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 time-dependent local-density approximation. The Na adlayer and a few outermost Al layers are described fully three dimensionally using norm-conserving 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) exhibits a single broad plasmon peak, which is a mixture of the Na multipole surface plasmon and the bulklike Na overlayer plasmon. In striking contrast, the Na plasmon peak disappears completely from the spectrum in the room-temperature phase (substitutional adsorption). These observations are in good agreement with recent experiments. It is predicted that the Al vacancies in the room-temperature phase induce a large change in the excitation spectrum of the Al substrate.