Vibrational energy transfer from levels below 410 cm−1 in <i>S</i>1 <i>p</i>-difluorobenzene. III. Different propensity rules for polyatomic partners

2003

J. Phys. Chem. A

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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.

Relaxation of the 6 Vibrational Level in ¹B₂_u Benzene by Polyatomic Colliders at Ultralow Temperatures

Waclawik

2003

J. Phys. Chem. A

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State-to-state vibrational energy transfer in S1 p-difluorobenzene at intermediate state densities: A change in propensity rules

Mudjijono

The Journal of Chemical Physics

1998

Collision-induced vibrational energy transfer has been studied from two vibrational levels at intermediate state density in S1 p-difluorobenzene in a supersonic free jet expansion at ∼30–40 K. Transfer was studied from the 7181 (Evib=751 cm−1) and 84 (Evib=733 cm−1) states where ρvib is ∼0.4 states per cm−1. Data were obtained for He, Ne, H2, and D2 for both levels and also for Ar and Kr for 7181. There is some doubt concerning the influence of predissociation of van der Waals complexes on the spectra for these latter partners. The data analysis for 7181 is quantitative for all collision partners. For 84 the analysis is quantitative for H2 and D2 but qualitative for He and Ne because of poor signal levels. The state-to-state propensity rules in this region of the vibrational manifold are compared with those observed at lower state densities, particularly those from 82. The main feature to emerge is a lack of predictability of the major relaxation pathways. There is a clear increase in the importance of transfers involving multiple changes in vibrational quanta for all situations studied and at times such transfers totally dominate. This occurs in spite of the possibility for loss of one quantum of ν8, which is a very efficient channel in transfer from 82. Collision partners that show similar state-to-state branching ratios for 82 show quite different branching ratios for 7181 and for 84.

Angular momentum influences on vibrational relaxation pathways from 61 benzene

Waclawik

The Journal of Chemical Physics

1998

Vibrational energy transfer from the 61 level of S1(1B2u) benzene has been studied at low collision energies in supersonic free jet expansions for the collision partners H2, D2, N2, CH4, C2H2, and c-C3H6. Three of the four accessible vibrational relaxation channels in S1 benzene are found to be significantly populated: the 162 level, the spectrally unresolved 111 and 161 levels, and the 00 level. A small amount of transfer to the 41 level was observed with H2 as a collision partner. It is found that: (i) transfer to 00 is generally efficient; and (ii) the state-to-state branching ratios change substantially with collision partner. This is quite different from the trends observed for monatomic collision partners, for which transfer to 00 is absent and the state-to-state branching ratios are largely independent of the collision partner’s identity [E. R. Waclawik and W. D. Lawrance, J. Chem. Phys. 102, 2780 (1995)]. It is further observed that the rotational contours of collisionally populated levels change. For a particular collision partner the extent of rotational excitation in the destination level increases with increasing vibrational energy gap. For a particular destination level there is considerable variation in rotational excitation amongst collision partners. The state-tostate propensity differences between monatomic partners and diatomics and small polyatomics are suggested to arise because angular momentum constraints are influencing the vibrational state-to-state branching ratios. 61→00 transfer is most affected: it is observed only when the collision partner can accept energy as rotational motion, and its branching ratio is particularly sensitive to the collision partner identity.