Electronic transitions of oxygen adsorbed on clean silicon (111) and (100) surfaces

Second-derivative energy-loss spectra of 100-eV electrons reflected from silicon (100) and (111) surfaces have been studied with increasing oxygen coverages. Two different binding states are found. The first state which occurs up to monolayer coverage is characterized by electron transitions at 3.5, 5, 7, and 11 eV. After electron irradiation, annealing, or high exposures, oxygen goes into an Si${\mathrm{O}}_2$-type binding state. The spectrum of a completely oxidized surface agrees with the second derivative of the loss function $-\mathrm{Im}\left(\frac{1}{\epsilon }\right)$ calculated from optical data. An interpretation of the spectra on the basis of dielectric-scattering theory also explains the shifts in peak positions below monolayer coverage.