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The production rates of neutral species by dissociative recombination (DR) of molecular ions with electrons in the ionosphere of Titan are quantified by a new model, including, for the first time, all the available kinetic data on this process. The calculation is based on the ion densities measured by the INMS instrument on Cassini orbiter during flyby T19 at 1100 km altitude. These production rates are compared with those predicted by photochemical models: we calculate that for many neutral species, DR has larger production rates than neutral chemistry. Concerning molecular growth in Titan’s ionosphere, DR is shown to have two antagonistic effects: (1) a global chemical lysis of ions through C–C and C–N bond breaking (missed by the “H-loss” DR paradigm); and (2) an enhancement of the neutral chemistry by production of reactive radicals, such as C 2 H or NH 2 . Further exploration of this chemistry requires the development of ionospheric coupled models taking explicitly into account the richness of the DR process and the strong impact of ions on the budget of neutral species. This study emphasizes also the urgent need of additional experimental studies about DR of molecular ions, with two priorities: evaluation of the impact of the temperature of ions on the rates and fragmentation patterns, and the systematic study of the fragmentation patterns of C x H y N z + ions with more than four heavy atoms ( m / z > 60 u).

Using pump–probe transient absorption spectroscopy, we studied the solvation dynamics of the electron in liquid polyalcohols: ethane-1,2-diol, propane-1,2-diol, propane-1,3-diol and propane-1,2,3-triol. Time-resolved absorption spectra ranging from 440 to 720 nm were measured. Our study shows that the excess electron in the diols presents an intense and wide absorption band in the visible and near-IR spectral domain at early time after two-photon ionization of the neat solvent. Then, for the first tens of picoseconds, the electron spectrum shifts toward the blue domain and its bandwidth decreases as the red part of the initial spectrum rapidly drops, while the blue part hardly evolves. In contrast, in the triol, the absorption spectrum of the electron is early situated in the visible range after the pump pulse and then solely evolves in the red part. The Bayesian data analysis of the observed picosecond solvation dynamics with different models is in favor of a heterogeneous continuous relaxation. That is corroborated by the analogy between the change in the absorption band with increasing time or decreasing temperature. That tends to indicate a similar organization disorder of the solvent. Moreover, the electron solvation dynamics is very fast in propane-1,2,3-triol despite its high viscosity and highlight the role of the OH-group in that process.

Using pump-probe transient absorption spectroscopy we studied the solvation dynamics of the electron in liquid polyalcohols: ethane-1,2-diol, propane-1,2-diol, propane-1,3-diol and propane-1,2,3-triol. First, transmission measurements allowed us to assess that electrons were produced via two-photon ionization of the solvent with 263 nm femtosecond laser pulses, and to determine the two-photon absorption coefficient of the polyols. Second, time-resolved absorption spectra ranging from 440 to 710 nm were measured. Our study shows that the excess electron in the diols presents an intense and wide absorption band in the visible and near-IR spectral domain at early time after photoionization. Then, for the first tens of picoseconds the electron spectrum shifts toward the blue domain and its bandwidth decreases as the red part of the initial spectrum drops rapidly while the blue part hardly evolves. Using Bayesian data analysis method, the observed picosecond solvation dynamics were reconstructed with three models: a two-step mechanism and two continuous relaxation models. Comparison between the ability of models to reproduce the experimental kinetics is in favor of a heterogeneous continuous relaxation. Recent results obtained in propane-1,2,3-triol show that the electron solvation dynamics is very fast in this solvent despite its high viscosity and highlight the role of the OH group in that process.

International audience; 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.

