Short-time rotational relaxation in supercooled and glassy o-terphenyl: a nonlinear electron spin resonance study by using molecular probes with different symmetries

The short-time rotational dynamics of nearly spherical and cylindrical spin probes in supercooled and glassy o-terphenyl is investigated by nonlinear electron spin resonance spectroscopy. At higher temperatures, the relaxation is exponential and the Debye-Stokes-Einstein law holds, tau proportional to (eta/T). tau is the rotational correlation time, eta the shear viscosity and T the temperature. On cooling, the coupling of the short-time rotational dynamics and the shear viscosity weakens, and a fractional Debye-Stokes-Einstein law tau proportional to (eta/T)(xi), with xi constant, is observed. At the lower temperatures, the rotation is activated. A scaling of the short-time rotational dynamics of the different molecular probes is obtained for the overall range of temperature.