Charge localization in multiply charged clusters and their electrical properties: Some insights into electrospray droplets

The surface composition of charged Lennard-Jones clusters A$_N^{n+}$, composed of N particles (55 \leq N \leq 1169) among which n are positively charged with charge q, thus having a net total charge Q = nq, is investigated by Monte Carlo with Parallel Tempering simulations. At finite temperature, the surface sites of these charged clusters are found to be preferentially occupied by charged particles carrying large charges, due to Coulombic repulsions, but the full occupancy of surface sites is rarely achieved for clusters below the stability limit defined in this work. Large clusters (N = 1169) follow the same trends, with a smaller propensity for positive particles to occupy the cluster surface at non-zero temperature. We show that these charged clusters rather behave as electrical spherical conductors for the smaller sizes (N \leq 147) but as spheres uniformly charged in their volume for the larger sizes (N = 1169).Comment: 10 pages and 4 figure

SEMICLASSICAL STUDY OF THE PHOTODISSOCIATION DYNAMICS OF VIBRATIONALLY EXCITED NH3_3(\ A) MOLECULES

A. W. Jasper, S. Nangia, C. Zhu and D. G. Truhlar, Acc. Chem. Res. 39D. A. McCormack and K. F. Lim, Phys. Chem. Chem. Phys. 1Author Institution: Department of Chemistry, University of Minnesota, 207 Pleasant Street S.E., Minneapolis, MN 55455-0431, USAThe photodissociation dynamics of ammonia upon excitation of the out-of-plane bending mode (mode $\nu_2$ with $n_2$=0,..,6 quanta of vibration) on its \~A electronic state is investigated by means of several semiclassical methods and compared to experiments. Five semiclassical methods are tested: one mean-field approach (the CSDM method), two surface-hopping methods (the FSTU and FSTU/SD methods) and two surface-hopping methods with zero-point energy preservation (the FSTU/SD/TRAPZ and FSTU/SD/mTRAPZ methods).}, 101 (2006); A. W. Jasper and D. G. Truhlar, J. Chem. Phys. \textbf{127}, 194306 (2007); D. Bonhommeau and D. G. Truhlar (in preparation)} We found a qualitative difference between distributions obtained for $n_2 = 0$ and $n_2 > 1$ which is experimentally observed. Distributions obtained for $n_2 = 1$ present an intermediate behavior between distributions obtained for smaller and larger n$_2$ values. The dynamics is also found to be highly electronically nonadiabatic with all these methods. NH$_2$ internal energy distributions may have a negative energy tail when the zero-point energy (ZPE) conservation is not ensured throughout the dynamics. Unlike the original TRAPZ (TRAjectory Projection onto Zero-point energy orbits) method,}, 1 (1999)} the mTRAPZ (modified TRAPZ) method does not lead to unphysical results and is much less time consuming. The effect of including ZPE corrections in semiclassical dynamics is also discussed in terms of agreement with experimental findings

Dynamics and thermodynamics of decay in charged clusters

We propose a method for quantifying charge-driven instabilities in clusters, based on equilibrium simulations under confinement at constant external pressure. This approach makes no assumptions about the mode of decay and allows different clusters to be compared on an equal footing. A comprehensive survey of stability in model clusters of 309 Lennard-Jones particles augmented with Coulomb interactions is presented. We proceed to examine dynamic signatures of instability, finding that rate constants for ejection of charged particles increase smoothly as a function of total charge with no sudden changes. For clusters where many particles carry charge, ejection of individual charges competes with a fission process that leads to more symmetric division of the cluster into large fragments. The rate constants for fission depend much more sensitively on total charge than those for ejection of individual particles

Structure and stability of charged clusters

When a cluster or nanodroplet bears charge, its structure and thermodynamics are altered and, if the charge exceeds a certain limit, the system becomes unstable with respect to fragmentation. Some of the key results in this area were derived by Rayleigh in the nineteenth century using a continuum model of liquid droplets. Here we revisit the topic using a simple particle-based description, presenting a systematic case study of how charge affects the physical properties of a Lennard-Jones cluster composed of 309 particles. We find that the ability of the cluster to sustain charge depends on the number of particles over which the charge is distributed---a parameter not included in Rayleigh's analysis. Furthermore, the cluster may fragment before the charge is strong enough to drive all charged particles to the surface. The charged particles in stable clusters are therefore likely to reside in the cluster's interior even without considering solvation effects.Comment: 15 pages, 6 figure

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)}

Expertise sur le projet d'arrêté visant à encadrer la pêche de loisir à Mayotte

Expertise de l'Ifremer DOI au sujet du projet d'arrêté visant à encadrer la pêche de loisir à Mayotte, en réponse à la demande de la DMSOI, reçue par courriel le 29/06/2016