Theoretical studies of collisional energy transfer in highly excited molecules: Temperature and potential surface dependence of relaxation in He, Ne, Ar + CS₂

In a previous paper we presented a method for studying the collisional relaxation of highly excited triatomic molecules based on trajectory simulations of successive collisions of the molecule with bath gas atoms. The method involves the microcanonical redistribution of vibrational coordinates and momenta between each collision to approximate the aging that occurs in gas-phase experiments. This method is expected to be accurate when the molecular internal energy is high so that molecular motions are chaotic. In this paper we use this redistributed successive collision (RSC) method and a related method based on single energy collisions (SEC) to study the relaxation of CS₂ by He, Ne, and Ar. For He + CS₂ we examine the temperature dependence of the average Vibrational energy transfer 〈ΔE〉 and find that, at 36000 cm^-1, 〈ΔE〉 increases monotonically with T between 300 and 2000 K. The temperature dependence is similar but stronger for Ne + CS₂ and Ar + CS₂. All of our results at 36000 cm^-1 are in agreement with experiment within the experimental uncertainties. The dependence of 〈ΔE〉 on the average vibrational energy 〈E〉 of CS₂ is approximately linear for He + CS₂ and Ne + CS₂ at all temperatures and for Ar + CS₂ at 1000 K. Except for He + CS₂ at 1000 K, this behavior differs from the stronger dependence that is found experimentally (often close to quadratic), and as a result the experimental values of 〈ΔE〉 at low 〈E〉 are smaller than those determined in the calculations. Ar + CS₂ at 300 K can be fit about equally well by either linear or quadratic dependence of (ΔE) on 〈E〉. For this case, theory and experiment agree well at all 〈E〉's. We also consider the dependence of 〈ΔE〉 on the potential surface used in the trajectory simulations. For the CS₂ intramolecular potential, we consider three surfaces: harmonic, sum of Morse functions, and a many-body expansion surface. We find that 〈ΔE〉 for He + CS₂ is insensitive to which surface we use. For the intermolecular potential we consider exponential-spline-Morse-spline-van der Waals (ESMSV) pair potentials and Lennard-Jones 6-12 and 6-20 pair potentials. We study the effect of well depth and repulsive wall steepness for the He + CS₂ system and find that different parametrizations of the potentials and variation of the van der Waals well depths lead to the same 〈ΔE〉's; however, an increase in steepness (as in changing from 6-12 to 6-20 potentials) leads to a significant change in 〈ΔE〉.