Tomoya Tamadate | Searcharxiv

arXiv2026

Post-Collision Thermal Excitation and Survival-Limited Cluster Growth in the Gas Phase

Gas-phase cluster growth by monomer addition is commonly modeled as an isothermal process. We develop a survival-limited framework in which association produces a thermally excited cluster that may dissociate before either cooling through bath-gas collisions or encountering the next monomer. A continuous-energy survival probability is first derived for an individual post-association thermal trajectory and is then marginalized over distributions of excitation energy, equilibrium energy, and monomer-arrival time using a trajectory functional. Molecular-dynamics simulations of water, silver, and gold clusters provide size-dependent caloric relationships and latent heats, while event-based Monte Carlo simulations independently test the survival formulation. Theory and Monte Carlo results agree closely. The ensemble-averaged survival probability exhibits strong and non-monotonic size dependence, with the largest thermal penalties generally occurring for the smallest clusters. Intermediate-size local maxima arise only when complete cluster-energy distributions are retained and result from competition between curvature-enhanced dissociation and the narrowing of the low-energy tail with increasing size. Surviving clusters are consequently drawn preferentially from the colder portion of the pre-collision energy distribution, and mean thermal trajectories can substantially underestimate population survival. To connect single-event survival to cumulative growth, we introduce a thermal forward-rate correction relative to an isothermal reference and incorporate it into a reversible birth--death model. Although the correction at each size may be moderate, its multiplicative accumulation can increase mean first-passage times by many orders of magnitude. The framework provides a general route for identifying post-collision stabilization as a control on gas-phase cluster growth.

physics.chem-ph↗

arXiv2020

Calculation of the Ion-Ion Recombination Rate Coefficient via a Hybrid Continuum- Molecular Dynamics Approach

Accurate calculation of the ion-ion recombination rate coefficient has been of long-standing interest, as it controls the ion concentration in gas phase systems and in aerosols. We describe the development of a hybrid continuum-molecular dynamics approach to determine the ion-ion recombination rate coefficient. The approach is based on the limiting sphere method classically used for transition regime collision phenomena in aerosols. When ions are sufficiently far from one another, ion-ion relative motion is described by diffusion equations while within a critical distance, molecular dynamics (MD) simulations are used to model ion-ion motion. MD simulations are parameterized using the AMBER force-field as well as by considering partial charges on atoms. Ion-neutral gas collisions are modeled in two mutually exclusive cubic domains composed of 103 gas atoms each, which remain centered on the recombining ions throughout calculations. Example calculations are reported for NH4+ recombination with NO2- in He, across a pressure range from 10 kPa to 10,000 kPa. Excellent agreement is found in comparison of calculations to literature values for the 100 kPa recombination rate coefficient (1.0 x 10-12 m3 s-1) in He. We also recover the experimentally observed increase in recombination rate coefficient with pressure at sub-atmospheric pressures, and the observed decrease in recombination rate coefficient in the high pressure continuum limit. We additionally find that non-dimensionalized forms of rate coefficients are consistent with recently developed equations for the dimensionless charged particle-ion collision rate coefficient based on Langevin dynamics simulations.

physics.chem-ph↗