Linear and non-linear electron spin resonance study of the rotational diffusion of a molecular tracer in supercooled o-terphenyl

The long-time and the short-time rotational relaxation of a molecular tracer in supercooled o-terphenyl (OTP) have been studied by linear and non linear electron spin resonance (ESR) spectroscopy, respectively. At higher temperatures the related correlation times, tau(l) and tau(s), are equal within the experimental errors, i.e. the relaxation is exponential, and well coupled to the viscosity, eta. On cooling, tau(s), and, at lower temperatures, tau(l), are partially coupled to the viscosity in agreement with the fractional Debye-Stokes-Einstein (DSE) law tau(i) proportional to (eta/T)(xi i), i = 1, s with xi(i) constant. For T < 330 K the rotational correlation function is not exponential. (C) 1998 Elsevier Science B.V. All rights reserved.