Scaling of the rotational relaxation of tracers in o-terphenyl:: A linear and nonlinear ESR study

The rotational dynamics at long and short times of nearly spherical and cylindrical tracers in supercooled and glassy o-terphenyl is investigated by linear and nonlinear electron spin resonance spectroscopy, respectively. The short-time and long-time dynamics are characterized by the effective correlation times tau(s) and tau(l), respectively. At higher temperatures, the relaxation is exponential and the Debye-Stokes-Einstein law (DSE) holds, tau(s) approximate to tau(l) proportional to eta. eta is the shear viscosity. On cooling, the rotational dynamics is partially coupled to the viscosity. At lower temperatures the reorientation is activated. Interestingly, on the short time scale the rotational correlation times of the two tracers (tau(s)) in a wide temperature range down to the activated regime collapse in a single curve by proper scaling. The scaling does not work at long times (tau(l)).