François G. Amar

Absolute classical densities of states for very anharmonic systems and applications to the evaporation of rare gas clusters

We have simulated the cluster dissociation reaction Arn→Arn−1+Ar (12≤n≤14) using molecular dynamics (MD) with well defined internal energy and total angular momentum. Reaction rates and kinetic energy release distributions are compared to the predictions of several statistical theories: Rice, Ramsperger, and Kassel (RRK), Engelking, and phase space theory (PST). We employ the Nose prescription for constant temperature dynamics coupled with the multiple histogram method of Labastie and Whetten to obtain highly accurate vibrational densities of states for the clusters. The absolute densities are determined by the adiabatic switching method of Reinhardt. Incorporation of these accurate anharmonic vibrational densities of states into classical PST allows us to make a direct comparison with the simulation results and eliminates any parameters from the theory. Then PST predictions for the kinetics of evaporation are given for the low energy (long time scale) regime where MD simulations are prohibitively expensi...

Spectral shifts and structural classes in microsolutions of rare gas clusters containing a molecular chromophore

We present the results of a series of molecular dynamics (MD) trajectory calculations of the structural, spectral, and dynamical properties of model clusters consisting of one impurity particle interacting with a pure host cluster. Our initial focus is on SF6Arn and SiF4Arn clusters in order to compare MD predictions of structure with the results of the IR studies of these systems performed by the Scoles group. We then show, more generally, how the preferred structural class (matrix or surface) of the heterogeneous cluster system depends on the interaction potential between guest and host molecules. The temperature dependence of the mobility of the impurity within the cluster is also investigated. Finally, the way in which our results can be adapted to interpret and predict solvation behavior for a wide variety of heterogeneous cluster systems is discussed.

Charge localization in negative ion dynamics: Effect on caging of Br−2 in Arn and (CO2)n clusters

We present results of molecular dynamics calculations of the recombination dynamics of a model system intended to mimic the main features of Br−2 in clusters of argon and CO2. The calculation displays a number of novel features, most notably, a consistent treatment of the asymptotic localization of the ‘‘extra’’ negative charge on one of the bromine atoms at large rBr–Br. We simulate the photoexcitation of Br−2 to the 2Πg state and present a summary of results for the dissociation dynamics and caging behavior of minimum energy structures of Br−2Xn, 5<n<17 where X can be Ar or structureless CO2. The structural control of caging is most pronounced for these minimum energy configurations. We then present results of thermal averaging over an ensemble of trajectories for each cluster. These results are compared with recent experiments on Br−2(CO2)n.

Quantum calculation of vibrational states in the aniline-argon Van der Waals cluster

Theoretical calculations of vibrational intermolecular states of the aniline–argon van der Waals complex for J=0 are reported. A fully‐quantum method (LCHOP) was used in order to describe the van der Waals cluster. Results in the first two electronic states S0 (X 1A1) and S1 (A 1B2) are presented; in the S1 state a comparison with available experimental data is made. We introduce an additive repulsive interaction between N and Ar in the S1 state in order to account for the spectral features observed in larger clusters. Several parametrizations of this term in the potential are discussed with a view to applications to semiclassical simulation of the spectra of the larger An–Arn clusters.

Two- versus three-dimensional melting and spontaneous reversing isomerization in isolated SF6-(Ar)9 van der Waals clusters

Abstract Molecular dynamics simulations for SF 6 -(Ar) 9 clusters at effective temperatures in the range 5–45 K show that the system undergoes two different “melting” transitions, one at T ≈15 K marking the onset of Ar atom mobility within a unimolecular layer around the SF 6 , and a second at T ≈35 K marking the onset of facile Ar atom motion out of and back into this layer. Moreover, on a narrow interval midway between these two points, the nine Ar atoms show a propensity for spontaneously isomerizing into and out of a long-lived near-rigid two-layer structure.

