PROPAGATION OF COLLECTIVE MODES IN LIQUID CESIUM

The dynamical structure factor S(q,omega) of liquid cesium near its me lting point is evaluated by using Mori's memory function formalism. Fo r the calculation of the memory function we have proposed a self-consi stent method based on expressing the third-stage memory function in te rms of the scaled second-order memory function. The short-time propert ies of S(q,omega) are exactly incorporated by the use of frequency sum rules up to sixth order, whereas its long-time property is fixed thro ugh the scaling parameter. It is found that our approach predicts the collective density excitation peak in S(q,omega) for q<1.2 Angstrom(-1 ) at a frequency that is in agreement with experimental results, thoug h with an overestimated peak height. For q=1.8 and 2.0 Angstrom(-1) we again found a omega not equal 0 peak and a shoulder in S(q,omega), re spectively. In this region, experimentally, only a shoulder has been s een.