Photon-excited collective modes in a surface alloy

We investigate the influence of modifications of the surface charge-density profile on photon-excited collective excitations of valence electrons, through a comparison of the angle- and energy-resolved photoyield of clean Al and the Al/Na "surface alloy." Although the charge-density profile is considerably different for the clean and surface alloy systems, the Al multipole plasmon energy is essentially similar, in contrast to what one might expect from simple charge-density arguments. A Na multipole plasmon is observed only when excess Na is deposited over the surface alloy. We base a qualitative explanation for these observations on results of surface electronic structure calculations using total-energy density-functional theory.