Rotational transfer, an angular momentum model

Self Cite

Abstract.We report separate experimental and theoretical studies that follow the equilibration of highly excited LiH (v=10;J=2) in H2 at 680K. Experiments that follow the time evolution of state-tostate population transfer in multi-collision conditions were carried out by Shen and co-workers at Xinjiang University and East China Institute of Science and Technology with µs resolution. At the same time, theoretical computations on the relaxation of this gas mixture were undertaken by McCaffery and co-workers at Sussex University. Rapid, near-resonant, vibration-vibration energy exchange is a marked feature of the initial relaxation process. However, at later stages of ensemble evolution, slower vibration-rotation transfer processes form the dominant relaxation mechanism. The physics of the decay process are complex and, as demonstrated experimentally here, a single exponential expression is unlikely to capture the form of this decay with any accuracy. When these separate studies were complete, the evolution of modal temperatures from the Sussex calculations were compared with experimental measurements of these same quantities from Shanghai and Urumqui. The two sets of data were found to be identical to within experimental and computational error. This constitutes an important experimental validation of the theoretical/computational model developed by the Sussex group and a significant experimental advance by the group of Shen et.al.

supporting

confidence: 48%

Section: The Computational Modelmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Energy transfer in multi-collision environments; an experimental test of theory: LiH (10;2) in H2(0;0)

Shen

Wang

Dai

et al. 2017

Self Cite

“…The probability of rotational state change falls rapidly with the magnitude of ⌬J, 18,19 an effect that may be masked by energy constraints. In the absence of specific constraints, transitions are most probable for lowest ⌬J and largest b n max .…”

Section: A Monatomic Collision Partnersmentioning

confidence: 99%

The role of angular momentum in collision-induced vibration–rotation relaxation in polyatomics

McCaffery

Osborne

Marsh

et al. 2004

Self Cite

Vibrational relaxation of the 6 1 level of S 1 ( 1 B 2u ) benzene is analyzed using the angular momentum model of inelastic processes. Momentum-͑rotational͒ angular momentum diagrams illustrate energetic and angular momentum constraints on the disposal of released energy and the effect of collision partner on resultant benzene rotational excitation. A kinematic ''equivalent rotor'' model is introduced that allows quantitative prediction of rotational distributions from inelastic collisions in polyatomic molecules. The method was tested by predicting K-state distributions in glyoxal-Ne as well as J-state distributions in rotationally inelastic acetylene-He collisions before being used to predict J and K distributions from vibrational relaxation of 6 1 benzene by H 2 , D 2 , and CH 4 . Diagrammatic methods and calculations illustrate changes resulting from simultaneous collision partner excitation, a particularly effective mechanism in p-H 2 where some 70% of the available 6 1 →0 0 energy may be disposed into 0→2 rotation. These results support the explanation for branching ratios in 6 1 →0 0 relaxation given by Waclawik and Lawrance and the absence of this pathway for monatomic partners. Collision-induced vibrational relaxation in molecules represents competition between the magnitude of the energy gap of a potential transition and the ability of the colliding species to generate the angular momentum ͑rotational and orbital͒ needed for the transition to proceed. Transition probability falls rapidly as ⌬J increases and for a given molecule-collision partner pair will provide a limit to the gap that may be bridged. Energy constraints increase as collision partner mass increases, an effect that is amplified when J i Ͼ0. Large energy gaps are most effectively bridged using light collision partners. For efficient vibrational relaxation in polyatomics an additional requirement is that the molecular motion of the mode must be capable of generating molecular rotation on contact with the collision partner in order to meet the angular momentum requirements. We postulate that this may account for some of the striking propensities that characterize polyatomic energy transfer.

“…[13][14][15][16][17] This formulation, which grew out of experimental findings, accurately reproduces a wide range of state-resolved, (single) collision-induced, inelastic, dissociative, and reactive processes, 18 giving new insight into the mechanisms of physical and chemical change. Its basis is the direct calculation of collision-induced linear-to-angular momentum (AM) via a lever-arm (b n ) of molecular dimension.…”

Section: Introductionmentioning

confidence: 99%

Quantum state-resolved, bulk gas energetics: Comparison of theory and experiment

McCaffery

2016

Self Cite

Until very recently, the computational model of state-to-state energy transfer in large gas mixtures, introduced by the author and co-workers, has had little experimental data with which to assess the accuracy of its predictions. In a novel experiment, Alghazi et al. [Chem. Phys. 448, 76 (2015)] followed the equilibration of highly vibrationally excited CsH(D) in baths of H 2 (D 2 ) with simultaneous time-and quantum state-resolution. Modal temperatures of vibration, rotation, and translation for CsH(D) were obtained and presented as a function of pump-probe delay time. Here the data from this study are used as a test of the accuracy of the computational method, and in addition, the consequent changes in bath gas modal temperatures, not obtainable in the experiment, are predicted. Despite large discrepancies between initial CsH(D) vibrational states in the experiment and those available using the computational model, the quality of agreement is sufficient to conclude that the model's predictions constitute at least a very good representation of the overall equilibration that, for some measurements, is very accurate. Published by AIP Publishing. [http://dx