Nouvelles perspectives dans les traitements classique et semiclassique de la dynamique réactionnelle

La théorie de la dynamique des processus chimiques élementaires cherche à décrire quantitativement les collisions réactives à l'échelle atomique. Les mouvements des noyaux étant extrêmement difficiles à traiter dans le formalisme quantique, les tomes sont souvent considérés comme des objets classiques. Cepandant, les effets purement quantiques jouent un rôle majeur dans certaines situations, alors que la description classique les néglige. Cette thèse apporte de nouvelles perspectives sur l'inclusion, dans le formalisme clasique, de forts effets quantiques, à savoir la quantification des mouvements internes des réactifs et produits.The goal of chemical reaction dynamics theory is the quantitative description of reactive molecular collistions at the atomic scale. Since nuclear motions are difficult to study quantum mechanically, nuclei are often considered as classical object. However, quantum effects may play a major role in some situation, and the standard classical description does not take them into account. This thesis brings new perspectives on the inclusion into the classical treatment of one of the strongest qunatum effects, the quantization of reagents and products.BORDEAUX1-Bib.electronique (335229901) / SudocSudocFranceF

A theoretical simulation of the resonant Raman spectroscopy of the H2O⋯Cl2 and H2O⋯Br2 halogen-bonded complexes

The resonant Raman spectra of the H2O⋯Cl2 and H2O⋯Br2 halogen-bonded complexes have been studied in the framework of a 2-dimensional model previously used in the simulation of their UV-visible absorption spectra using time-dependent techniques. In addition to the vibrational progression along the dihalogen mode, a progression is observed along the intermolecular mode and its combination with the intramolecular one. The relative intensity of the inter to intramolecular vibrational progressions is about 15% for H2O⋯Cl2 and 33% for H2O⋯Br2. These results make resonant Raman spectra a potential tool for detecting the presence of halogen bonded complexes in condensed phase media such as clathrates and ice.Fil: Franklin Mergarejo, Ricardo. Université Paris Sud; Francia. Centre National de la Recherche Scientifique; Francia. InSTEC; Cuba. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Rubayo Soneira, Jesús. InSTEC; CubaFil: Halberstadt, Nadine. Université Paris Sud; Francia. Centre National de la Recherche Scientifique; FranciaFil: Janda, Kenneth C.. University of California at Irvine; Estados UnidosFil: Apkarian, V. Ara. University of California at Irvine; Estados Unido

Quasi-classical trajectories study of Ne2Br2(B) vibrational predissociation: Kinetics and product distributions

The vibrational predissociation of the Ne2Br2(B) van der Waals complex has been investigated using the quasi-classical trajectory method (QCT), in the range of vibrational levels v' = 16-23. Extensive comparison is made with the most recent experimental observations [Pio et al., J. Chem. Phys. 133, 014305 (2010)], molecular dynamics with quantum transitions (MDQT) simulations [Miguel et al., Faraday Discuss. 118, 257 (2001)], and preliminary results from 24-dimensional Cartesian coupled coherent state (CCCS) calculations. A sequential mechanism is found to accurately describe the theoretical dynamical evolution of intermediate and final product populations, and both QCT and CCCS provide very good estimates for the dissociation lifetimes. The capabilities of QCT in the description of the fragmentation kinetics is analyzed in detail by using reduced-dimensionality models of the complexes and concepts from phase-space transport theory. The problem of fast decoupling of the different coherent states in CCCS simulations, resulting from the high dimensionality of phase space, is tackled using a re-expansion scheme. QCT ro-vibrational product state distributions are reported. Due to the weakness of the vdW couplings and the low density of vibrational states, QCT predicts a larger than observed propensity for \Delta v' = -1 and -2 channels for the respective dissociation of the first and second Ne atoms.Comment: 16 pages, 6 figures, 4 tables. Accepted for publication in J. Chem. Phy

La disciplina Física teorica en la formación Del Profesor de Física

En el presente trabajo se muestra, primeramente, una breve reseña histórica de la presencia de la disciplina Física Teórica en la formación de profesores de Física en Cuba desde 1977 hasta 1992. A continuación se presenta una concepción de la disciplina a partir del objeto de la misma, su papel en la formación del profesor de Física, y sus objetivos generales. Algunas ideas acerca de la importancia de los métodos variacionales, como principio rector de la estructura de la disciplina Física Teórica son tambien expuestas en el trabajo. Por último se ilustra un modelo de curso de Física Teórica para profesores de Física, incluyendo la mención de algunas dificultades que pueden presentarse durante el proceso de enseñanza-aprendizaje con la concepción antes expuesta y nuestra experiencia en la impartición de los cursos componentes de la disciplina

