Editing Ionization energy (section)

== Determination of ionization energies ==
[[File:Measurement of ionization energy of atoms - schematic.svg|thumb|304x304px|Ionization energy measurement apparatus. |alt=]]
The ionization energy of atoms, denoted ''E''<sub>i</sub>, is measured<ref>{{Cite web|last=Mahan|first=Bruce H.|date=1962|title=Ionization Energy|url=https://archive.org/details/ionization_energy|access-date=2020-09-13|publisher=College of Chemistry, University of California Berkeley}}</ref> by finding the minimal energy of light quanta ([[photon]]s) or electrons accelerated to a known energy that will kick out the least bound atomic electrons. The measurement is performed in the gas phase on single atoms. While only noble gases occur as [[monatomic gas]]es, other gases can be split into single atoms.{{fact|date=February 2025}} Also, many solid elements can be heated and vaporized into single atoms. Monatomic vapor is contained in a previously evacuated tube that has two parallel electrodes connected to a voltage source. The ionizing excitation is introduced through the walls of the tube or produced within.

When ultraviolet light is used, the wavelength is swept down the ultraviolet range.  At a certain wavelength (λ) and frequency of light (ν=c/λ, where c is the speed of light), the light quanta, whose energy is proportional to the frequency, will have energy high enough to dislodge the least bound electrons. These electrons will be attracted to the positive electrode, and the positive ions remaining after the [[photoionization]] will get attracted to the negatively charged electrode. These electrons and ions will establish a current through the tube. The ionization energy will be the energy of photons ''hν''<sub>i</sub> (''h'' is the [[Planck constant]]) that caused a steep rise in the current: ''E''<sub>i</sub> = ''hν''<sub>i</sub>.

When high-velocity electrons are used to ionize the atoms, they are produced by an [[electron gun]] inside a similar evacuated tube. The energy of the electron beam can be controlled by the acceleration voltages. The energy of these electrons that gives rise to a sharp onset of the current of ions and freed electrons through the tube will match the ionization energy of the atoms.