Binding energy The energy (measured in eV) of the exiting photoelectron produced by the photoionization process of core level (first shell) electrons, which contain discrete chemical information. Peaks arising from energy level of various orbitals allow for qualitative oxidation state identification.

Core level shift Precise photoelectron binding energy peak positions of the peak centers denote the chemical oxidation and/or electronic states of orbitals from which the photoelectrons emanate. Minute variations in the binding energy arising from differing electronic environments of the ejected photoelectrons are manifested as shifts in the peak position (also referred to as chemical shifts).

Fermi level A reference point (taken as zero eV) with which binding energies of photoelectrons are measured.

Inelastic mean free path The average distance that a particle at a given energy (photoelectrons) can travel along a trajectory between inelastic collisions within a solid.

Intra- and extra-atomic relaxation The contraction of outer electronic orbitals toward the nucleus resulting from core hole vacancies following the photoemission process. The outgoing photoelectrons can be screened by either valence electrons within the atom (intra-atomic) or from local environment electrons just outside the atom (extra-atomic). These effects contribute to a surface analyte's core-level binding energy position observed in the X-ray photoelectron spectra.

Monolayer A one-molecule thick quantity of adsorbate on the surface.

Shake-up satellites Excitation of valence electrons (that accompanies relaxation processes) to an unfilled level at higher binding energy. The loss of kinetic energy of the outgoing photoelectron into a discrete state appears as a peak along with the main core-level photoionization.

Surface sensitivity The ability to probe the topmost (on the order of angstroms) atomic layers of a solid.

Work function The energy required to remove an electron from the solid. This energy level above the Fermi reference is known as the vacuum level.

X-ray-excited Auger emission A secondary electron emission process that follows the photoionization and appears as a peak in the X-ray photoelectron spectrum. After the initial photoemission, an upper level valence electron relaxes into the vacant core-level state, followed by an ejection of another electron in the valence level.

X-RAY photoelectron spectroscopy (XPS) is an important and widely used surface analysis method in a many fields of study in physics and chemistry (e.g., microelectronics, heterogeneous catalysis, environmental geochemistry, etc.). The technique probes the energy distribution of electrons ejected from solids via irradiation by X-rays and the photoelectric effect; the electrons contain information regarding chemical oxidation state, electronic structure, and atomic composition of the analyte being studied. Thus, surface composition as well as the electronic environment can nondestructively (in many cases) be determined. X-ray photoelectron spectroscopy is also useful for quantitative analysis, capable of probing ultrathin layers of material (0.1% of a monolayer). Since the photoelectrons analyzed emanate only from the topmost atomic layers of the solid surface being studied (<100 A), the technique is an invaluable tool for studying interfacial phenomena at the solid-solid and solid-gas boundaries.

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