Rotational and vibrational state dependence on intramolecular coupling and dynamics in the S₁ state of pyrazine

In this paper we report on the absolute fluorescence quantum yields, Y, from photoselected rotational and vibrational states within the S₁ manifold of pyrazine cooled in planar, pulsed, supersonic expansions of Ar. The Y data from the S₁ origin exhibit a marked J′ dependence, which can be fit by the empirical relation Y=0.124/(2J′+1) for J′=5-22. The vibrational state dependence of Y on the excess vibrational energy, E _v, exhibits an approximate exponential decrease with increasing E_v in the range E_v=0-1300 cm^-1 with a possible saturation at E_v=1300-1800 cm^-1. The low absolute Y values can be attributed to intramolecular nonradiative decay of S₁-T molecular eigenstates. The marked rotational state dependence of Y from the S₁ origin can be interpreted in terms of the intermediate level structure (ILS) provided that the dilution factor is N_eff= 16(2J′+1) for J′=5-22. This N_eff exhibits a qualitatively different J′ dependence than the number of effectively coupled states originating from a single J′K′ doorway state N (spectroscopic)=12-15 for J′=0-3, which was inferred by Kommandeur et al. [Chem. Phys. Lett. 92, 565 (1980)] from high resolution spectroscopic studies. In order to reconcile our Y data, together with the available psec time-resolved data and the spectroscopic information in terms of the ILS model, one has to assert that the range J′=3-5 marks a crossover from J′ independent to J′ dependent N_eff.