V. Bernshtein

COLLISIONAL ENERGY TRANSFER BETWEEN AR AND NORMAL AND VIBRATIONALLY AND ROTATIONALLY FROZEN INTERNALLY EXCITED BENZENE-TRAJECTORY CALCULATIONS

Quasiclasical trajectory calculations of energy transfer between an exited benzene molecule and an argon atom were performed. Values of average energy transferred per collision, 〈ΔE〉, were calculated. Three cases were investigated. (a) Collisions with unconstrained “normal” initial conditions. (b) Collisions where the rotations of the benzene molecule are initially “frozen.” (c) Collisions where the out-of-plane vibrations of the benzene molecule are initially “frozen.” The distributions of 〈ΔE〉 vs collision durations and the values of 〈ΔE〉 for collisions with frozen degrees of freedom are different than those obtained in normal collisions. This indicates the effects these modes have on the energy transfer process. The effect of rotations was found to be the largest. This indicates the predominant role rotations play in the energy transfer process. The effect of out-of-plane vibrations on the efficiency of energy transfer corroborates quantum mechanical calculations which show that out-of-plane motions ar...

Trajectory calculations of relative center of mass velocities in collisions between Ar and toluene

Average velocities of Ar relative to the center of mass of toluene in bimolecular collisions were determined using quasiclassical trajectory calculations. The collision durations were binned in 20 fs and 100 fs bins and for each bin the velocities of all trajectories were averaged. 10 000 trajectories were calculated. About 64% of all collisions were elastic and the rest were inelastic collisions. The remaining 36% inelastic collisions can be classified into four types. (a) Impulsive collisions of duration 0–300 fs (62%). (b) Chattering collisions of duration longer than 300 fs but shorter than intramolecular vibrational relaxation (IVR) times (≳30%). (c) Complex forming collisions which last longer than molecular IVR times but less than complex (molecular+transition modes) IVR times and complex forming collisions which last longer than complex IVR times. The latter may lead to statistical distribution of energy in the collision complex. These long lived trajectories have negligible contribution to the va...

Intermolecular energy transfer probabilities from trajectory calculations: A new approach

A new method to calculate intermolecular energy transfer probability density function P(E′,E) from trajectory calculations is proposed. The method distinguishes between effective trajectories that contribute to P(E′,E) and those with very large impact parameter which do not. The P(E′,E) thus found obeys conservation of probability and detailed balance and is independent of the impact parameter. The method is demonstrated for benzene–Ar collisions at various temperatures and internal energies. With this method it is possible to combine ab initio inter and intramolecular potentials with trajectory calculations, obtain P(E′,E) and use that in master equation calculations to obtain rate coefficients and populations distributions without resorting to any a priori assumptions and energy transfer models. In addition, the effects of internal energy, temperature and rotations on the average energy transferred are discussed. Global potentials in center-of-mass and minimal distance coordinates which are obtained by ...

Formation and emission of gold and silver carbide cluster ions in a single C60− surface impact at keV energies: Experiment and calculations

Impact of fullerene ions (C(60)(-)) on a metallic surface at keV kinetic energies and under single collision conditions is used as an efficient way for generating gas phase carbide cluster ions of gold and silver, which were rarely explored before. Positively and negatively charged cluster ions, Au(n)C(m)(+) (n = 1-5, 1 ≤ m ≤ 12), Ag(n)C(m)(+) (n = 1-7, 1 ≤ m ≤ 7), Au(n)C(m)(-) (n = 1-5, 1 ≤ m ≤ 10), and Ag(n)C(m)(-) (n = 1-3, 1 ≤ m ≤ 6), were observed. The Au(3)C(2)(+) and Ag(3)C(2)(+) clusters are the most abundant cations in the corresponding mass spectra. Pronounced odd/even intensity alternations were observed for nearly all Au(n)C(m)(+/-) and Ag(n)C(m)(+/-) series. The time dependence of signal intensity for selected positive ions was measured over a broad range of C(60)(-) impact energies and fluxes. A few orders of magnitude immediate signal jump instantaneous with the C(60)(-) ion beam opening was observed, followed by a nearly constant plateau. It is concluded that the overall process of the fullerene collision and formation∕ejection of the carbidic species can be described as a single impact event where the shattering of the incoming C(60)(-) ion into small C(m) fragments occurs nearly instantaneously with the (multiple) pickup of metal atoms and resulting emission of the carbide clusters. Density functional theory calculations showed that the most stable configuration of the Au(n)C(m)(+) (n = 1, 2) clusters is a linear carbon chain with one or two terminal gold atoms correspondingly (except for a bent configuration of Au(2)C(+)). The calculated AuC(m) adiabatic ionization energies showed parity alternations in agreement with the measured intensity alternations of the corresponding ions. The Au(3)C(2)(+) ion possesses a basic Au(2)C(2) acetylide structure with a π-coordinated third gold atom, forming a π-complex structure of the type [Au(π-Au(2)C(2))](+). The calculation shows meaningful contributions of direct gold-gold bonding to the overall stability of the Au(3)C(2)(+) complex.

