Long-time scale spectral diffusion in polymer glass

Spectral diffusion (SD) in polymethylmethacrylate (PMMA) doped with free-ba se tetraphenylporphine is investigated at 0.5-4.2 K on a time scale of 3-10 (6) s via optical hole burning. Two contradictory results-within the framew ork of the two level system (TLS) model-are obtained. The first one is the absence of aging effects at temperatures near 4 K which puts the upper limi t for the TLS relaxation times at tens of minutes. The second one is an int ensive superlogarithmic SD on the whole time scale of the experiment, which is evidence for the presence of very slow relaxations, independent of the sample history on a time scale of up to two months. The results presented p rovide clear evidence of a deviation of SD behavior from the TLS model pred ictions at moderately low temperatures. The concept of structural relaxatio ns is applied for a qualitative interpretation of the experimental data. (C ) 2000 American Institute of Physics. [S0021-9606(00)50126-3].