The pentapeptide methionine-enkephalin (Met-enk) is a natural opiate that inhibits signals of pain. The N-terminal tyrosyl residue is important in the recognition of the peptide by its receptor. In oxidative stress, this residue can be oxidized by reactive oxygen species. The one-electron oxidation of Met-enk and of tert-butoxycarbonyl-methionine-enkephalin (Boc-Met-enk) was studied by gamma- and pulse radiolysis in the absence and in the presence of superoxide radical anions (O(2)(.-)) and oxygen, using azidyl radicals as oxidants. Without oxygen, both peptides behaved similarly. The tyrosyl radical resulting from the oxidation of tyrosyl residue produced the dimer linked by dityrosines. Methionine was also oxidized to its sulfoxide; however, this reaction is of minor importance. When O(2)(.-) was present, it added to tyrosyl radical giving a hydroperoxide. For Met-enk, this adduct cyclized via an intramolecular Michael addition of the amine on the aromatic ring. Conversely, for Boc-Met-enk, the adduct eliminated oxygen which led to 97% regeneration of the nonmodified peptide. Blocking the terminal amine group had thus a key role in protection of the tyrosyl residue. This finding might be exploited in the search for new pain inhibitors.

We produced solvated electron in ethylene glycol by two photon ionization of the solvent with a 263 nm femtosecond laser pulse and followed its dynamics by pump–probe transient absorption spectroscopy. For the first time, time resolved absorption spectra ranging from 450 to 725 nm and around 900 nm were measured revealing the rapid formation of a solvated electron’s precursor which absorbs around 450 nm. We also analysed the geminate recombination of the solvated electron.

The influence of the internal energy on the reactivity of iron carbonyl cations with dimethylether CH 3 OCH 3 (DME) has been studied using a triple cell Fourier Transform Ion Cyclotron Resonance mass spectrometer. The experimental set-up as well as the data analysis are briefly presented before being detailed on the example of the reactivity of Fe(CO) 2 + . The strong energy dependence upon the reactivity of the ion is shown: when working with thermalised ions, the only channels observed are the two successive substitutions of a CO ligand by one DME molecule, whereas other channels are opened up for excited ions (the cleavage of the C–O bond may be homolytic or due to a rearrangement). A reaction mechanism of the C–O bond activation is then proposed.

The potent activity exhibited by the antitumoral camptothecin (CPT) and its analog irinotecan (CPT-11) is known to be related to a close contact between the drug and the nucleic acid base guanine. This specificity of interaction between these two chromophores was examined by following changes in the photophysical properties of the drug using steady-state as well as time-resolved absorption and fluorescence methods. The observed effects on absorption, fluorescence emission and singlet excited state lifetimes give evidence for the occurrence of a stacking complex formation restricted to the quinoline part of CPT or CPT-11 and the guanine base but also with the adenine base. The triplet excited state properties of the drugs have been also characterized in absence and in presence of guanosine monophosphate and reveal the occurrence of an electron transfer from the guanine base to the drug. Support for this conclusion was obtained from the studies of a set of biological targets of various oxido-reduction potentials, adenosine monophosphate, cytidine, cytosine, tryptophan, tyrosine and phenylalanine. This finding gives an interpretation of the CPT-induced guanine photolesions previously reported in the literature. These data taken together are discussed in connection with the drug activity. The stacking complex CPT/guanine is necessary but not sufficient to explain the role of the chirality and of the lactone structure in the function of the drug. A stereospecific interaction with the enzyme topoisomerase I seems necessary to stabilize the stacking complex. The first experiments using time-resolved fluorescence by two-photon excitation confirms that CPT does not bind to the isolated enzyme.

The reaction kinetics of electron impact ionization generated FeCO + cations with O 2 is investigated using a triple-cell Fourier Transform Ion Cyclotron Resonance spectrometer. Two reaction channels are observed: (1) exothermic formation of FeO + with oxidation of the CO ligand; (2) slightly endothermic ligand substitution giving FeO 2 + . Residual water also reacts on FeCO + and FeO 2 + . Varying the relaxation delay of ions before reaction allowed us to study the radiative relaxation of FeCO + . Three groups of differently excited states of FeCO + , termed as (FeCO + ) ** , (FeCO + ) * and (FeCO + ) 0 by order of decreasing internal energy, can be differentiated according to their different reactivities. While (FeCO + ) ** and (FeCO + ) * lead to FeO 2 + , only (FeCO + ) 0 gives FeO + .

