Energy transfer in collisions of small gas phase clusters. Comparison of molecular dynamics and statistical limit estimates (original) (raw)
Related papers
Molecular dynamic simulations of atom–cluster collision processes
The Journal of Chemical Physics, 2004
Monomer-cluster collisions of Lennard-Jones argon atoms have been studied using molecular dynamics simulation for target cluster sizes of 2, 3, 4, 5, 10, and 20 atoms. Capture probability of monomers by clusters and the lifetimes of the resulting clusters have been calculated as a function of impact parameter and the total energy of the target cluster. Cluster lifetime is further integrated over all impact parameters to obtain the average lifetime for each cluster size and energy. The average lifetime of the smallest aggregates is shown to be short compared to the collision time between monomers and clusters unless the vapor is highly supersaturated. The formation probability of a new cluster decreases steeply if a minimum lifetime is required for the cluster.
A molecular dynamics investigation of energy transfer efficiency in collisions of diatomic molecules
Chemical Physics, 1993
Diatomic collisional energy transfer has been studied wtth the aim of aiding the development of accurate representation of the activation-deactivation mechanism in gas phase reaction rate theory. We seek to determine the dependence of the energy transfer efficiency on the parameters describing the colliding molecules, i.e. mass, vibrationaf frequency, inte~olecular potential and initial energy or temperature. While weak van der Waals type interactions give rise to very poor energy transfer efftciency scaling up the mteractions to the strength of weak chemical bonds increases the energy transfer efftciency dmmatic~ly up to nearly statistical bmit values. The dependence on mass and vibrattonal frequency is found to be weaker. The interaction strength is resolved into two factors, hardness of encounter and attractive strength. Varying these factors independently by a modification of the intermolecular atomic pair potential, both are found to contribute strongly to the observed energy transfer efftciency. Two temperatures, 160 and 1500 K, are consIdered and the sensitivity to the lntermo~ecuiar potential 1s found to be distinctly greater at the lower temperature. The average energy transferred per colliston (AE) is obtamed as a function of the internal energy of the target molecule at normal and high interaction strength.
Progress on the modeling of the collisional energy transfer mechanism in unimolecular reactions
Berichte der Bunsengesellschaft für physikalische Chemie
Words: Computer Experiments / Elementary Reactions / Energy Transfer / Molecular Collisions Statistical Theory The RRKM theory of unimolecular reaction rates is a statistical mechanical theory based on an assumption of microcanonical equilibrium in the reactant phase space. The energy transfer in reactant medium collisions was originally described by a canonical strong collision assumption, i.e., an assumption of full thermal equilibration in each collision. In our work we first introduce a microcanonical strong collision assumption which gives the RRKM theory a consistent form. we then introduce parametrizations of the degree of weakness (nonergodicity) of the collisions. A concept of collision efficiency is defined. The weakness of the collision is expressed in terms of reduced subsets of active reactant and medium degrees of freedom. The corresponding partially ergodic collision theory (PECT) yields physical functional forms of the collisional energy transfer kernel P (E ' , E). In order to resolve the energy and temperature dependence and the dependence on interaction strength a multiple encounter theory is introduced (PEMET). Initially each encounter may be described by a semiempirical PECT model. Eventually the encounters may be resolved by quantum dynamical calculations of the semiclassical or CAQE (classical approach/quantum encounter) type. Simple statistical collision models only distinguish between "hits and misses". In reality the energy transfer efficiency exhibits characteristic fall off with increasing impact parameter b. This b-dependence can be explicitly accounted for in the master equation for the reaction rate coefficient.
Chemical Physics Letters, 1992
Molecular dynamics simulation is used to study the activation/deactivation mechanism in the unimolecular decomposition of a diatomic gas. The strength parameter ¢ in the Lennard-Jones representation of the intermolecular potential is varied to reveal the dependence of the energy transfer rate on the strength of interaction in the collision complex. A dramatic increase is found with increasing ¢ and the energy transferred per collision approaches the ergodic collision limit at high ¢.
