FRAGMENTATION DYNAMICS OF IONIZED RARE-GAS CLUSTERS: NEW ACHIEVEMENTS

D. Bonhommeau, N. Halberstadt and U. Buck, Int. Rev. Phys. Chem. 26F. Calvo, D. Bonhommeau and P. Parneix, Phys. Rev. Lett. 99Author Institution: Department of Chemistry, University of Minnesota, 207 Pleasant Street S.E., Minneapolis, MN 55455-0431, USA; LCAR-IRSAMC, Universite Paul Sabatier and CNRS, 118 route de Narbonne, F-31062 Toulouse CEDEX 09, France; Max-Planck Institut fur Dynamik und Selbstoganisation, Busenstr. 10, D-37073 Gottingen, Germany; LCPQ-IRSAMC, Universite Paul Sabatier, 118 route de Narbonne, F-31062 Toulouse, France; Laboratoire de Photophysique Moleculaire, CNRS Bat. 210, Universite Paris-Sud, F-91405 Orsay, FranceThe fragmentation of rare-gas clusters Rg$_n$ ($2\le n\le 14$ and Rg = Ne, Ar and Kr) upon electron-impact ionization has been studied theoretically and compared to experiments}, 353-390 (2007)}. The dynamics of these ionic clusters has been modeled by means of a trajectory surface hopping method, the Tully's Fewest Switches (TFS) method, in which all the relevant electronic states of the ions and their couplings are taken into account. A very good qualitative agreement is found for all types of clusters, concerning the extensive character of the dissociation and the tendency to form larger fragments when the parent ion size increases. For instance, no trimer fragments are found for clusters smaller than the pentamer. In addition, a very good quantitative agreement is obtained for argon clusters. On the other hand, some discrepancies are found between experiment and theory for krypton clusters: the production of monomers seems underestimated in the simulation. Theoretical results also show that the parent ion dissociation occurs within the first picoseconds, and that most of the dynamics is completed within 10 picoseconds. Despite their success, TFS-like and adiabatic dynamics methods are based on classical mechanics and cannot reach experimental time scales, in the microsecond or millisecond range, whereas large clusters may carry on losing atoms after several nanoseconds. This issue was specifically examined on Ar$_n^+$ clusters (n=20 and 30): a new method that combines a TFS dynamics for the internal conversion, an electronic ground state adiabatic dynamics and phase space theory (PST) was designed and allows to reach the millisecond time scale}, 083401 (2007)}

Anharmonicité et spectroscopie IR de systèmes moléculaires complexes (application aux hydrocarbures aromatiques polycycliques)

Mon travail de thèse, de nature théorique, a porté sur la spectroscopie IR de systèmes moléculaires complexes. Je me suis focalisée sur les Hydrocarbures Aromatiques Polycycliques (HAPs), dont la présence dans le Milieu InterStellaire (MIS) est bien établie à partir de leur signature spectrale dans le domaine IR. Le but de cette étude est de sonder les effets d'anharmonicité de la surface d'énergie potentielle de l'état électronique fondamental sur les profils spectraux. Les spectres simulés sont confrontés aux données expérimentales et/ou observationnelles afin de tester la présence de molécules HAPs isolées dans le MIS. Dans un premier temps, j'ai simulé les spectres d'absorption et d'émission dans l'ensemble microcanonique, à partir d'une procédure Monte Carlo multicanonique. Pour ce faire, j'ai développé une méthode originale, basée sur l'algorithme Wang-Landau, permettant de calculer la densité d'états rovibrationnels anharmonique quantique. Le spectre d'absorption IR d'une molécule à une température donnée peut alors être calculé simplement, et l'étude de l'évolution du profil spectral des différentes bandes (ro-)vibrationnelles en fonction de la température interne de la molécule est aisée. Les spectres d'émission résultant d'une cascade d'émission IR dans l'état électronique fondamental ont également été simulés par une procédure Monte Carlo cinétique. Le même schéma théorique a permis de calculer les spectres d'absorption multiphotonique de HAPs, et de les comparer aux spectres d'absorption. Finalement, la simulation du spectre d'absorption IR de HAPs en fonction de la température a été abordée par dynamique moléculaire ab-initio de type Car-Parrinello.The subject of my thesis concerns a theoretical treatment of Infra-Red (IR) spectroscopy of complex molecular systems. l have focussed in particular on Polycyclic Aromatic Hydrocarbons (PAHs). These molecules are known to be present in the InterStellar Medium (ISM) on the basis of their spectral signature in the IR region. The objective of my thesis is to probe the anharmonicity of the potential energy surface of the molecular ground states and the effect on the spectral profiles. The simulated spectra are compared to experimental measurements or observations to determine if isolated PAHs exist in the ISM. The first step was to simulate the absorption and emission spectra within the microcanonical ensemble using a multicanonical Monte Carlo procedure. In order to achieve this, l had to develop an original method based on the Wang-Landau algorithm which allows a quantum anharmonic calculation of the rovibrational density of states. For a given temperature, the IR absorption spectra of a molecule can be calculated relatively simply, and the evolution of any particular spectral profile of the different rovibrational bands can be easily studied as a function of the internaI temperature of the molecule. The emission spectra generated by an IR cascade in the electronic ground state can also be simulated by a kinetic Monte Carlo process. The same theoretical scheme has enabled the multiphotonic absorption spectra of the PAHs to be compared with the absorption spectra. Finally, the PAH IR absorption spectra as a function of temperature were also simulated using ab-initio molecular dynamics calculations (Car-Parrinello method).ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

