The Fluorescence Lifetime of Tetracene Vapour

The study of the vibrational energy dependence of radiative and radiationless processes has been a rich field for experiment and theory alike [l-S]. Although a good deal of progress has been made in both areas, there are many aspects of the problem which remain unclear. One of the most important of these aspects is the relative importance of internal conversion and intersystem crossing in determining the lifetime of the higher vibrational levels of the excited singlet state. From a theoretical point of view the problem seems insoluble at present because no accurate values of the electronic matrix elements for internal conversion and intersystem crossing have yet been calculated [9]. Consequently, theoretical calculations of radiationless transition rates have concentrated on the vibrational part of the problem and much effort hhs been put into calculations of Franck-Condon factors and densities of vibrational levels either separately [IO, 1 l] or, more recently, as weighted_ sums [2]. It is foind both experimenjally and theoretically that the rate of radiationless processes ,usually increases with increasing vibrational energy content of the excited singlet state. This has been explained [2,7] in terms 6.9 the increas. .irig ability of thk Optical (totally symmetric) C-C : modes to act as accepting modes as the vibrational energy content of the molecule is increased.