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Photoelectron spectroscopy is a method of analyzing substances using the photoelectric effect. When a photon interacts with an atom or molecule, it can — if it has enough energy — cause an electron to be ejected. The electron is ejected with a kinetic energy that depends upon its initial energy state and the energy of the incoming photon. The wavelength of the photon determines its energy, with shorter wavelengths having higher energies. By irradiating a substance with photons of a known wavelength, it is possible to obtain information about its chemical composition, and other properties, by measuring the kinetic energies of the ejected electrons.
When a negatively charged electron is ejected from an atom, a positive ion is formed and the amount of energy required to eject an electron is known as the ionization energy or binding energy. Electrons are arranged in orbitals around the atomic nucleus, and more energy is required to dislodge those close to the nucleus than those in more distant orbitals. The ionization energy of an electron depends mainly upon the charge on the nucleus — each chemical element has a different number of protons in the nucleus and therefore a different charge — and upon the electron’s orbital. Each element has its own unique pattern of ionization energies and in photoelectron spectroscopy, the ionization energy for each electron that is detected is simply the energy of the incoming photon minus the kinetic energy of the ejected electron. Since the first value is known and the second can be measured, the elements present in a sample can be determined from the patterns of ionization energies observed.
Relatively energetic photons are needed to eject electrons, which means that radiation toward the high energy, short wavelength end of the electromagnetic spectrum is required. This has given rise to two main methods: ultraviolet photoelectron spectroscopy (UPS) and x-ray photoelectron spectroscopy (XPS). Ultraviolet radiation is only able to eject the outermost, valence electrons from molecules, but x-rays can eject core electrons close to the nucleus due to their higher energy.
X-ray photoelectron spectroscopy is carried out by bombarding a sample with x-rays at a single frequency and measuring the energies of the electrons emitted. The sample must be placed in an ultra-high vacuum chamber in order to prevent photons and emitted electrons being absorbed by gases and to ensure there are no adsorbed gases on the sample surface. The energy of the emitted electrons is determined by measuring their dispersal within an electric field — those with higher energies will be deflected to a lesser extent by the field. Since the ionization energies of core electrons are shifted to slightly higher values when the element concerned is in an oxidized state, this method can not only provide information about the elements present, but also about their oxidation states. X-ray photospectroscopy cannot be used for liquids due to the requirement for vacuum conditions and is normally used for surface analysis of solid samples.
Ultraviolet photoelectron spectroscopy works in a similar way, but using photons in the ultraviolet range of the spectrum. These are most commonly produced by a gas discharge lamp using one of the noble gases, such as helium, to provide photons of a single wavelength. UPS was first used to determine ionization energies for gaseous molecules, but is now often employed to investigate the electronic structure of materials.
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