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

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Abstract
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>qc=1.3 AA-1 (qc=critical wavevector) and reach q approximately=2.2 AA-1. For qqc, 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.