Relaxation of the 6 Vibrational Level in ¹B_2u Benzene by Polyatomic Colliders at Ultralow Temperatures

Abstract

Vibrational energy transfer from the 61 level of S1 (1B2u) benzene has been studied at low collision energies in supersonic free jet expansions with 11 polyatomic collision partners. Each of the collision partners has available low lying vibrational levels so that relaxation can proceed via transfer of vibration in benzene to vibration in the collision partner (V → V transfer). Transfer to the 00 level of benzene is significant with each of these partners. From (a) comparisons with the behavior of the monatomic, diatomic, and small polyatomic partners studied previously and (b) previous results at room temperature [C. S. Parmenter, C. S.; Tang, K. Y. Chem. Phys. 1978, 27, 127], it is deduced that V → V transfer is responsible for the dominance of the 61 → 00 pathway. It is observed that the branching ratios for transfer to 00 are consistently largest for straight chain partners. The rotational contours of collisionally populated levels are fairly broad, revealing that significant rotational excitation accompanies vibrational relaxation. Boltzmann distribution fits to the rotational contours for 00 give temperatures that are on average slightly lower than those found for small polyatomics. The observation that a reasonable amount of energy is transferred into benzene rotation suggests that it is the low-frequency modes of the collision partner that are excited. Resonant or near resonant transfer appears to be inefficient. V → V and V → R transfer both appear to be operating in relaxation of 61 benzene yet are absent in relaxation from the related case of 61 p-difluorobenzene [Mudjijono; Lawrance, W. D. J. Chem. Phys. 1996, 105, 9874]. It is suggested that this difference arises because of a reduced efficiency for V → T transfer in the benzene case due to the 112 cm-1 higher frequency of ν6 in this molecule.

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

“…These show how VRT probabilities change when di-and polyatomic molecules are involved. These follow explanations given earlier, 5,6 but arguments are quantified and demonstrate an emerging pattern that stresses the importance of AM in determining collision outcomes. v r -⌬J plots may be constructed for J-and K-changing collisions separately and (J,K) changing is readily incorporated if data are available.…”

“…4,5 Analysis indicates that relaxation is controlled by factors related to the disposal of angular momentum and is unrelated to detailed features of the intermolecular potential. These conclusions were drawn from experiments in the absence of full resolution of rotational features.…”

“…Experiments on collision-induced 6 1 →0 0 vibrational relaxation of the S 1 level of benzene by monatomic and polyatomic collision partners 4,5 showed the nuclear dynamics of momentum disposal to be the controlling factor in this VRT process. Parmenter and co-workers, 16 in a series of crossed beam experiments, convincingly demonstrated the central role played by kinematic factors in RT and VRT involving glyoxal.…”

“…1 However the number of additional excitation and relaxation routes in polyatomic molecules brings new possibilities, raising fundamental questions concerning the dynamical behavior of molecules. [2][3][4][5][6] Parmenter, 2 Rice 3 among others have drawn attention to the extraordinarily selective nature of vibrational transfer ͑VT͒ in large molecules. Waclawik and Lawrance 4,5 demonstrated that angular momentum ͑AM͒ factors exercise a controlling influence on VT in excited benzene.…”

“…[2][3][4][5][6] Parmenter, 2 Rice 3 among others have drawn attention to the extraordinarily selective nature of vibrational transfer ͑VT͒ in large molecules. Waclawik and Lawrance 4,5 demonstrated that angular momentum ͑AM͒ factors exercise a controlling influence on VT in excited benzene. Alwahabi et al 6 showed that rotational transfer ͑RT͒ rate constants in NH 2 fall exponentially with transferred AM and are unrelated to the amount of energy transferred.…”