Chemical Physics Letters, 1993
Efficient Monte Carlo sampling methods are used to investigate the effect ofthe conservation of energy and angular momentum on energy transfer in Br,+Ar and Br,+Br, collisions. The effect of the conservation of spatial configuration in the collision is also considered. The results show that angular momentum conservation has a small ( 10%20%) but significant restraining effect on the energy transfer rate while the conservation of the spatial configuration reduced it by nearly 50%. The effects of anharmonicity in the bond potential of the diatomic molecule are also studied and found to be significant. Comparison with trajectory calculations reemphasizes the observation that for small molecules the energy transfer approaches the statistical limits only for strong interaction potentials.
Energetic and thermodynamic size effects in molecular clusters
The Journal of Chemical Physics, 1989
In this paper we explore the interrelationship between the energetics and the thermodynamic properties of molecular clusters. We advance simple models for the energy spectrum, which are used to derive analytical results for the thermodynamic properties of these clusters. The energy spectrum is characterized by the distribution of the energies of the local minima of the nuclear potential energy hypersurface, i.e., the inherent structures. On each of these energy levels the vibrational density of states of the particular inherent structure is superimposed. The energy spectra were specified in terms of the energy gap, a, between the (single) ground state and the excited-state inherent structures, the number, R, of the inherent structures and their energetic spread W. Four classes of energy spectra were considered. (1) A large energy gap with nearly degenerate excited-state manifold, i.e., a~ W. (2) A large energy gap with a considerable spread of the excited-state manifold, i.e., a < Wand W / R < a. (3) A gapless spectrum with W / R ~ A. (4) A multiple bunched spectrum with several energy gaps. Explicit analytical relations for the temperature dependence of the internal energy were derived for energy spectra of types (1), (2), and (3) both for the canonical and for the microcanonical ensembles. For energy spectra oftypes and the caloric curve exhibits a single inflection, which marks the "transition". A unified description of multistate isomerization with large R, which corresponds to rare-gas clusters, and of molecular isomerization with R= 1, which prevails for alkali-halide clusters, was provided. For energy spectra of type (3) the transition disappears, while for energy spectra of type (4) hierarchical isomerization is exhibited. Our analytical models have established the ensemble dependence of the transition for types (1) and (2), which is manifested by a considerable broadening of the transition range for the canonical ensemble, reflecting the role of energy fluctuations in the finite system.
Collisions of Molecules with Clusters: A Quasiclassical Study
NATO ASI Series, 1994
DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, re, commendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereoL
Theoretical investigation of collisional energy transfer in polyatomic intermediates
International Reviews in Physical Chemistry, 2013
Quantum scattering calculations on collisional rotational and vibrational energy transfer in small hydrocarbon reactive intermediates are highlighted. This review focuses on recent theoretical studies of energy transfer in methylene (CH 2 ), in both its ground tripletX 3 B 1 and low-lying singletã 1 A 1 electronic states, and in the methyl (CH 3 ) radical. Propensities in the state-to-state cross sections are shown to depend upon the two types of anisotropies that are present in potential energy surfaces of systems involving nonlinear polyatomic molecules. Computed rate constants for rotational and vibrational relaxation are compared with available experimental data. In addition, collisional transfer between the CH 2X andã states is addressed. Collision-induced intersystem crossing is shown to be mediated by spectroscopically perturbed rotational levels of mixed electronic character.
The Journal of Physical Chemistry Letters, 2011
E lucidation of the dynamics of the interaction of energetic particles with solids has been of long-standing fundamental scientific interest. Knowledge about the details of these interactions is important for understanding such diverse phenomena as the doping of semiconductor devices and the shaping of the topography of extraterrestrial surfaces. The emergence of cluster ion sources has expanded the scope of this field even further to include the smoothing and modification of semiconductor surfaces, 1À4 the cleaning of specific polymers, 5 biological mass spectrometry, 6À8 depth profiling of molecular solids, 9À12 and three-dimensional chemical imaging on the nanoscale. 13,14 The unique nature of the cluster/solid interaction has been elegantly illustrated using molecular dynamics (MD) computer simulations. 1,15À17 A key feature is that the kinetic energy of each incident particle in the cluster is equal to the total energy of the cluster divided by the number of atoms. For 20 keV C 60 , for example, each C atom imparts only 333 eV into the solid. Since there are a large number of lower energy collisions occurring nearly simultaneously, traditional analytical theories based upon