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Esboui M.,Laboratoire Of Spectroscopie Atomique | Jouvet C.,University Paris - Sud | Dedonder C.,University Paris - Sud | Ebata T.,Hiroshima University
Journal of Physical Chemistry A | Year: 2010

The excited-state lifetimes of different vibrational levels of the 2-hydroxypyridine-ammonia complex have been recorded with the picosecond pump probe technique. These lifetimes decrease with increasing energy. The decrease of the lifetime is discussed based on the possible enol keto isomerization computed using RICC2 methods. © 2010 American Chemical Society. Source

Esboui M.,Laboratoire Of Spectroscopie Atomique | Esboui M.,P.A. College
Journal of Chemical Physics | Year: 2015

The stepwise and concerted excited state intermolecular proton transfer (PT) and hydrogen transfer (HT) reactions in 2-hydroxypyridine-(NH3)2 complex in the gas phase under Cs symmetry constraint and without any symmetry constraints were performed using quantum chemical calculations. It shows that upon excitation, the hydrogen bonded in 2HP-(NH3)2 cluster facilitates the releasing of both hydrogen and proton transfer reactions along ammonia wire leading to the formation of the 2-pyridone tautomer. For the stepwise mechanism, it has been found that the proton and the hydrogen may transfer consecutively. These processes are distinguished from each other through charge translocation analysis and the coupling between the motion of the proton and the electron density distribution along ammonia wire. For the complex under Cs symmetry, the excited state HT occurs on the A″(1πσ∗) and A′(1nσ∗) states over two accessible energy barriers along reaction coordinates, and excited state PT proceeds mainly through the A′(1ππ∗) and A″(1nπ∗) potential energy surfaces. For the unconstrained complex, potential energy profiles show two 1ππ∗-1πσ∗ conical intersections along enol → keto reaction path indicating that proton and H atom are localized, respectively, on the first and second ammonia of the wire. Moreover, the concerted excited state PT is competitive to take place with the stepwise process, because it proceeds over low barriers of 0.14 eV and 0.11 eV with respect to the Franck-Condon excitation of enol tautomer, respectively, under Cs symmetry and without any symmetry constraints. These barriers can be probably overcome through tunneling effect. © 2015 AIP Publishing LLC. Source

Ahmed J.B.,Laboratoire Of Spectroscopie Atomique | Cowpe J.,University of Salford
Applied Optics | Year: 2010

Using well-known expressions describing radiative transfer, we have established an expression predicting the spectral profile of a self-absorbed Ca++ 393:4nm emission line as emitted by a transient laser-induced plasma. In this approach, the plasma was approximated as comprising five distinct layers, each of thickness 0:5mm, and each characterized by a unique uniform electron density, electron temperature, and optical depth. The validity of the theoretical model was confirmed by successful comparison with experimental data. Inhomogeneous laser-induced plasmas were produced on the surface of an aqueous CaCl2 (0:01mol=l) solution using a frequency-doubled Nd:YAG laser. Optical emission spectra were collected in such a way as to allow for temporal and spatial diagnostics of the plasma plumes. © 2010 Optical Society of America. Source

Ben Fredj A.,Laboratoire Of Spectroscopie Atomique
Helvetica Chimica Acta | Year: 2016

Dimeric structures chlorophyll (a) (Chla) and their mono- and dihydrated have been suggested to play an important role in the mechanism of photoreaction center chlorophyll special pairs PSI and PSII. Despite their functional importance, the molecular basis structures for interacting two Chla molecules and the structural stabilization role of H2O in the formation of hydrated Chla dimer complexes is poorly understood. In this article, the different coordination modes between two interacting Chla molecules and the configurational (orientation and distance) features between the dimer and bound H2O molecules are characterized by means of super molecule approach the density functional theory DFT. An estimation of the thermodynamic quantities is made for Chla dimerization and hydration processes. The results indicate that structure including ester linkages via H2O hydrogen bonding is the most favorable conformation for the dihydrated chlorophyll (a) dimer at B3LYP/6-31G∗-DCP level of calculation. The dispersion interaction is shown to be of great significance for the Chla dimer stabilization. In aqueous nonpolar solvent, the thermodynamics show that Chla has a slightly stronger driving force for full hydration than for dimerization and that hydration of the dimers is rather weakly exergonic. The tetrahydrated dimers having a similar arrangement to that in crystals of ethyl chlorophyllide (a) dihydrate are found to be more stable than the Chla dihydrated dimer. The data underscore the key role of H-bonding in the stability of Chla-H2O adducts and, in particular, the great importance of the Chla monomeric dihydrated species in the hydration and dimerization of Chla in aqueous media. Clearly, the Chla dihydrates (Chla-2 H2O) are found more stable than the monohydrates (Chla-H2O) and the Chla dimers (Chla2), owing to a particular structure in which cooperative interactions occur between the H2O molecules and Chla. Calculations also indicate that the most thermodynamically preferred pathway for the formation of Chla dimer hydrates can be represented by two steps: the first corresponds to the formation of Chla monomeric dihydrates and the second is the dimerization of the dihydrates on to tetrahydrated Chla dimers. These results allow to obtain a new possible pathway for Chla dimer formation processes and could provide new insights to the aggregation of chlorophyll (a) in solution. © 2016 Verlag Helvetica Chimica Acta AG, Zürich. Source

Ahmed J.B.,Laboratoire Of Spectroscopie Atomique | Fouad F.,University Mohammed Premier
IEEE Transactions on Plasma Science | Year: 2014

In this paper, we propose and use an experimental method permitting the measurement of optical depth of a self-absorbed spectral line emitted from laser-induced plasma on the surface of an aqueous solution. Using well-known equations describing the radiatif transfer and the evolution with the optical depth of the relative uncertainty on the measurement of line intensity and on the half width at half maximum, we have also deduced the uncertainty on the measurement of electron temperature and the electron density. An application to the diagnostics of this plasma has been realized, and we show the reliability of the spectroscopic measurements. 0018-9456 © 2014 IEEE. Source

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