ELECTRONIC EXCITATIONS IN ALKALI-METAL OVERLAYERS - II - EFFECTS OF SUBSTITUTIONAL ADSORPTION OF NA AL(001)/

The collective excitations in the c(2X2) Na overlayers on Al(001) are investigated by a dynamical linear-response calculation within the tim e-dependent local-density approximation. The Na adlayer and a few oute rmost Al layers are described fully three dimensionally using norm-con serving pseudopotentials while the interior of the metal is treated as semi-infinite jellium with an electron density of Al. The excitation spectrum in the low-temperature phase (hollow-site adsorption) exhibit s a single broad plasmon peak, which is a mixture of the Na multipole surface plasmon and the bulklike Na overlayer plasmon. In striking con trast, the Na plasmon peak disappears completely from the spectrum in the room-temperature phase (substitutional adsorption). These observat ions are in good agreement with recent experiments. It is predicted th at the Al vacancies in the room-temperature phase induce a large chang e in the excitation spectrum of the Al substrate.