In XPS using Al Kα radiation (energy 1486.6 eV), the kinetic energy of Si 2p photoelectrons was measured as 1378.8 eV. If the spectrometer work function is 4.5 eV, which value is closest to the binding energy?

Answer: C. About 103.3 eV

From energy conservation in the photoelectric effect, the binding energy is E_B = hν − E_K − φ = 1486.6 − 1378.8 − 4.5 = 103.3 eV. The Si 2p binding energy of elemental Si is about 99 eV, so this value, about 4 eV higher, is typical of SiO₂ (Si⁴⁺), indicating that the surface is oxidized. About 107.8 eV forgets to subtract the work function, and about 112.3 eV adds it where it should be subtracted. When measuring insulators, charging shifts the peaks, so binding energies are often corrected by referencing the C 1s peak of adventitious carbon on the surface (about 284.8 eV).