VIBRATIONS OF ALKALI-METAL ATOMS CHEMISORBED ON THE AL(111) SURFACE

By use of high-resolution electron-energy-loss spectroscopy, atomic vibrations of the sodium and the lithium chemisorbed Al(111) surfaces have been investigated as a function of coverage. Owing to the screening of the adsorbate-adsorbate interaction by electrons of the Al atoms located between the alkali-metal atoms, the vibrational energy of Na on Al(111) remains almost constant (11.6-12.6 meV) over the coverage range of theta = 1/15-1/3. The vibrational energy of Li on Al(111) also remains constant (17.6-18.1 meV) from theta = 1/30 to theta = 1, which strongly suggests a substitutional adsorption geometry for this system. Good agreement has been found between a recent ab initio calculation and the present experiment for the vibrational energy of the Al(111)(root 3 X root 3)R30 degrees-Na surface.