Dynamics of liquidN2atT=66.4K studied by neutron inelastic scattering

Neutron inelastic scattering from liquid ${\mathrm{N}}_2$ at $T=66.4\mathrm{}$ K and saturated vapor pressure is presented in the form of the normalized coherent nuclear-scattering law $S_{n,\mathrm{coh}}(\kappa , \omega )$. Our results at the wave vector, $\kappa =0.1\mathrm{}$ ${\mathrm{\AA }}^{-1\mathrm{}}$ are consistent with recent computer simulation showing that the sound mode continues to propagate for larger wave vectors than described by linear hydrodynamic theory. At larger wave vectors no evidence is found of propagating phononlike excitations in this liquid, but the observed spectra are broad and centered around the mean recoil energy of a Boltzmann gas. Using Sears' partial-wave expansion to separate the scattering owing to rotational motion from that owing to the motion of the molecular centers, we compare the latter to the result obtained from liquid argon, appropriately scaled according to the principle of corresponding states. The over-all agreement is good, but significant deviations from scaling are observed and briefly discussed in terms of anisotropic interactions. Further, the information obtained on rotational relaxation is compared with recent experiments and theories.