Process whereby atoms lose one or more electrons to become cations. Ionization occurs by ionizing radiation or if an atom suffers a sufficiently violent collision. The “ionization potential” is the minimum amount of energy needed to remove an electron to infinity from the ground state. If the electron has already been excited to a higher level, less energy is needed to remove it. Where an atom has two or more electrons, the ionization potential for the second and subsequent electrons is greater than for the first electron.

Astronomers identify an atom that has lost a single electron by the Roman numeral II (the neutral atom, by I); whereas, cosmoschemists use the superscript +. For example, neutral hydrogen is denoted HI and ionized hydrogen by HII or H+. Doubly ionized He (helium that has lost both electrons) is denoted by He III or He2+. Very high degrees of ionization occur in some astrophysical settings (e.g., very high-temperature gases).

Ions that retain at least one bound electron can absorb or emit radiation, so producing spectral lines (emission and absorption lines) that differ in wavelength from those produced by neutral atoms. Photons are also emitted when an ion captures, or recaptures, an electron (this process is called “recombination”).

Some or all content above used with permission from J. H. Wittke.

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