What is photoelectron spectroscopy?
Photoelectron spectroscopy is a method of analysis of substances using a photoelectric effect. When the photon interacts with an atom or a molecule, it can - if it has enough energy - to cause the electron. The electron is released by kinetic energy, which depends on its initial energy condition and the energy of the incoming photon. The photon wavelength determines its energy, while shorter wavelengths have higher energy. By irradiating the substance with photons of the well -known wavelength, it is possible to obtain information about its chemical composition and other properties by measuring kinetic energies of the drained electrons.
When a negatively charged electron is fired from an atom, a positive ion is formed and the amount of energy required to extend the electron is called ionizing energy or binding energy. Electrons are arranged on orbitals around the core and more energy is needed to release those nearby cores than those on the more distant orbitál. The electron ionization energy depends mainly on the hub on the core - each chemical element has a different number of protons in the core, and therefore a different charge - and the electron orbital. Each element has its own unique formula of ionization energies and in photoelectron spectroscopy, ionization energy for each detected electron is simply the energy of the incoming photon minus kinetic energy of the drained electron. Because the first value is known and the second can be measured, the elements present in the sample may be determined from the observed formulas of ionization energies.
relatively energy photons are needed to eject electrons, which means that radiation is required towards high energy, a short end of the electromagnetic spectrum. This led to two main methods: ultraviolet photoelectron spectroscopy (UPS) and X -ray photoelectron spectroscopy (XPS). Ultraviolet radiation is only able to eject the farthest valence electrons from molecules, but X -ray rays can pull out core electronsnear the core due to their higher energy.
X -ray photoelectron spectroscopy is carried out by bombing the sample with an X -ray at one frequency and measuring the energy of emitted electrons. The sample must be placed in an ultra -high vacuum chamber to prevent photons and emitted electrons of absorbed gases and to ensure that there are no adsorbed gases on the sample surface. The energy of the emitted electrons is determined by measuring the scattering in the electric field - those with higher energy will be diverted to a lesser extent in the field. Given that the ionization energy of the core electrons is shifted to slightly higher values when the conceived element is in the oxidized state, this method can not only provide information about the elements present, but also their oxidation conditions. X -ray photospektroscopy cannot be used for liquids due to vacuum requirements and is usually used for surface analysis of solid samples.
Ultraviolet photoelectron spectroscopy worksIn a similar way, it uses photons in the ultraviolet range of the spectrum. They are most often produced by a gas drain lamp using one of the noble gases, such as helium, to provide photons of one wavelength. UPS was first used to determine ionization energies for gaseous molecules, but now it is often used to explore the electronic material structure.