Spectral diffusion and individual two-level systems probed by fluorescence of single terrylene molecules in a polyethylene matrix

Abstract

We study the influence of low-energy matrix excitations on the line widths and fluorescence correlation of single terrylene molecules in polyethylene at helium temperatures. The histogram of line widths has a cutoff at the natural line width of terrylene, showing that for some molecules dephasing and spectral diffusion are negligible on the measurement time scale. The shape of the histogram can be qualitatively interpreted by means of a simple model for spectral diffusion. The line widths of different molecules show different temperature dependences. The correlation method is then applied to a time-resolved study of the intensity fluctuations of single molecule fluorescence. Many possible shapes of correlation functions appear, spanning many decades of relaxation 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 spectrum. They present the same time constant in their correlation functions and jump together to a new frequency. The dependence of the correlation on exciting flux shows that jumps can be spontaneous or photo-induced. The study of the rate as a function of temperature shows clear power 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 activation of the rate, which could be explained by dressing the standard TLS model with slow matrix modes.