Behaviour of collective volume excitations in aluminum near the 'critical wavevector' (transition into single particle excitation)

From energy loss experiments with 51 keV electrons the dependence of the aluminum plasmon energy on the wavevector q has been derived. The measurements are extended to q>q c =1.3 AA -1 (q c =critical wavevector) and reach q approximately=2.2 AA -1 . For q<q c the excitation energy increases approximately with q 2 ; for q>q c , however, the loss energy shows only a weak dependence on the wavevector. The results are compared with calculations of the loss function. Im(- epsilon ( omega ,q) -1 ), using a dielectric function epsilon ( omega , q) that takes into account finite electron lifetime and local field corrections. It is shown that for large q local field effects have a strong influence. The halfwidth of the loss peak has also been obtained as a function of q. It shows a nearly steplike increase near the critical wavevector.