Spectral Trajectories of Single Molecules

Spectral shifts of single-molecule lineshapes are common in many systems, including crystals, polymers, and even polycrystalline Shpol skiii matrices [75, 76], Spectral diffusion effects will be described in detail experimentally in Section 1.4, with theoretical analysis in Section 1.5. In this section and in the next, the principal experimental methods for studying this behavior will be described. Again taking pentacene in p-terphenyl as an example. Fig. 10(a) shows a sequence of fluorescence excitation spectra of a single molecule taken as fast as allowed by the available SNR. The laser was scanned once every 2.5 s with 0.25 s between scans, and the hopping of this molecule from one resonance frequency to another from time to time is clearly evident. 

The first question which should be asked when such behavior is observed is this is the effect spontaneous, occurring even in the absence of the probing laser radiation, or is it light-driven, i.e., produced by the probing laser itself. To answer this question, it is usually necessary to observe the spectral shifting behavior as a function of the 

Spectral trajectories contain much information about the stochastic behavior of the single molecule. If a simple measure of the average time scale of spectral shifts is required, it is useful to calculate the autocorrelation of the spectral trajectory, C u(t), given by 

---