Minimal separation distance in energy transferring collisions. Trajectory calculations

Abstract Classical trajectory calculations of toluene-Ar collisions were performed. The energy transferred in up, down and all collisions is reported as a function of the minimal distance, MD, of approach between the Ar atom and the nearest atom to it in the toluene molecule. There is a distribution of MD at which energy transfer takes place and most energy transfer occurs in the range 0.29–0.35 nm. Increasing the temperature shifts the average to lower values. The collision duration and the MD are independent. The average MD is 0.308 nm at 300 K and 0.299 nm at 1500 K. Supercollisions were found but no correlation could be made between their duration and their MD.

Dynamics and energy release in benzene/Ar cluster dissociation

Energy disposal distributions and cluster lifetimes of Ar–benzene clusters (ABC) were studied by quasiclassical trajectory calculations. Four intermolecular potentials, Lennard-Jones, ab initio, and two Buckingham-type potentials, were used in the calculations. The Ar atom was placed in one of the five minima of the potential surface at 0 K. The benzene monomer in ABC at 0 K was excited to various internal energies, and internal energy loss of the monomer following dissociation was calculated. The average energy removed, 〈ΔE〉, depends on the well depth of the potential and on the initial structure of the cluster. The highest value was obtained when the cluster was formed at the deepest well, in which the Ar atom is above the center of the ring. Regardless of the initial structure, it was found that the atom migrated from well to well including the deepest, and dissociation occurred from a structure different from the initial one. No correlation was found between the energy removed and the cluster lifetime...

Endohedral formation, energy transfer, and dissociation in collisions between Li+ and C60

Quasiclassical trajectory calculations were performed on Li+ ion collisions with a C60 molecule. The probabilities of endohedral formation and escape from the cage are reported. It is found that endohedral formation depends on the relative translational energy and it is independent of the internal energy. The average energy transferred per collision of a Li+ with a fullerene molecule is reported and its dependence on the relative translational energy is given. The collisional energy transfer probability density function, P(E′,E), is calculated for two translational energies and the results are used to calculate the degree of dissociation of the fullerene molecule following a collision with Li+. Details of the intramolecular vibrational energy redistribution, IVR, are reported. It is found that following an exciting collision, energy relaxes by moving from one moiety to another within the molecule. Initial partial relaxation can be as fast as ∼67 fs but total redistribution of energy takes ∼1.5 ps.

Energy release in benzene–argon cluster dissociation – quasiclassical trajectory calculations

Abstract Results of quasiclassical trajectory calculations of the dissociation of a highly excited argon–benzene van der Waals cluster are reported. Two intermolecular potentials were used. The average energy removed from the excited benzene molecule, 〈ΔE〉, is −886 cm−1 for Lennard-Jones potential and −436 cm−1 for ab initio potential. The average trajectory lifetimes are 36.3 and 80.4 ps, respectively. The ΔE distribution shows a supercollision tail. No correlation was found between the lifetime of the cluster and the value of ΔE. The effects of the nature of the potential on the values of 〈ΔE〉 and on the long lifetime of the cluster are discussed.

Termolecular collisions between benzene and Ar

Termolecular collisions between a benzene molecule and two Ar atoms were studied by quasiclassical trajectory calculations. The calculations show that termolecular collisions form termolecular complexes and occur by three mechanisms: (a) the Chaperon mechanism, in which the first Ar in is the first Ar out of the termolecular complex, is the dominant one at high pressures. Two-thirds of all termolecular collisions go by this mechanism. (b) The energy transfer mechanism, in which the first Ar in is the last Ar out of the termolecular complex, comprises about a quarter of all termolecular collisions at high pressures. (c) The concerted channel, in which both argon atoms depart from the benzene simultaneously and does not lead to products in reactive systems, comprises about 10% of all termolecular collisions. Energy transfer quantities and collision complex lifetimes in binary and termolecular collisions are evaluated and their dependence on inter- and intramolecular harmonic and anharmonic potentials, tempe...

INTRAMOLECULAR ENERGY REDISTRIBUTION IN C60 FOLLOWING HIGH-ENERGY COLLISIONS WITH LI+

Abstract Quasiclassical trajectory calculations are used to investigate the dynamics of intramolecular energy redistribution, IVR, following a sudden excitation of a C 60 molecule by a high-energy Li + ion. During the collisions, ∼6 and ∼14 eV of translational energy were transferred to the C 60 and converted to internal energy. Only four normal modes, identified by fast Fourier transforms, out of the available 174 modes, were excited and participated in the initial phase of IVR. It took 60 fs for the excitation to move from the front to the back of the molecule. Total relaxation was obtained, however, only after few ps.