Electronic transitions on UHV-cleaved Si (111) adsorbed with oxygen

The electron energy loss spectra have been measured of oxygen-adsorbed Si (111) surfaces at the primary energies of 10–200 eV. The observed peaks are at the loss energies of 2.9, 3.6, 4.8, 5.7, 6.9, 8.2, 9.6, 12.0, and 13.0 eV. All the peaks are attributed to the one-electronic transitions related to the adsorbed oxygen, except for the 9.6 eV peak which could possibly be due to the relaxed surface plasmon excitation. It is shown that the “splitting” of surface plasmon reported by Ibach and Rowe does not exist.