Plasmons in the Heavy Alkali Metals: Strong Deviations from RPA

The q -dependence of plasmon energy and width for the heavy alkali metals K, Rb, and Cs were measured by electron energy loss spectroscopy. For Rb, we found almost no plasmon dispersion while for Cs, the small- q dispersion turned out to be even negative. These results can be explained qualitatively but not quantitatively by recent electron liquid models including exchange and correlations. Going from K to Rb and Cs, the q -dependence of the plasmon width changes from quadratic to linear.