Chemical Physics 19 (1977) 377-386 0 North-Holland Publishing Company NON-EXPONENTlALDECAYINPYIUZINE SlNGLEVlBROh'ICLEVELFLUORESCENCE Gad FISCHER and Ron NAAMAN zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDC Department ofChemistry, Ben Gurion University of the Negev, Beer Shevo. Isrrrel Received 23 August 1976 A new theoretical analysis of the fhtorescencc decay from low single vibronic levels of pyratine is presented in terms of its known electronic states. Pressure, or more generally the environment, plays a key role in zyxwvutsrqponmlkjihgfedcbaZYXWVU governing the radiationless de- cay. 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 < 104 s which is shorter than the radiative one, while the other is in- directly to the ground state via the sparse triplet manifolds with a characteristic lifetime Ionger 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 ir- reversible decay to it is allowed with a characteristic lifetime considerably shorter than that of both the radiative and inter- nal conversion. The measured lifetimes and quantum yields can be understood in terms of this model and without the need to invoke unrealistic molecutar characteristics. 1. Introduction The occurrence of two component fluorescenceis now a welldocumented fact [l-7] . It has been estab- lishedfor pyrazine at low vapor pressures(< I torr) and both kinetic and quantum mechanical interpreta- tions have been put forward. Despite the measure of success achieved in account- ing for the experimentallymeasuredquantum yields and lifetimesof the two components of the bi-expo- nential fluorescence decay, and in interpreting their pressureand excessvibrationalenergy dependencies, many questions remain unanswered. More specitically, Frad et aI. [3] point out that they are unabIe to ac- commodate the derived density of states with that corresponding to the vibrationalstructure of pyrazine. The problem is the more serioussince even the density of ‘strongly coupled states - the ones purported to be responsible for the long lived fluorescence decay com- ponent -is largerthan that available. An associated difficulty concernsthe constancy of the derived dens- ity of strongly coupled states over a particular energy range despite an actual increaseby an order of rnagni- tude in the overall state density. These are not isolated problems restricted to pyra- zine but are encountered when a similar approach is applied to other moleculesthat display related behavi- or. In the absence of external perturbations (pressure, crystal etc.) their behavior may be classified in the in- termediate case [S]. A multiexponentialfluorescence decay obtains. Under the action of the perturbation, their behavior correspondsto the statisticallimit [8] and only exponential fluorescencedecay appears. AI- though Nitzan, Jortner and Rentzepis [2] gavetheoret- ical expressionto this problem the application of the theory to real physicalsystemsis not straight-forward. Tramer and co-workers[3] interpreted the bi-expo- nential decay in terms of a model consisting of an in- itially excited state strongIy coupled to a number of states of a quasi-continuum and weakly coupled to the remaining states of the quasi-continuum. All the states lie within the relaxation linewidthof the initial- ly excited state. Kommandeur and co-workers[6] em- ployed a similar model to explain the fluorescencede- cay from glyoxal and biacetyl and McDonald and Brus [4] and Soep and Tramer [S] applied the model to quinoxaline fluorescence. All these works were plagued by a common problem. The derived density