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 alpha stru ctural relaxation processes which can couple to the longitudinal and s hear acoustic modes are analyzed phenomenologically in terms of a dist ribution of relaxation times f(L)(tau(L),T) or f(S)(tau(S),T), respect ively, in the temperature range of 270-240 K. The two distributions ap pear to be identical, and undergo marked broadening as the sample is c ooled. They are best described by a stretched exponential relaxation f unction whose exponent decreases with temperature. The molecular orien tational relaxation times tau(or) follow an Arrhenius temperature depe ndence and are longer than the average relaxation times <tau(L)> and < tau(S)> at high temperature, but converge as the temperature is lowere d. We conclude that the observed orientational relaxation is decoupled from the alpha structural relaxation process and is a manifestation o f the beta-orientational relaxation process. The low frequency intramo lecular mode observed in the experiment was found to be essentially in dependent of temperature. A picture of structural and orientational re laxation processes in supercooled TPP consistent with recent theories of structural relaxation processes in supercooled liquids is proposed.