Ultraviolet Photoelectron Spectroscopy

Abstract

Abstract Ultraviolet photoelectron spectroscopy (UPS) probes electronic states in solids and at surfaces. It relies on the process of photoemission, in which an incident photon provides enough energy to bound valence electrons to release them into vacuum. Their energy E , momentum ℏ k , and spin σ provide the full information about the quantum numbers of the original valence electron using conservation laws. Depicts the process in an energy diagram. Essentially, the photon provides energy but negligible momentum (due to its long wavelength λ=2π/| k |), thus shifting all valence states up by a fixed energy (“vertical” or “direct” transitions). In addition, secondary processes, such as energy loss of a photoelectron by creating plasmons or electron‐hole pairs, produce a background of secondary electrons that increases toward lower kinetic energy. It is cut off at the vacuum level E V , where the kinetic energy goes to zero. The other important energy level is the Fermi level E F , which becomes the upper cutoff of the photoelectron spectrum when translated up by the photon energy. The difference ϕ = E V ‐ E F is the work function. It can be obtained by subtracting the energy width of the photoelectron spectrum from the photon energy. Photoemission is complemented by a sister technique that maps out unoccupied valence states, called inverse photoemission or bremsstrahlung isochromat spectroscopy (BIS). Inverse photoemission represents the reverse of the photoemission process, with an incoming electron and an outgoing photon. The electron drops into an unoccupied state and the energy is released by the photon emission. Both photoemission and inverse photoemission operate at photon energies in the ultraviolet (UV), starting with the work function threshold at ∼4 eV and reaching up to 50‐ to 100‐eV photon energy, where the cross‐section of valence states has fallen off by an order of magnitude and the momentum information begins to get blurred. At kinetic energies of 1 to 100 eV, the electron mean free path is only a few atomic layers, making it possible to detect surface states as well as bulk states.