Simulating the photoelectron spectra of rare-gas clusters

Motivated by the recent experiments of the Swedish group [M. Tchaplyguine, R. R. Marinho, M. Gisselbrecht et al., J. Chem. Phys. 120, 345 (2004)], we simulate the photoelectron spectra of pure xenon and argon clusters. The clusters are modeled using molecular dynamics with Hartree-Fock-dispersion type pair potentials while the spectrum is calculated as the sum of final state energy shifts of the atoms ionized within the cluster relative to the isolated gas phase ion. A self-consistent polarization formalism is used. Since signal electrons must travel through the cluster to reach the detector, we have accounted for the attenuation of the signal intensity by integrating an exponentially decaying scattering expression over the geometry of the cluster. Several different approaches to determining the required electron mean free paths (as a function of electron kinetic energy) are considered. Our simulated spectra are compared to the experimental results.

The shapes of first‐stage sinters

The shape of first‐stage sinters is derived within a framework in which a solid skeleton is wetted by a mobile‘‘liquid’’ pool. Several lattices are considered as possible solid skeletons, corresponding to various degrees of surface versus grain boundary and volume transport; the effect of a nonzero dihedral angle at the interface between crystal faces is also treated. In some cases, significant differences exist between shapes calculated by minimizing the free energy and the shapes assumed by previous workers.

ISOMER SPECIFIC EVAPORATION RATES : THE CASE OF ANILINE-AR2

This paper reports the results of molecular dynamics (MD) simulations of isomerization and evaporation processes of the aniline–Ar2 cluster. The trajectory results are analyzed in terms of a simple unimolecular kinetics scheme in order to extract isomer‐specific evaporation rate constants. The less stable isomer, denoted (2/0) is found to have an evaporation rate constant that is about 25% smaller than that for the more stable isomer, (1/1). This result is explained in terms of the densities of states associated with each isomer. We present preliminary results on the aniline–Ar3 cluster and connect this latter system to possible experiments.

Structural motifs and stability of small argon–nitrogen clusters

The molecular dynamics (MD) simulation method is used to study Arm(N2)n clusters. Using realistic pair potentials for the argon–argon, nitrogen–nitrogen, and argon–nitrogen interactions, the structures and thermodynamics of these clusters are investigated. The initial focus of the study is the series of thirteen particle clusters of Arm(N2)13−m (0⩽m⩽13). These icosahedral argon–nitrogen clusters display systematic changes in energetics when argon is substituted by nitrogen in the central position. The relative stability of argon-centered clusters over nitrogen-centered clusters is further investigated by defining and calculating a “species-centric” order parameter which can be monitored during a MD simulation. These results are interpreted in terms of frustration effects due to anisotropy in the N2–N2 and N2–Ar potentials. The consequences of these observations for cluster stability and for dynamical behavior, such as melting and evaporation, are investigated. The dynamical studies of larger clusters reve...

On the use of evaporation dynamics to characterize phase transitions in van der Waals clusters: investigations in aniline–(argon)n up to n=15

Abstract The evaporation process in small van der Waals aniline–(argon) n clusters (with n =3, 4, 5, 8 and 15) has been investigated by molecular dynamics (MD) simulations. MD results have been compared to three statistical theories: classical RRK, the Engelking model and phase space theory (PST). The latter gives results in good agreement with the results derived from classical trajectories. The sensitivity of the average kinetic energy release to the thermodynamic phase of the product sub-cluster is put into evidence for small aniline–Ar n clusters. This finding opens new perspectives on the experimental characterization of phase transitions in atomic or molecular clusters. A complete study of the final populations in the different product isomers has been performed by MD simulations and compared to an extended phase space theory (EPST) in which the densities of states of individual isomers are explicitly taken into account. Finally the use of PST in the low energy regime has allowed to address the question of the cluster temperature resulting from evaporation down to time windows in the microsecond domain of experimental interest. The thermodynamic state of the cluster is predicted to change at a critical size.