Comparative configurational study for He, Ne, and Ar trimers

11 pages, 5 figures, 7 tables.Helium trimer bound states are calculated by means of a variational method described in terms of atom pair coordinates and distributed Gaussian basis functions for zero total angular momentum. To show the feasibility of this method, we also apply it to the calculation of the first vibrational levels of the Ar3 and Ne3 clusters. Special emphasis is made on the study of the possible Efimov behavior of the first excited state found in the 4He3 trimer. Geometrical configurations of the ground and first excited states of these rare gas trimers have been exhaustively studied owing to the proper symmetry of the coordinates chosen.This work has been supported in part by DGICYT (Spain) under Contract No. PB95-0071, Spanish-Cuban Project between CSIC and Agencia de Ciencia y Tecnologia Cubana, and the Research Network with Contract No. FMRX-CT96-0088.Peer reviewe

Quasiclassical dynamics of the I2-Ne2 vibrational predissociation: A comparison with experiment

The vibrational predissociation dynamics of the I2(B,v)-Ne2 complex is investigated for several vibrational levels of I2, using a quasiclassical trajectory approach. The time evolution of the population of nascent I2 fragments is calculated. A model is proposed which reproduces the results of the classical trajectories, and allows to obtain the lifetimes associated with the dissociation of the two van der Waals (vdW) bonds. The classical lifetimes are higher in general than the experimental ones of Zewail and co-workers [J. Chem. Phys. 97, 8048 (1992)]. The classical method appears to overestimate mechanisms of energy redistribution between the modes, which slow down the dissociation of the cluster. However, the behavior of the lifetimes with the initial iodine vibrational excitation is in very good agreement with experiment. A sequential path of fragmentation of the two weak bonds via direct predissociation is found to dominate, producing I2(B,v-2)+2Ne fragments. Although with smaller probability, alternative dissociation paths are observed involving statistical mechanisms of internal energy redistribution. In these paths, the energy initially transferred by the iodine heats the vdW modes without breaking the complex. Further energy transfer produces either simultaneous or sequential dissociation of the two weak bonds in a rather evaporative way, populating the v-2 and v-3 exit channels. © 1996 American Institute of Physics.This work was supported by DGICYT, Spain, under Grant No. PB92-0053, and by Comunidad Auto´noma de Madrid, under Grant No. 064/92.Peer Reviewe

Capture approximations beyond a statistical quantum mechanical method for atom-diatom reactions

Statistical techniques constitute useful approaches to investigate atom-diatom reactions mediated by insertion dynamics which involves complex-forming mechanisms. Different capture schemes based on energy considerations regarding the specific diatom rovibrational states are suggested to evaluate the corresponding probabilities of formation of such collision species between reactants and products in an attempt to test reliable alternatives for computationally demanding processes. These approximations are tested in combination with a statistical quantum mechanical method for the S + H2(v = 0,j = 1) → SH + H and Si + O2(v = 0,j = 1) → SiO + O reactions, where this dynamical mechanism plays a significant role, in order to probe their validity

Dynamics of exciplex formation in rare gas media

A hopping-surface algorithm has been used to simulate the dynamics induced in rare gas matrices due to the photoexcitation (1S0 → 3P1) of atomic mercury embedded in them. Especially, the study of the dynamics of an exciplex formation in a model system consisting of solid xenon doped with atomic mercury. The process starts upon the photoexcitation of the Hg atom to its 3P1 electronic excited state. Diatomics-in-Molecule approach has been used for constructing the adiabatic potential surfaces. In all trajectories we show that a triatomic Xe-Hg*-Xe complex is formed, but in two conformations: bent and linear. The mechanisms leading to the formation of one or the other are identified. Mainly, are noted the thermal fluctuations of the Hg impurity and the shape of the potential surfaces. Furthermore, we show that non-radiative intrastate relaxation occurs via a conical intersection between the excited state surfaces. The simulated spectra are in very good agreement with the experimental data. © 2009 Elsevier B.V. All rights reserved.G. Rojas-Lorenzo acknowledges the Ministerio de Ciencia e Innovación (Spain) for the project SB2006-0011.Peer Reviewe

Description théorique de la dynamique de recombinaison Eley-Rideal de l'azote moléculaire à la surface du tungstène

La recombinaison hétérogène de molécules diatomiques influence de manière déterminante la réactivité chimique de certains plasmas froids vis à vis des matériaux. Dans ce travail la dynamique de formation de molécules d'azote (N2) sur des surfaces modèles de tungstène (W) est théoriquement décrite. La méthode des trajectoires quasi-classiques est employée pour simuler l'abstraction directe d'atomes d'azote initailement pré-adsorbés sur la surface, par les atomes provenant de la phase gazeuse : la recombinaison Eley Rideal. L'utilisation d'un modèle d'interactions suffisamment précis, basé sur un fit multidimensionnel de calculs ab initio, révèle les mécanismes de recombinaisons à l'échelle moléculaire et en particulier le rôle prépondérant joué par la répulsion à moyenne lors de l'approche des deux atomes d'azote. La possible dissipation aux phonons du matériau ainsi que le rôle de la température de surface sont pris en compte par un modèle effectif d'oscillateur de Langevin généralisé. L'influence de la symétrie de surface est enfin abordée par la comparaison de la dynamique de recombinaison pour deux surfaces d'orientation cristallographique différente W (100) et W (110).AbstractBORDEAUX1-Bib.electronique (335229901) / SudocSudocFranceF