Theoretical studies of van der Waals dimer depletion mechanisms in free jet expansions: The Ar2+X (X=CO2, CO, N2) systems

Collisional energy transfer, exchange, and complex formation mechanisms for Ar2 dimer depletion in free jet expansions have been investigated using quasiclassical trajectory methods on several different potential-energy surfaces. Computed Ar2 dissociation cross sections show that V → V energy transfer is an unimportant mechanistic pathway for Ar2 dissociation in collisions with CO2, N2, and CO. An R → V energy transfer pathway is found to be important at translational energies of 0.03 eV. However, there is very little difference among the results obtained for CO2, N2, and CO. At higher translational energies, around 0.10 eV, the importance of an R → V energy transfer mechanism in Ar2 dissociation decreases. The results are found to be insensitive to moderate variations in the pairwise LJ(12,6) potential parameters. Three-body potential terms are shown to be of negligible importance. Rate coefficients for collisional dissociation, exchange, and complex formation have been computed for (CO2, Ar2) and (N2, Ar2) systems under conditions that approximate those existing in the experiments reported by Yamashita et al. [J. Chem. Phys. 75, 5355 (1981)]. For the CO2 system, collisional dissociation is the major mechanistic pathway for Ar2 depletion. Exchange plays only a minor role. Complex formation does not occur. For the N2 system, collisional dissociation predominates for a rotational temperature equal to 298 K. At lower rotational temperatures, exchange becomes the major process. Complex formation does not occur. Yield ratios computed from a simple pseudo-first-order rate model are found to be in good accord with the experimental data for most systems. The exceptions are (C2H4, Ar2) when the mole fraction of C2H4 is 0.10 or greater, and the [C4H6, (CO2)2] system.