Are the spectral trails of single molecules consistent with the standard two-level system model of glasses at low temperatures?

The spectral trail of a single molecule is a 2D plot of many repeated spect ra displayed as a function of time. Here, we present an analysis of the spe ctral trails of some 70 tenylene molecules in polymers. We compare the feat ures of individual trails to the standard model of two-level systems (TLSs) in glasses at low temperatures, and we find a good overall agreement for a majority of molecules. However, mon than 30% of the molecular trails canno t be reconciled with the standard model. For example, we found three-level systems, evidence for a strong coupling between some TLSs, and spectral dri fts. Whereas our data confirm that the standard TLS model describes large e nsembles of TLSs quite satisfactorily, this may result only from a cancella tion of the deviations. Therefore, the application of the model to explain the spectral diffusion of individual molecules must be questioned in each p articular case. (C) 1999 Elsevier Science B.V. All rights reserved.