The Mechanism of Occurrence of Dual Fluorescence in Intermediate Case Molecules as Viewed From Rotational Effects

Fluorescence decays of jet-cooled pyrazine excited at various positions of rovibronictransitions within the 0–0 band were measured by the use of a pulsed dye laser pumpedby an excimer or a nitrogen laser. The experimental data thus obtained on the number(N) of the triplet states coupled to the initially excited singlet rovibronic state wereanalyzed and interpreted by the aid of computer simulation using an asymmetric rotorprogram. The results indicate that the extraordinarily great variation of N with excitationenergy and unreasonably small values of N hitherto reported in the literature can be understoodin terms of the conventional theory based on the singlet–triplet mixed statemodel which has been developed for elucidating the mechanism of the occurrence ofdual fluorescence in intermediate case molecules.