Fast Relaxation in a Fragile Liquid under Pressure

The incoherent dynamic structure factor of orthoterphenyl is studied by quasielastic neutron scattering to 240 MPa. Tagged-particle correlations in the compressed liquid decay in two steps. The α-relaxation line shape is independent of pressure, and the relaxation time proportional to viscosity. A kink in the amplitude f_Q\(P\) reveals the onset of β relaxation. The β-relaxation regime can be described by the mode-coupling scaling function; amplitudes and time scales allow a consistent determination of the critical pressure P_c\(T\). α and β relaxation depend in the same way on the thermodynamic state; close to P_c\(T\), this dependence can be parametrized by an effective coupling γ~nT^-1/4.