Dimer-formation rate coefficients from measurements of terminal dimer concentrations in free-jet expansions

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Abstract
Dimer formations in free jets of rare gases are considered as a simple example of recombination processes. In order to extract values of the local recombination‐rate coefficient kf from measurements of the terminal dimer mole fraction xA2, a sudden‐freeze model is used. Of the several kinetic models considered, only Bunker’s model of stabilization of virtual orbiting pairs by collisions with third bodies yields realistic values of the collision effectiveness. Procedures are developed for (1) predicting the terminal value of xA2 for given source conditions and known kf, (2) deducing kf for given source conditions and measured xA2, and (3) selecting appropriate source conditions for specified xA2 and known kf. Data for Kr, Ar, and Ne from six experimental studies and one computer study are correlated by a steric factor of 0.4 and an activation energy of 0.6 times the potential‐well depth ε. The simplified relation xA2?0.5[noσ3(ε/kTo)7/5 (d*/σ)2/5]5/3, where no is the source number density, To is the source temperature, d* is the source‐orifice diameter, σ is the zero‐potential interaction radius, and k is Boltzmann’s constant, predicts terminal dimer mole fractions to an approximation suitable for technical applications. It is suggested that a critical value of xA2, namely, about 0.01, is associated with the initiation of significant trimer formation.

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