or porphyrins but by various localization of the EA 50 Papanicolaou stain as revealed by both the spectral and time-resolved parameters. The present results demonstrate that the use of single-cell endofluorescence emission of Papanicolaou-stained urothelial cytological slides can allow an early ex vivo diagnosis of low-grade bladder cancers.
Abstract: Solvated electrons have been produced in ethylene glycol by two-photon ionization of the solvent with 263 nm femtosecond laser pulses. The two-photon absorption coefficient of ethylene glycol at 263 nm is determined to be beta = (2.1 +/- 0.2) x 10(-11) m W(-1). The dynamics of electron solvation in ethylene glycol has been studied by pump-probe transient absorption spectroscopy. So, time-resolved absorption spectra ranging from 430 to 710 nm have been measured. A blue shift of the spectra is observed for the first tens of picoseconds. Using the Bayesian data analysis method, the observed solvation dynamics are reconstructed with different models: stepwise mechanisms, continuous relaxation models, or combinations of stepwise and continuous relaxation. Comparison between models is in favor of continuous relaxation, which is mainly governed by solvent molecular motions.
Abstract: The fragmentation mechanism of iron complexes bearing a bidentate ligand, dimethoxyethane (CH(3)OCH(2)CH(2)OCH(3), labeled as DXE) has been investigated by means of FT-ICR mass spectrometry (ion-molecule reactions) and infrared multiphoton dissociation spectroscopy. Two possible reaction mechanisms were envisioned for the Fe(DXE)(+) + DXE reaction, leading to the formation of the Fe(CH(2)O)(DXE)(+) ion. The two mechanisms differ in the nature of the neutral molecules formed: CH(3)OC(2)H(5) or CH(2)=CH(2) + CH(3)OH. The combination of ion-molecule reactions, thermochemistry considerations, and IRMPD spectra leads us to suggest that the mechanism involves successive elimination of the neutrals CH(2)=CH(2) and CH(3)OH, the first step of the mechanism being the insertion of the iron atom in the O-C(central) bond.
