SPECTRAL DIFFUSION AND INDIVIDUAL 2-LEVEL SYSTEMS PROBED BY FLUORESCENCE OF SINGLE TERRYLENE MOLECULES IN A POLYETHYLENE MATRIX

We study the influence of low-energy matrix excitations on the line wi dths and fluorescence correlation of single terrylene molecules in por yethylene at helium temperatures. The histogram of line widths has a c utoff at the natural line width of terrylene, showing that for some mo lecules dephasing and spectral diffusion are negligible on the measure ment time scale. The shape of the histogram can be qualitatively inter preted by means of a simple model for spectral diffusion. The line wid ths of different molecules show different temperature dependences. The correlation method is then applied to a time-resolved study of the in tensity fluctuations of single molecule fluorescence. Many possible sh apes of correlation functions appear, spanning many decades of relaxat ion times. We believe single two-level systems (TLSs) are the cause of the well defined exponential steps we observe. In some cases, the two positions of a single molecule's line can be identified in the spectr um. They present the same time constant in their correlation functions and jump together to a new frequency. The dependence of the correlati on on exciting flux shows that jumps can be spontaneous or photo-induc ed. The study of the rate as a function of temperature shows clear pow er laws which we attribute to tunnelling of the TLS assisted by one (T -1) or two (T-3) acoustic phonons. In one case, we find an Arrhenius a ctivation of the rate, which could be explained by dressing the standa rd TLS model with slow matrix modes.