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Schematic of the photoemission (PES) and inverse photoemission (IPES) process. In PES (A) an electron is excited by an incoming photon from a previously occupied valence state (lower shaded region) into the continuum (gray shaded region, starting above the vacuum level Evac). In IPES (B) an injected electron with kinetic energy Ekin undergoes a radiative transition into an unoccupied state (upper shaded region) thus emitting a photon in the process.
