Low energy photoelectron spectroscopy of solids. Aspects of experimental methodology concerning metals and insulators

Both He(I) and He(II) ultraviolet photoelectron spectra (UPS) for metals (Ag, Au) and thin solid films of organic (anthracene) and inorganic compounds (LiF, LiCl, LiBr, LiI, NaF, NACl, Li₂SO₄, NaOH) are presented. The results of measurements on two different spectrometers have shown that both the bias voltage, V_b, applied to the solid sample, and the angle of incidence, Ø, of the photon beam with the sample are crucial to the accurate determination of the work function (metals) and binding energies (insulators). The true binding energy of a band in the UPS with respect to the vacuum level, E_b^vac, is defined as the minimum value of (hv - ΔE), where ΔE is the energy difference between the threshold of the SEED, E_T, and the band maximum. ΔE is maximized at low values of ø and of V_b. The intensity of photoelectron emission (signal) increases with both ø (2–30°) and V_b (5–20 V). It is recommended that the variation of ΔE with both ø and V_b should be observed in any determination of E_B^vac.