FAST RELAXATION IN A FRAGILE LIQUID UNDER PRESSURE

The incoherent dynamic structure factor of orthoterphenyl is studied b y quasielastic neutron scattering to 240 MPa. Tagged-particle correlat ions in the compressed liquid decay in two steps. The alpha-relaxation line shape is independent of pressure, and the relaxation time propor tional to viscosity. A kink in the amplitude f(Q)(P) reveals the onset of beta relaxation. The beta-relaxation regime can be described by th e mode-coupling scaling function; amplitudes and time scales allow a c onsistent determination of the critical pressure P-c(T). alpha and bet a relaxation depend in the same way on the thermodynamic state; close to P-c(T), this dependence can be parametrized by an effective couplin g Gamma proportional to nT(-1/4).