Abstract

All spectroscopies, whether explicitly time-resolved or steady-state, are sensitive to fluctuations in the spectroscopic environment of the chromophore on the time scale of the radiation-matter interaction(s). Simple "two-state jump" models that assume random hopping between just two spectroscopically distinct environments have been well studied. Such models can provide some qualitative insight into the influence of fluctuations on a particular spectroscopy even if the actual system accesses a continuous distribution of states, as is usually the case for chromophores in liquids. The usual two-state models assume that the states differ in their transition frequencies to one or more accessible excited states. In linear spectroscopies, such models predict the well-known coalescence from two discrete resonances to a single broad one which then motionally narrows as the fluctuation rate increases. For multiphoton spectroscopies the effects are more complicated; in particular, for monochromatically excited spontaneous emission, increasing the fluctuation rate causes evolution from a sharp, "resonance Raman-like" spectrum to one having increasing contributions from broad emission.¹ The transition frequency fluctuations constitute a source of electronic pure dephasing at the level of the chromophore's density matrix, generating a "fluorescence" component to the emission.

© 1996 Optical Society of America