Photoemission from Ar, Kr, and Xe on Pb(111)

Angle-resolved photoelectron spectra were measured for the valence p states of Ar, Kr, and Xe on Pb(111). The thickness of the rare-gas layers was varied between 0.05 and about 5 monolayers. For each layer the work function was deduced from the width of the photoelectron energy-distribution curve. On Pb(111) tightly packed monolayers were prepared, exhibiting energy dispersion of the valence p states, which is discussed in the framework of the known two-dimensional band structure. The spectral features are very sharp, as can be seen from the full width at half maximum value of 175 meV for the Xe 5$p_{1/2}$ peak. For Xe the submonolayer regime was also studied, where single atoms are adsorbed together with small three-dimensional clusters. The single atoms exhibit no dispersion and no split of the $p_{3/2}$ level into two sublevels. This demonstrates that these features are derived from lateral interaction in the rare-gas monolayer. On top of the monolayer a second completely filled layer can be prepared for Ar, Kr, and Xe. The second-layer shift in binding energy is 720 meV for Ar and Kr and 525 meV for Xe. It is about 6 times as large as the work-function changes induced by the monolayer. This observation can only be explained in the known final-state interpretation of the layer-dependent shifts in binding energy.