Jump reorientation of a molecular probe in the glass transition region of o-terphenyl

The reorientation process of a molecular probe dissolved in the fragile glass former o-terphenyl is studied via electron spin-resonance spectroscopy. Owing to the good resolution of the ESR lineshape in the so-called slow-motion regime, we are able to discriminate between diffusive and jump rotations. In the region of the glass transition (T-g - 31 K < T ( T-g + 17 K) the results are at variance with the diffusion models and are fairly well reproduced by assuming that the probe jumps with steps phi = 80 degrees(-5 degrees)(+10 degrees). The temperature dependence of the correlation time is well described, in the temperature range investigated, by a double Arrhenius law broken at T-g. The findings are compared with previous NMR, dielectric, fluorescence and molecular dynamics studies.