Dynamics of amorphous polymers in the temperature region 2–7 K where the standard model of low-temperature glasses begin to fail: studies by single molecule spectroscopy and comparison with photon echo data

Spectra of single tetra-tert-butylterrylene molecules incorporated in purely amorphous polyisobutylene matrix have been measured at 2, 4.5 and 7 K (244, 381 and 187 molecules, correspondingly). This is a temperature region, where the main assumptions of the standard two-level system (TLS) model of low-temperature glasses begin to be not valid. At T=2 K the main parameters of most of the registered spectra were found to be consistent with the standard TLS model. At T=4.5 and 7 K some deviations from predictions of this model were observed. The detailed analysis reveals that increasing of temperature leads to additional, in comparison with the predictions of the standard TLS model, line broadening of spectral peaks. This additional line broadening was attributed to the influence of quasi-local low-frequency modes (LFMs) of the amorphous matrix in system under study at T=4.5 and 7 K.