Size-controlled CdS clusters, with subnanometric radii, have been produced by γ-irradiation of aqueous solution containing Cd 2+ and thiol. Absorption and fluorescence spectra show that the mean CdS cluster size increases in the distribution with the dose or the post-irradiation time. On the contrary, at a given dose, the effect of the dose rate has been clearly demonstrated to lower the cluster size. The mechanism of CdS cluster growth is discussed via coalescence reactions. A numerical simulation taking into account the coalescence reactions between monomers produced by irradiation and then between clusters of any size is presented. Calculations show how the final nuclearity depends on the dose rate effect. The observation of isosbestic points during the slow coalescence (100 M −1 s −1 ) allowed us to derive the absorption spectra of oligomeric CdS clusters, successively formed with maxima at 270, 282 and 320 nm, respectively. At high dose rate and low dose, only the smallest clusters of CdS absorbing at 270 nm (and never observed previously) are formed.

The reactivity of Fe(CO) n + cations with methanol has been investigated using a triple-cell Fourier transform ion cyclotron resonance apparatus. Successive substitutions of the CO ligands by methanol occur rapidly with similar rates. For n = 3–5 the last CO ligand is not replaced, and the terminal substitution products are FeCO(CH 3 OH) n −1 + . A second reaction pathway is observed for n = 3–5, in which two CO ligands are replaced by one methanol molecule. This pathway occurs even if the Fe(CO) n + ions have been radiatively relaxed before reaction, providing information on the bond energies in the product ions. Strong internal energy effects are observed on the reaction rates and branching ratios for further substitution of the intermediate products. The substitution product Fe(CH 3 OH) + from FeCO + reacts further, with C–O bond cleavage followed by substitution. The resulting Fe[C 2 , H 7 , O 2 ] + ion undergoes either isomerization or collisional deactivation, then slow association with methanol. The reaction of Fe + , due to excited states only, begins with formation of FeOH + and involves the same final steps as in the preceding case.

The possibility of extracting the initial location of probe molecules in a micelle from the dynamics of their diffusion towards the interface has been studied. Advantages and drawbacks of the simulation of this dynamics with a Smoluchovski-type equation are discussed on the example of bianthryl in a cetyl trimethyl ammonium chloride micelle. It appears that the size and shape of the domain where the extinction of the probe molecules occurs, has a critical role in this approach. Its accurate knowledge is a prerequisite to any precise determination of the initial location of the probes.

We present the use of Nested Dirichlet distributions to represent uncertain branching ratios in chemical networks. The interest is twofold: (1) to preserve the structure of experimental data by imposing sum-to-one representations; and (2) to be able to introduce totally unknown subsets of branching ratios (missing data). These points are central to sound uncertainty propagation and sensitivity analysis in complex chemical networks.

A theoretical analysis of the quenching of Ar* in its two metastable states (3P2 and 3P0) by H is presented. It is shown that a simplified treatment of the electronic structure permits a discussion of the dynamics of that type of excitation transfer in terms of the relative position of a crossing (RC) of potential curves and of the region (RD) of decoupling of atomic states by the molecular field. In the present case we explain why the 3P2 state is more reactive than the 3P0 state.

A new method for computing polarized atomic orbitals (PAOs) is described: this method leads to very easy calculations. The space of the resulting PAOs is close to that of MC SCF MOs. Using these PAOs in the frame of a VB calculation has led to the same level of accuracy as the comparable MC SCF calculation for the dissociation energies and the lowest electronic transition energies of H2, H3 and N2.

A quantum time-dependent treatment of the He + H2 (B 1Σ+u) encounter provides new insight on the dynamics of the system. Time-resolved product distributions of electronic quenching to the ground state and vibrational energy transfer probabilities for the excited electronic state are obtained for a fixed-collision-angle model. The formation and evolution of an intermediate complex on the excited potential energy surface is found to govern the dynamics of formation of the products.