Abstract
Spectral diffusion in polymethylmethacrylate (PMMA) doped with the chromophore zinc meso-tetraphenylporphine (ZnTPP) was investigated by stimulated photon echo spectroscopy (SPE) at three temperatures: 0.75, 1.5 and 3.0 K. Above 1 K, a distinctly non-logarithmic line broadening is observed. The data can be well described by a recent theoretical model which assumes a modified distribution of two-level systems (TLS) in amorphous solids. There is no evidence for a gap in the distribution of TLS relaxation rates. This previously observed phenomenon [H.C. Meijers and D.A. Wiersma, Phys. Rev. Lett. 68 (1992) 381; J. Chem. Phys. 101 (1994) 6927] is, most likely, related to sample heating. We present echo data below 1 K showing that our model is consistent.
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