Fluorescence and nonradiative relaxation from vibronic levels of S1 pyrimidine : Effects of molecular rotation and intramolecular vibrational redistribution.

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Emission characteristics of pyrimidine vapor which were measured after excitation into various S1 vibronic levels with relatively low excess vibrational energies (ΔE) below 2650 cm−1 were found to depend markedly upon ΔE. The emission from a vibronic level with ΔE<1000 cm−1 (e.g., 6a1 or 11) is sharp and attributable to the fluorescence from the initially prepared vibronic level (IPL), and that from a vibronic level with ΔE>1000 cm−1 (e.g., 121, 6a2, or 122) involves broad fluorescence besides the IPL fluorescence. At low pressure, the broad fluorescence consists of fast and slow components with lifetimes comparable to those for the corresponding components of the IPL fluorescence. The broad fluorescence exhibits another slow component which is characterized by a less tendency to undergo collisional quenching. The quantum yield of the slow component of the IPL fluorescence clearly depends upon molecular rotation, whereas that of the broad fluorescence appears to show little or no rotational-level dependence. On the basis of these observations, the broad fluorescence is assigned to the one originating from S1 vibronic levels that can be reached directly from the IPL through intramolecular vibrational redistribution (IVR) or indirectly via triplet levels. There exists no evidence to show that the IVR is affected by molecular rotation.
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