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 $\alpha$-relaxation line shape is independent of pressure, and the relaxation time proportional to viscosity. A kink in the amplitude $f_Q\left(P\right)$ reveals the onset of $\beta$ relaxation. The $\beta$-relaxation regime can be described by the mode-coupling scaling function; amplitudes and time scales allow a consistent determination of the critical pressure $P_c\left(T\right)$. $\alpha$ and $\beta$ relaxation depend in the same way on the thermodynamic state; close to $P_c\left(T\right)$, this dependence can be parametrized by an effective coupling $\Gamma \propto {\mathrm{nT}}^{-1\mathrm{}/4}$.

• Received 5 June 1997

DOI:https://doi.org/10.1103/PhysRevLett.80.2374