Abstract

A central open fundamental question in exciton dynamics in molecular crystals concerns transport coherence of excitons, particularly at low temperatures. A large amount of effort has been spent over many years on this issue but clear answers have not emerged. Old suggestions that the temperature dependence of energy transfer rates is indicative of transport coherence have been shown to be untenable, or at least inconclusive, since a variety of alternate factors having little to do with coherence can be shown to lead quite reasonably to explanations of those observations. Careful reanalysis^1,2 of sensitized luminescence experiments is making clear that the role that capture plays in them (relative to exciton motion) can be considerable and that that category of observations is not well suited to investigations of coherence. We have recently found^3,4 that Ronchi grating experiments constitute an ideal candidate for definitive coherence studies. The observable is the time-dependent delayed fluorescence arising from mutual annihilation of triplet excitons. The parameter under experimental control is the extent of the spatial inhomogeneity of the triplets manipulated through the ruling period of the Ronchi grating covering the crystal during illumination.⁵ We present a detailed analysis of the suggested experiment and describe the expected effect of two separate factors on the outcome of the experiments: (i) finite degree of coherence and (ii) finite rate of vibrational relaxation.

© 1984 Optical Society of America