QUANTUM CALCULATIONS FOR ROTATIONAL ENERGY-TRANSFER IN NITROGEN MOLECULE COLLISIONS

Rotational energy transfer in collisions of nitrogen molecules has bee n studied theoretically, using the N-2-N-2 rigid-rotor potential of va n der Avoird et al. [J. Chem. Phys. 84, 1629 (1986)]. For benchmarking purposes, converged close coupling (CC) calculations have been carrie d out to a total energy of about 200 cm(-1). Coupled states (CS) appro ximation calculations have been carried out to a total energy of 680 c m(-1), and infinite order sudden (IOS) approximation calculations have also. been carried out. The CC and CS cross sections have been obtain ed both with and without identical molecule exchange symmetry, whereas exchange was neglected in the IOS calculations. The CS results track the CC cross sections rather well: between 113-219 cm(-1) the average deviation is 14%, with accuracy improving at higher energy. Comparison between the CS and IOS cross sections at the high energy end of the C S calculations, 500-680 cm(-1), shows that IOS is sensitive to the amo unt of inelasticity and the results for large Delta J transitions are subject to larger errors. State-to-state cross sections with even and odd exchange symmetry agree to better than 2% and are well represented as a sum of direct and exchange cross sections for distinguishable mo lecules, an indication of the applicability of a classical treatment f or this system. This result, however, does not apply to partial cross sections for given total J, but arises from a near cancellation of the interference terms between even and odd exchange symmetries on summin g over partial waves. In order to compare with experimental data for r otational excitation rates of N-2 in the n=1 excited vibrational level colliding with ground vibrational level (n=0) bath N-2 molecules, it is assumed that exchange scattering between molecules in different vib rational le;els is negligible and direct scattering is independent of n so that distinguishable molecule rigid rotor rates may be used. With these assumptions good agreement is obtained. Although the IOS approx imation itself is found to provide only moderately accurate values for rate constants, IOS/ECS scaling methods, especially if based on funda mental rates obtained from coupled channel results, are found to provi de generally good accuracy. (C) 1995 American Institute of Physics.