What’s Photoelectron Spectroscopy?

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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 sufficient energy, cause an electron to be ejected. The electron is ejected with a kinetic energy which depends on 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 known wavelengths, 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 remove those close to the nucleus than those in more distant orbitals. The ionization energy of an electron depends mainly on the charge on the nucleus – each chemical element has a different number of protons in the nucleus and therefore a different charge – and on the orbital of the electron. 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 former value is known and the latter can be measured, the elements present in a sample can be determined from observed ionization energy patterns.

Relatively energetic photons are needed to eject the electrons, which means that radiation towards the high-energy, short-wavelength end of the electromagnetic spectrum is needed. This gave rise to two main methods: ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). Ultraviolet radiation is capable of ejecting only the outermost valence electrons from molecules, but X-rays can eject electrons from the nucleus near the nucleus due to their higher energy.

X-ray photoelectron spectroscopy is performed by bombarding a sample with single frequency X-rays and measuring the energies of the emitted electrons. The sample must be placed in an ultra-high vacuum chamber to prevent the emitted photons and electrons from being absorbed by gases and to ensure that there is no gas adsorbed on the surface of the sample. The energy of the emitted electrons is determined by measuring their dispersion within an electric field: those with higher energies will be less deflected by the field. Because the ionization energies of the nucleus’ electrons are shifted to slightly higher values ​​when the element in question is in an oxidized state, this method can provide information not only about the elements present, but also about their oxidation states. X-ray photospectroscopy cannot be used for liquids due to the need 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 which uses one of the noble gases, such as helium, to deliver photons of a single wavelength. The UPS was first used to determine ionization energies for gaseous molecules, but is now often used to study the electronic structure of materials.

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