Density fluctuations in liquid rubidium. I. Neutron-scattering measurements

We report neutron-scattering measurements of the coherent scattering function $S(Q,\omega )$ of liquid rubidium at 315 K, in the range of wave vectors $1.25\le Q\le 5.5$ ${\mathrm{\AA }}^{-1\mathrm{}}$. In this range there is no evidence of peaks at finite $\omega$ in $S(Q,\omega )$ plotted at constant $Q$. On the other hand the Fourier transform $F(Q,t)\mathrm{}$ exhibits structure, notably for $Q=2.0\mathrm{}$ ${\mathrm{\AA }}^{-1\mathrm{}}$, which indicates at least two characteristic (wavelength-dependent) relaxation times in the liquid. For wave vectors >3.0 ${\mathrm{\AA }}^{-1\mathrm{}}$, $F(Q,t)\mathrm{}$ may be characterized by a single relaxation time. These results, in conjunction with our results for $Q<\mathrm{}1.0\mathrm{}$ ${\mathrm{\AA }}^{-1\mathrm{}}$, offer the possibility of detailed comparisons with models of the liquid state and with molecular-dynamics calculations.