INTERMEDIATE CASE RADLATIBNLESS DECAY: THE EXCmD STATE DYNaMICS OF PYRAZINE

This paper reports new measurements of the non-exponential fluorescence decay of pyrazine, covering the time range 0.1-200 ns. The results of our measurements remove the inconsistencies between experiment and the Frad-LahmaniTramer-Tric model of the radiationless decay in this molecuIe. In particular, our data show that the triplet component of the mixed singlet-triplet levels does increase with increasing triplet density of states. The elective density of tripler levels determined from our experimental data exceeds the theoretical density of vibrational levels. We propose that at the excitation levels achieved in the triplet manifold there is extensive scrambling of rotational states, but that conservation of nuclear spin states permits a level with total angular momentum quantum number 1 to couple to only (2J+ 1)/4 of the 2J-tI rotational levels built on one vibration. The appropriate theoretical density of states to be compared with experiment is then obtained by multiplying the vibrational density of states by J/2. Good agreement is found between experimentally determined and calculated densities of states.