Chemical timing 4. The rovibronic level structure associated with intramolecular vibrational redistribution in S1 p-difluorobenzene

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Abstract
The method of chemical timing for obtaining picosecond time‐resolved fluorescence spectra has been applied to the measurement of intramolecular vibrational redistribution (IVR) in S1 p‐difluorobenzene. In this report, a more detailed examination of the spectroscopy of some vibrationally mixed levels is used in conjunction with the previously measured IVR parameters to construct a generalized picture of the rovibronic level structure giving rise to IVR. Strong, off‐resonance interactions are observed to mix levels separated by several cm−1, contributing additional discrete structure to the spectrum but not directly affecting the IVR dynamics. The dynamics, instead, reflect on‐resonance interactions between levels separated only by a few tenths of a wave number that give rise to congested background emission. Evidence is seen in comparisons between bulb and jet spectra for the involvement of vibration‐rotation coupling in both the off‐resonance and the on‐resonance interactions. A particularly important and subtle rotational effect is the change of the coupled vibrational field with rotational excitation due to the variation of rotational constants with vibrational excitation, leading to a smoother congested background.