Thermal irreversibility in optically labeled low-temperature glasses

We present an investigation of irreversible features of thermal broadening of persistent spectral holes. The investigation is based on temperature-cycling hole-burning experiments performed with a variety of organic glasses doped with rather different probe molecules. The results show a rich temperature dependence. They can, however, be interpreted in terms of the well-known spectral diffusion models, in which we introduced a freezing condition to account for thermal irreversiblity. There is a tunneling regime for low temperatures and an activated regime for high temperatures. In the tunneling regime the broadening is linear in T; in the activated regime it increases with $T^{3/2}$ and logarithmically with time. From the transition region the quantity ${\mathrm{md}}^2$, with m being the tunneling mass and d the distance, can be determined.