He-atom scattering study of the vibrational modes of alkalis on Al(1 1 1)

He-atom scattering measurements were performed for various sodium, potassium and cesium adsorption structures on Al(1 1 1). Constant energy vibrational modes were observed to be a common feature for these structures. Such modes were observed for both the intermixed and the on-surface adsorption structures of Na, K, and Cs. This implies that the adatoms vibrate essentially independently of each other in a wide variety of structures and adsorption geometries, including top-site and substitutional-site geometries. The energies of these modes depend strongly on their polarization and the adsorption geometry, however. Connections are made between these data and those previously reported for alkali adsorption vibrational modes.