Valence bands
Many of the molecular orbitals from which valence band photoelectron signal originates possess a high degree of hybridization, therefore the shifts in peak binding energy are far more varied and subtle than those observed for core level photoemission peaks. For this reason valence band spectra are predominantly used for material characterization through spectral fingerprinting, and individual peak assignment is either performed on surfaces with well-known electronic structure, or in conjunction with computational studies. Due to this ambiguity in the assignment of valence band peaks, these spectra are not used for quantification.
Ultraviolet photoelectron spectroscopy is also widely used to collect valence band spectra, the combination of both XPS and ultraviolet photoelectron spectroscopy to investigate the valence band can be extremely powerful as the ionization cross section of an orbital is dependent on the incident photon energy, therefore different electronic transitions and states can be probed by using different photon energies.
Ultraviolet photoelectron spectroscopy also exhibits greater surface sensitivity than XPS, the inherent surface sensitivity of XPS is due to the short inelastic mean free path (IMFP, or λ) of free electrons within a solid, with the so-called “information depth” from which > 99% of a photoemission signal originates conventionally being defined at 3 mean free path lengths from the surface, which in XPS is often quoted as 10 nm. This is an approximation as the IMFP of an electron is determined by the material properties of the solid media through which it is travelling and its kinetic energy, with electrons of lower kinetic energy having shorter path lengths. The lower incident photon energies used in ultraviolet photoelectron spectroscopy give emit photoelectrons of much lower kinetic energies than those measured in XPS, therefore giving ultraviolet photoelectron spectroscopy an approximate information depth of 2-3 nm.
