SINGLET TRIPLET INTERACTION AND MAGNETIC-FIELD EFFECTS ON FLUORESCENCE IN PYRIMIDINE VAPOR

Zeeman quantum beats, magnetic depolarization (the so-called Hanle effect) and magnetic quenching are observed in fluorescence of pyrimidine vapor on excitation into S1 and interpreted in terms of singlet-triplet interaction between a zero-order singlet state of S1 which carries the absorption intensity from the ground electronic state and zero-order triplet states located near S1. Fluorescence depolarization at zero field is attributed to an internal magnetic field resulting from the hyperfine interaction at the excited singlet-triplet mixed level, and the hyperfine quantum beats are clearly observed on excitation at the R(0) line of the 0-0 band of pyrimidine-d4.