Improved data analysis for molecular dynamics in liquid CCl4.

Previously reported inelastic x-ray scattering spectra of a typical van der Waals molecular liquid CCl 4 were reanalyzed using a generalized Langevin formalism with a memory function including a thermal and two viscoelastic relaxation processes and simple sparse modeling. The obtained excitations of longitudinal acoustic phonons show a largely positive deviation from the hydrodynamic value by about 57%, much larger than about 37% by the previously reported damped harmonic oscillator result. Such large values of fast sounds in molecular liquids larger than 15-20% of typical liquid metals are interpreted as extra energy losses for terahertz phonons by molecules' vibrational and rotational motions. The rates of the fast and slow viscoelastic relaxations in the memory function at low Q indicate large values, about 0.5 and 2 ps, which correspond to the vibrational and rotational motions of CCl 4 molecules, respectively. These values are larger than those of the typical polar molecular liquid acetone, which may reflect the heavier atomic mass of CCl 4. The Q dependences of the viscoelastic relaxation rates are discussed in terms of the lifetime and propagating length of the terahertz phonon oscillations. The microscopic kinematic longitudinal viscosity was obtained from the viscoelastic relaxation magnitudes and times, rapidly decreasing with Q from a macroscopic value at Q → 0. • Generalized Langevin formalism was applied to inelastic x-ray scattering spectra of liquid CCl 4. • A large deviation from hydrodynamic prediction was found in the phonon dispersion relation. • Fast sound in terahertz oscillations originates from viscoelastic damping by vibrational and rotational molecular motions. [ABSTRACT FROM AUTHOR]