Recombination Reactions of Atomic Chlorine in Compressed Gases. 3. Molecular Dynamics and Smoluchowski Equation Studies with Argon Pressure up to 6 kbar

Abstract

The photodissociation/recombination reactions of chlorine molecules under 355 nm laser radiation in compressed argon gas were studied by canonical molecular dynamics with the Nose-Hoover thermostat. The thermal relaxation and the final thermalized spatial chlorine atom-pair distribution over the dissociating electronic potential were analyzed and also compared with the predictions of the Langevin equation approach. The geminate recombination probability rate constants and quantum yields for all the bound electronic states of the chlorine molecule were calculated. The recombination in the ground electronic state was also studied by the Smoluchowski equation approach. Comparisons between the results of the molecular dynamics and the Smoluchowski equation approaches suggest that the solutions of the simplest diffusion equation consistently underestimate both the geminate rate constants and the quantum yields. Factors that may affect the recombination rate constants and quantum yields in the Smoluchowski equation approach were evaluated. They include local temperature jump due to initial hot atom thermalization, non-Markovian effect, meanforce potential, Cl-Cl long-range potential, and hydrodynamic interactions. For the Cl 2 /Ar system, it was found that a near quantitative agreement could be obtained by just taking the non-Markovian effect into account in the framework of the generalized diffusion equation approach. The contribution of mean-force potential, Cl-Cl long-range potential, and hydrodynamic interactions could account for the remaining mismatch in the short time and medium time domains. The effect of local temperature jump due to initial hot atom thermalization was found to be insignificant.

Recombination Reactions of Atomic Chlorine in Inert Gases: A Vibrationally Resolved Transient Kinetics Study at Pressures below 1 atm

“…The recombination rate constant ratio between the B and A states is either equal to or less than 0.2. This is consistent with the conventional assumption that the major contribution to the recombination reactions is through the four bound states: A, A‘, B‘, and X . For the above general observations, to date there were no related reports available for comparisons.…”

“…To the authors' knowledge, to date there are no experimental R A values available in the literature. Nevertheless, theoretically, by taking the electronic degeneracies into account, the A state recombination quantum yield has been calculated to be 34% by the energy transfer model in this laboratory . For the present purpose, R A was directly set at the value of 34% of the total experimental recombination rate constant.…”

“…The recombination reactions of the chlorine atoms and their accompanied physical processes, such as the electronic quenching and the vibrational energy relaxation in the electronic excited states, have been investigated by a number of research groups since the 1960s. − The rate constants of the recombination, the photon emission of the B state of the chlorine molecule, and the electronic quenching by some third bodies have been measured by a variety of techniques over a quite wide pressure range. Nevertheless, it was only quite recently that the chemiluminescence of the recombination reactions was known quantitatively to be composed of the emissions of the B and A states instead of the pure B state as traditionally assumed…”

Molecular Dynamics Simulation of Dissociation Kinetics

“…7,8,9 Dissociation takes place when the potential between two atoms of an oxygen molecule exceeds some critical value. The point at which this occurs is relatively ambiguous because it is difficult to explicitly define the point on the Morse potential curve where the molecule dissociates.…”

“…When the potential between two dissociated atoms decreases below 0.5 kT, the atoms combine to form an oxygen molecule. 8 When recombination occurs the potential between the two atoms is switched from the Lennard-Jones to the Morse potential.…”