Fluorescence polarization from ¹B₁ pyrimidine: Evidence for intramolecular vibration-rotation energy transfer in a highly excited molecule

We have measured the polarization of ensemble-averaged fluorescence from the Og and 5 vibronic regions of 1B1 pyrimidine, in order to determine the extent of intramolecular vibration-rotation energy transfer at high energies. The polarization of the 000 is 15.54%±0.35%, and decreases smoothly to 6.93%±0.22% at E vib=3700 cm-1. From comparisons with model calculations, we conclude that highly excited pyrimidine undergoes nearly statistical rotational motion during its fluorescence lifetime. Additional experiments in a supersonic expansion show that the rotational temperature dependence of the polarization is weak. © 1986 American Institute of Physics.