A simple but accurate potential for the naphthalene-argon complex: Applications to collisional energy transfer and matrix isolated IR spectroscopy

International audienceAn explicit polarizable potential for the naphthalene-argon complex has been derived assuming only atomic contributions, aiming at large scale simulations of naphthalene under argon environment. The potential was parametrized from dedicated quantum chemical calculations at the CCSD(T) level, and satisfactorily reproduces available structural and energetic properties. Combining this potential with a tight-binding model for naphthalene, collisional energy transfer is studied by means of dedicated molecular dynamics simulations, nuclear quantum effects being accounted for in the path-integral framework. Except at low target temperature, nuclear quantum effects do not alter the average energies transferred by the collision or the collision duration. However, the distribution of energy transferred is much broader in the quantum case due to the significant zero-point energy and the higher density of states. Using an ab initio potential for the Ar-Ar interaction, the IR absorption spectrum of naphthalene solvated by argon clusters or an entire Ar matrix is computed via classical and centroid molecular dynamics. The classical spectra exhibit variations with growing argon environment that are absent from quantum spectra. This is interpreted by the greater fluxional character experienced by the argon atoms due to vibrational delocalization

Multiscale Dynamics of Cluster Fragmentation

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Accurate Modeling of Infrared Multiple Photon Dissociation Spectra: The Dynamical Role of Anharmonicities

International audienceThe dynamical response of a molecular system to a macropulse typically produced by a free-electron laser is theoretically modeled over experimentally long times, within a realistic kinetic Monte Carlo framework that incorporates absorption, stimulated emission, spontaneous emission, and dissociation events. The simulation relies on an anharmonic potential energy surface obtained from quantum chemistry calculations. Application to cationic naphthalene yields a better agreement with measurements than the anharmonic linear absorption spectrum, thus emphasizing the importance of specific dynamical effects on the spectral properties

Finite-temperature stability of hydrocarbons: Fullerenes vs flakes

International audienceThe effects of a finite temperature on the equilibrium structures of hydrocarbon molecules are computationally explored as a function of size and relative chemical composition in hydrogen and carbon. Using parallel tempering Monte Carlo simulations employing a reactive force field, we find that in addition to the phases already known for pure carbon, namely, cages, flakes, rings, and branched structures, strong changes due to temperature and the addition of little amounts of hydrogen are reported. Both entropy and the addition of moderate amounts of hydrogen favor planar structures such as nanoribbons over fullerenes. Accurate phase diagrams are proposed, highlighting the possible presence of multiple phase changes at finite size and composition. Astrophysical implications are also discussed

Extracting vibrational anharmonicities from short driven molecular dynamics trajectories

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