Evidence for hybrid classical - quantal behaviour in state- and angle-resolved atom - diatom scattering

The Journal of Chemical Physics

1999

A kinematic interpretation for quasiresonant vibration-rotation collisional transfer ͑QRT͒ is outlined based on the angular momentum ͑AM͒ theory. QRT provides a particularly stringent test since as rotational AM increases, energy decreases ͑or vice versa͒. We demonstrate using velocity-AM plots for (A) 1 ͚ u Li 2 -Ne that although experimentally spectacular, in kinematic terms it constitutes only a slightly unusual energetic constraint to the linear-to-angular momentum conversion.

Section: An Angular Momentum Interpretationmentioning

confidence: 61%

Quasiresonant vibration–rotation transfer: A kinematic interpretation

The Journal of Chemical Physics

1999

“…This highly detailed collision dynamics experiment yielded results that on analysis, corroborated the point made by Korsch and Ernesti [12], namely, that very simple models can reproduce the results of the most sophisticated of experiments. The most probable scattering angle could be predicted from a simple Newtonian vector model connecting initial relative velocity (v r ), threshold, or channel-opening velocity (v th ), a quantity readily calculated from DE for the transition, and sin y where y is the (measured) most probable scattering angle [32,33]. The relationship is sin y ¼ v th /v r and as Fig.…”

Section: Angular Momentum In Collision Dynamicsmentioning

confidence: 99%

From Ligand Field Theory to Molecular Collision Dynamics: A Common Thread of Angular Momentum

2011

Structure and Bonding

Interest in, and appreciation of, the role played by angular momentum in chemical physics was first aroused by a Carl Ballhausen lecture early in the author's scientific career. Later came deeper understanding of the fundamental nature of angular momentum and the power of its formal algebraic expression. Spectroscopy using light of precisely defined energy and (z-component of) angular momentum represents a unique experimental probe with the potential to reveal the underlying physics of chemical processes. Experiments using circularly polarised emission of gas phase molecules led to new insights in the field of molecular collision dynamics. Further work, and that of others, suggested an alternative formulation of the mechanics of bimolecular collisions. A theoretical model was developed guided throughout by experiment. Its basis is linear-to-angular momentum conversion within the constraint of state-to-state energy conservation. An evolutionary process guided by experiment is described, with illustrations that demonstrate the power of the angular momentum theory of molecular collisions developed by the author and co-workers. Examples include very recent work on multicollision models of gas ensembles of potential value in, e.g., modelling planetary atmospheres.

“…[6][7][8] We have shown 1 that for many sys-tems, particularly those in which the collision partner is a light atom and the molecule a diatomic initially in a low rotational state, the AM constraint generally dominates. In this situation, the full anisotropy of the intermolecular potential may be explored in the collisional event and the probability for each ⌬ j readily predicted using a simple relationship in which the repulsive anisotropy ͑which may be equated to the maximum available effective impact parameter or torque-arm b n max ) plays a key role.…”

Section: Energy and Angular Momentum Constraintsmentioning

confidence: 99%

“…In the case of entirely forward scattering and for narrow distributions of relative velocity, accurate prediction of scattering angles may be made. 7,8 This separation into regions of forward and backward scattering is based on the availability in Eq. ͑1͒, of collision momentum up to double that initially present through the process of backscattering while that available via Eq.…”

Section: Energy and Angular Momentum Constraintsmentioning

confidence: 99%

“…The steric effects in which trajectories are restricted to those end-on to the ellipsoid for low ⌬ j values are more appropriately termed stereokinematics since the process concerns the motions of the nuclei and it is a little more than the operation of the vector relationships of Newtonian mechanics, constrained by the needs of the quantised molecular eigenstates. 7,8 In this process, the final state linear and angular momenta determine the direction of a successful trajectory once the relative velocity is specified. FIG.…”

Section: A Stereokinematic Considerationsmentioning

confidence: 99%

See 1 more Smart Citation

Rotational and vibrotational transfer in H*–CO2 collisions: The influence of stereokinematic restrictions

Clare

Marks

The Journal of Chemical Physics

1999

Articles you may be interested inVibrational and rotational energy transfers involving the CH B Σ − 2 v = 1 vibrational level in collisions with Ar, CO, and N 2 O We describe a quantitative angular momentum ͑AM͒ model for predicting rotational transfer ͑RT͒ and vibrotational transfer ͑VRT͒ in collisions between CO 2 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 ͑001͒ 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 (b n max ) for individual ⌬ j channels and the effect is manifest as reduced probability for populating low ⌬ j channels. In CO 2 -H* this leads to a shift in the peak of ͑VRT͒ ⌬j probabilities away from zero as found experimentally for the ͑001͒ vibrational mode. We report a Monte Carlo trajectory calculation similar to that of Kreutz and Flynn ͓J. Chem. Phys. 93, 452 ͑1990͔͒ but predict an exponential-like dependence of pure RT on ⌬ j. For VRT to ͑001͒ the constraint-induced restrictions on b n max 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.