Structural and orientational relaxation in supercooled liquid triphenylphosphite

Structural and orientational relaxation processes in the glass-forming liquid triphenylphosphite (TPP) were studied by impulsive stimulated scattering, a time-resolved four-wave mixing technique. The a structural relaxation processes which can couple to the longitudinal and shear acoustic modes are analyzed phenomenologically in terms of a distribution of relaxation times f_L(τ_L,T) or f_S(τ_S,T), respectively, in the temperature range of 270-240 K. The two distributions appear to be identical, and undergo marked broadening as the sample is cooled. They are best described by a stretched exponential relaxation function whose exponent decreases with temperature. The molecular orientational relaxation times τ_or follow an Arrhenius temperature dependence and are longer than the average relaxation times 〈τ_L〉 and 〈τ_S〉 at high temperature, but converge as the temperature is lowered. We conclude that the observed orientational relaxation is decoupled from the α structural relaxation process and is a manifestation of the β orientational relaxation process. The low frequency intramolecular mode observed in the experiment was found to be essentially independent of temperature. A picture of structural and orientational relaxation processes in supercooled TPP consistent with recent theories of structural relaxation processes in supercooled liquids is proposed.