Tracer diffusion in salol in the supercooled state

The tracer diffusion coefficient of camphorquinone in salol has been measured by forced Rayleigh scattering technique at temperatures from 266 to 220 K. Over this temperature range, the shear viscosity of salol changes by nearly nine orders of magnitude. The diffusion coefficient is found to be much faster than the value predicted by the Stokes–Einstein equation. The enhancement of the diffusion coefficient is interpreted as due to the rotation–translation coupling mechanism, a mechanism commonly invoked to account for the intensity of the shear wave doublet in the depolarized Rayleigh scattering spectrum of a rotationally anisotropic molecular liquid in the supercooled state. The rotation–translation coupling parameter extracted from the translation diffusion measurement is compared with that obtained from the depolarized Rayleigh scattering spectrum of salol. Qualitative agreement between the two types of experiments is obtained.