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.

Li2 , HF, and H2and other light diatomics at high vibration-rotational excitation exhibit an unusual form of vibration-rotation transfer generally associated with near energy resonance. This quasi-resonant transfer (QRT) gives rise to narrow rotational distributions. Near resonance in angular momentum is also a necessary requirement for the occurrence of QRT. However, the underlying physical processes differ little from those governing the more common forms of collisional transfer which, along with QRT, can be rationalized via the mechanism of linear to angular momentum interconversion within boundary conditions set by energy conservation. Velocity-jplots illustrate that these boundary conditions moderate the mechanism in a unique fashion in the case of QRT since they are sharply defined around a limited set of jvalues. The occurrence of QRT will be widespread in the high lying states of light diatomic molecules, the hydrides for example, and may readily be identified using plots of the energy and angular momentum conservation relations. Energy conservation forces a reduction in the maximum available torque arm in the angular momentum mechanism for all but a narrow range of jtransitions. This analysis of the primary physical mechanism is confirmed via multiellipsoid Monte Carlo calculations for Li2and for H2 . In HF-Ar we show that QRT is a much more likely process than pure rotational transfer giving rise to collisional pumping which will be enhanced in a multicollision environment.

We describe a quantitative angular momentum (AM) model for predicting rotational transfer (RT) and vibrotational transfer (VRT) in collisions between CO2 and hot H atoms. This molecule is important in several contexts, not least as a bridge between the relative simplicity of diatomic molecules and the complexities of polyatomic RT and VRT. We show that for pure RT, an AM constraint dominates but that this changes to a dominant energetic constraint in the case of VRT. The requirement that the (001) vibrational channel be opened simultaneously with the generation of AM imposes special restrictions which effectively limit the trajectories that lead to VRT. The origin of this is a constraint-induced restriction on the effective impact parameter (bnmax) for individual Δj channels and the effect is manifest as reduced probability for populating low Δj channels. In CO2–H* this leads to a shift in the peak of (VRT) Δj probabilities away from zero as found experimentally for the (001) vibrational mode. We report a Monte Carlo trajectory calculation similar to that of Kreutz and Flynn [J. Chem. Phys. 93, 452 (1990)] but predict an exponential-like dependence of pure RT on Δj. For VRT to (001) the constraint-induced restrictions on bnmax are incorporated quantitatively and the vibrational channel-opening velocity is treated as a vector quantity. The results of these calculations are in good agreement with experiment. The underlying mechanism, likely to be general in VRT, is clearly revealed in plots of relative velocity versus rotational AM change.