ELECTRONIC EXCITATIONS IN ALKALI-METAL OVERLAYERS - I - UNRECONSTRUCTED LOW-TEMPERATURE PHASE OF LI AL/

The influence of interband transitions on collective excitations in ad sorbed alkali-metal layers is investigated for Li on Al in the unrecon structed low-temperature phase. The three-dimensional Li overlayer is described in terms of nonlocal pseudopotentials while the Al substrate is treated within the jellium model. The finite-frequency response ap propriate for incident photons is calculated within the time-dependent density-functional approach. In striking contrast to previous results for Na and K layers on Al, which reveal multipole surface and bulklik e overlayer plasmons, we find for Li overlayers only one dominant adso rbate-induced collective excitation associated with the Li multipole s urface plasmon. The bulklike Li overlayer plasmon is greatly suppresse d due to single-particle transitions induced by the Li lattice potenti al. The theoretical spectra compare well with recent photoyield measur ements for Li on Al(111).