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

Abstract

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.

Recoil energy distributions for dissociation of the van der Waals molecule p-difluorobenzene–Ar with 450–3000cm−1 excess energy

“…47,48 It has been shown that by using an equivalent rotor representation of polyatomics this model can reproduce the total angular momentum in the benzene fragment following dissociation of benzene-Ar. 9 The model also predicts the significant rotational excitation observed in 0°p DFB following dissociation of pDFB-Ar from 5 1 .…”

Rotational distributions following van der Waals molecule dissociation: Comparison between experiment and theory for benzene–Ar

“…1,37 b n max is simply calculated in each case from the rotational constants for the corresponding equivalent rotor. 15 The initial J has been set to zero. The predictions are remarkably consistent with the experimental results.…”

“…The model was found to successfully reproduce RT and VRT relative rate constants in glyoxal-Ne collisions, RT relative rate constants in C 2 H 2 -He collisions, and distributions extracted from experimental lineshapes for C 6 H 6 -H 2 , -D 2 , and -CH 4 collisions. 15 The more difficult task of predicting simultaneous J-and K-changing processes was not attempted though it was noted that experiment indicates that either Jor K-changing processes tend to dominate inelastic transfer in large molecules. The model used here for VP in benzene-Ar is a direct analog of that used successfully in diatomics in which dissociation is induced by an internal collision between the molecule and weakly bound species as each executes quasiindependent vibrational motion.…”

“…The method utilizes the equivalent rotor model introduced as a means of calculating rotational distributions following inelastic collisions in polyatomic species. 15 In this approach Jor K-changing processes are simulated using a rigid ellipsoid, the so-called equivalent rotor. The equivalent rotor is essentially a homonuclear diatomic representation of the polyatomic.…”

“…However, we have recently introduced a kinematic "equivalent rotor" model that allows the quantitative prediction of rotational distributions from inelastic collisions in polyatomic molecules for changes along one of these axes, i.e., for either J-or K-changing transitions. 15 Here we explore the use of this model to predict the J distribution for the benzene product in the dissociation of benzene-Ar. The calculated results are compared with the experimentally determined distribution and shown to provide an excellent fit.…”