Temperature dependence of collective modes in liquid sodium

We present results for the purely coherent dynamic structure factor of liqu id sodium obtained from inelastic X-ray scattering experiments. Measurement s were carried out at four temperatures between 388 and 1173 K. The dispers ion relations for longitudinal collective excitations in the liquid could b e determined for each temperature. It is found that the adiabatic sound vel ocity given as the slope of the dispersion at lower Q is increased compared to the bulk value. This 'positive dispersion' still exists even at the hig hest temperatures attained in this experiment. From the data an estimate is given for the shear modulus of the liquid along the temperature range cove red by our experiment. (C) 1999 Elsevier B.V. All rights reserved.