Condensation and dissociation rates for gas phase metal clusters from molecular dynamics trajectory calculations

Abstract

In gas phase synthesis systems, clusters form and grow via condensation, in which a monomer binds to an existing cluster. While a hard-sphere equation is frequently used to predict the condensation rate coefficient, this equation neglects the influences of potential interactions and cluster internal energy on the condensation process. Here, we present a collision rate theory-molecular dynamics simulation approach to calculate condensation probabilities and condensation rate coefficients. We use this approach to examine atomic condensation onto 6-56-atom Au and Mg clusters. The probability of condensation depends upon the initial relative velocity (v) between atom and cluster and the initial impact parameter (b). In all cases, there is a well-defined region of b-v space where condensation is highly probable, and outside of which the condensation probability drops to zero. For Au clusters with more than 10 atoms, we find that at gas temperatures in the 300-1200 K range, the condensation rate coefficient exceeds the hard-sphere rate coefficient by a factor of 1.5-2.0. Conversely, for Au clusters with 10 or fewer atoms and for 14- and 28-atom Mg clusters, as cluster equilibration temperature increases, the condensation rate coefficient drops to values below the hard-sphere rate coefficient. Calculations also yield the self-dissociation rate coefficient, which is found to vary considerably with gas temperature. Finally, calculations results reveal that grazing (high b) atom-cluster collisions at elevated velocity (>1000 m s) can result in the colliding atom rebounding (bounce) from the cluster surface or binding while another atom dissociates (replacement). The presented method can be applied in developing rate equations to predict material formation and growth rates in vapor phase systems.

Rate enhancement in collisions of sulfuric acid molecules due to long-range intermolecular forces

“…Based on our MD simulations using the OPLS force field, the average moment of inertia I of a vibrating sulfuric acid molecule is 100.04 amuÅ 2 , which deviates slightly from the values of 100.66 and 104.94 amuÅ 2 obtained experimentally (Kuczkowski et al, 1981) and from quantum chemical calculations (Zapadinsky et al, 2019), respectively. The fitting function is obtained from classical trajectory calculations and is expressed as…”

“…A total of 99 % of the distribution lies within this range at T = 300 K. For each value of the impact parameter b and the relative velocity v, 1000 simulations were carried out starting with different initial atomistic velocities, to ensure good sampling. The simulation setup is very similar to the one recently used by Yang et al (2018). Sulfuric acid molecules bind to each other via the formation of one or more hydrogen bonds.…”

“…Atomistic simulations have been used to study collisions of Lennard-Jones clusters and atmospherically relevant molecules, but these studies did not analyse or report thermal collision rate coefficients (Napari et al, 2004;Loukonen et al, 2014). Recently, Yang et al (2018) studied the condensation rate coefficients for Au and Mg clusters at various gas temperatures using molecular dynamics (MD) calculations. The influence of van der Waals forces on the collision rate has also been considered in Brownian coagulation models of ultrafine aerosol particles (Marlow, 1980;Sceats, 1986Sceats, , 1989.…”

Atomistic insights into the dynamics of binary collisions between gaseous molecules and polycyclic aromatic hydrocarbon dimers

“…4,15 Collisions between soot nuclei and surrounding gaseous moelcules are quite common in combustion, which are also of great importance to particle formation especially at high pressures. 16,17 Serving as the soot nuclei, PAH dimers should survive the collision from surrounding gaseous molecules and be stable enough to grow with other PAH monomers or dimers. As the collision time between N 2 molecules in engines (1600 K, 10 bar) is comparable to the PAH dimer lifetime, Schuetz and Frenklach 18 stated that collisions with gaseous molecules is sufficient for PAH dimers to survive long enough for soot nucleation.…”

“…Since higher peak pressures are becoming the norm in internal combustion engines, studying the interplay between the gaseous molecules and soot nuclei is ever more demanding. 17,19 The objective of the present study is to provide a better understanding of the dynamics and kinetics of gaseous molecules colliding with PAH dimers. By performing a series of MD simulations, the influence of key factors, such as temperature, PAH dimer type, on the residence time and the scattering behavior of gaseous molecules is extensively examined.…”