Abstract

As was recently demonstrated¹ using highly efficient fluorescence excitation spectroscopy with N.A. = 0.98 collection optics, single molecules of pentacene in a p-terphenyl crystal can be detected with signal-to-noise ratios (SNR) as high as 30 in a 1 -Hz bandwidth at low temperatures. As a result, it has become possible to study the electronic absorption spectra of individual impurity molecules in solids in great detail. Our measurements include precise determination of the lifetime-limited linewidth and the temperature dependence of the optical dephasing, both of which agree with earlier measurements on large assemblies of molecules. Surprisingly, it was also found that some molecules absorbing in the wing of the inhomogeneous line change their absorption frequency with time in a random fashion. This spectral jumping behavior (which was quite unexpected in the crystalline p-terphenyl matrix but reminiscent of amorphous systems) is termed spectral diffusion and is thought to be caused by an instability in the local environment of the single pentacene absorbers that is driven by phonons present at 1.5 K. The experiments revealed that the jumps in frequency space in general are not restricted to occur between a small number of accessible states. In fact, the spectral diffusion processes observed are very rich and complex and range from discontinuous jumps between several states to quasicontinuous wandering in frequency space.

© 1991 Optical Society of America