Why are collision induced rotational distributions unresponsive to kinematic differences?

An unanswered question in collision-induced rotational transfer (RT) centers on the similarities that characterize the distributions of Delta j 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 (b(n)) of molecular dimension. The mechanism operates strictly within boundary conditions set by energy conservation and, in certain kinematic circumstances, the range of b(n) values that may be accessed is constrained. These constraints are particularly marked when initial rotor state, j(i) much greater than 0 and when reduced mass (mu) is large. The occurrence of constraints is clearly seen in velocity-AM plots and the reduction of b(n) that results is readily quantified. Insights obtained from velocity-AM plots for j(i) > 0 and large mu 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 j(i) not equal 0 and introduces poorly defined parameters that appear unnecessary for a full description.