Why Are Collision Induced Rotational Distributions Unresponsive to Kinematic Differences?

Abstract

An unanswered question in collision-induced rotational transfer (RT) centers on the similarities that characterize the distributions of Δj states despite very large differences in mass and chemical composition of collision partners (Clegg, S. M.; Burrill, A. B.; Parmenter, C. S. J. Phys. Chem. A 1998, 102, 8447). We show these observations to be consistent with a kinematic model whose mechanism is the conversion of linear momentum of relative motion into rotational angular momentum (AM) via a torque arm (bn) of molecular dimension. The mechanism operates strictly within boundary conditions set by energy conservation and, in certain kinematic circumstances, the range of bn values that may be accessed is constrained. These constraints are particularly marked when initial rotor state, ji ≫ 0 and when reduced mass (μ) is large. The occurrence of constraints is clearly seen in velocity−AM plots and the reduction of bn that results is readily quantified. Insights obtained from velocity−AM plots for ji > 0 and large μ are confirmed through multi hard ellipsoid Monte Carlo calculations. The analysis presented here indicates that the energy corrected form of the IOS scaling relation does not adequately represent the RT mechanism for ji ≠ 0 and introduces poorly defined parameters that appear unnecessary for a full description.