Non-exponential decay in pyrazine single vibronic level fluorescence

Abstract

A new theoretical analysis of the flurescence decay from low single vibronic levels of pyrazine is presented in terms of its known electronic states. Pressure, or more generally the environment, plays a key role in governing the radiationless decay. At low pressures two radiationless decay channels are prominent. One is direct and involves internal conversion to the ground electronic state with a characteristic lifetime < 10 −9 s which is shorter than the radiative one, while the other is indirectly to the ground state via the sparse triplet manifolds with a characteristic lifetime longer than the radiative one. Only the ground state provides a sufficiently dense manifold to allow for irreversible, radiationless decay. At higher pressures an additional decay channel is available. The sparse manifold of the lowest triplet state becomes a quasi-continuum and irreversible decay to it is allowed with a characteristic lifetime considerably shorter than that of both the radiative and internal conversion. The measured lifetimes and quantum yields can be understood in terms of this model and without the need to invoke unrealistic molecular characteristics.