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

Abstract

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, EV, exhibits an approximate exponential decrease with increasing EV in the range EV=0–1300 cm−1 with a possible saturation at EV=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 Neff=16(2J′+1) for J′=5–22. This Neff 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 Neff.