Electron affinity (data page)

Elements

Electron affinity can be defined in two equivalent ways. First, as the energy that is released by adding an electron to an isolated gaseous atom. The second (reverse) definition is that electron affinity is the energy required to remove an electron from a singly charged gaseous negative ion. The latter can be regarded as the ionization energy of the −1 ion or the zeroth ionization energy. Either convention can be used.

Negative electron affinities can be used in those cases where electron capture requires energy, i.e. when capture can occur only if the impinging electron has a kinetic energy large enough to excite a resonance of the atom-plus-electron system. Conversely electron removal from the anion formed in this way releases energy, which is carried out by the freed electron as kinetic energy. Negative ions formed in these cases are always unstable. They may have lifetimes of the order of microseconds to milliseconds, and invariably autodetach after some time.

† A quantum offset of the velocity imaging-based measurements was revealed in 2025, which could make a revision of all electron affinities marked with a dagger ^† necessary. The value of the downward correction to be applied is determined by the intensity of the electric field that was used in the experiment, which was not published with the original measurements, but can be estimated to be of the order of −20 μeV. However, subsequent studies have called both the magnitude and the very existence of such a quantum offset into question. A 2026 evaluation that combined simulations and high-precision experiments found no evidence for a quantum offset as large as 0.2 cm⁻¹ in properly analyzed slow‑electron velocity‑map imaging (SEVI) data. The original claim of a quantum offset in SEVI measurements rests on incorrect assumptions made in the LPM method. Moreover, the electron affinity of selenium determined by the LPM method deviates by approximately 0.5 cm⁻¹ from the values obtained with SEVI and with earlier laser photodetachment threshold (LPT) measurements. This discrepancy suggests that the LPM method alone may not provide a sufficiently reliable benchmark for validating other experimental techniques.