Structural relaxations in disordered solids below Tg: Study by thermal-cycling single-molecule spectroscopy

Structural relaxation processes in disordered solids on the length scale of nanometers have been studied in a broad temperature range, from 4.5 K up to the glass transition, with single-molecule spectroscopy and thermal cycling experiments. Irreversible changes of single-molecule spectra after thermal cycles were observed and attributed to relaxation processes in the local environment of the probe molecules of tetra-tert-butylterrylene in polyisobutylene (PIB) and terrylene in solid ortho-dichlorobenzene. The effects of the relaxation processes on the individual parameters of low-energy matrix excitations (tunneling two-level systems and low-frequency vibrational modes) were analyzed, as well as the temperature dependence of the efficiency of relaxation processes in PIB. The results prove that the local matrix relaxations are characterized by a spatially varying distribution of activation energies.