Oxygen adsorption on a Cs-monolayer-covered Si(100) surface has been studied by ${\mathrm{Li}}^{-}$ ion spectroscopy and normally emitted secondary electron emission (SEE) spectroscopy. It is clearly shown that the oxygen lies above and below the Cs atoms, respectively, at low and high O exposures, disproving the dipole model for the work function change. The initial O adsorption induces a shoulder at the low-energy edge of the SEE spectra, indicating the existence of patches of a lower work function on the surface. The patches are explained as due to the Cs${\mathrm{O}}^{*}$ complexes formed by nonadiabatic chemisorption.

• Received 20 September 1996

DOI:https://doi.org/10.1103/PhysRevB.54.R17347