Ionization

Process whereby atoms lose one or more electrons to become cations. Ionization occurs by ionizing radiationRadiation with sufficient energy to eject electrons from electrically neutral atoms, leaving behind an ion. There are four basic types of ionizing radiation: • α particles (He nuclei) • β particles (electrons) • Neutrons - neutrons are not themselves ionizing but their collisions with nuclei lead to the ejection of 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 stateEnergy state of an atom, or of a solid, when its total energy equals the minimum possible energy for that atom or solid. For solids, this only occurs at a temperature of absolute zero. Individual atoms may be in their ground state at non-zero temperatures, but they will not stay there. 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 hydrogenElectrically neutral hydrogen atom with one proton and one electron. It is commonly referred to as HI (pronounced H-one), and is located throughout galaxies as HI clouds or external to galaxies as part of the intercloud gas. Neutral H is detected via the spin-flip transition at 21 cm, and HI is denoted HI and ionized hydrogenLightest and most common element in the universe (~92% by atoms; ~75% by mass). Hydrogen's isotopes are: • H (99.9885 %)
• H (0.0115 %), also called deuterium.
• H, also called Tritium, is a radioactive (t½ = 12.32 y) by-product of atmospheric thermonuclear tests in Earth's hydrosphere and atmosphere.
by HII or H⁺. Doubly ionized He (heliumHelium (He) Second lightest and second most abundant element (after Hydrogen) in the universe. The most abundant isotope is He (99.9998%), He is very rare. Helium comprises ~8% of the atoms (25% of the mass) of all directly observed matter in the universe. Helium is produced by hydrogen burning inside 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 absorptionTransfer of energy to a medium as a particle or electromagnetic radiation passes through it. Absorption of electromagnetic radiation is the combined result of Compton scattering, σ, and photoelectric absorption, τ. It may be quantified: where, t = thickness, ρ = density, and μ = mass absorption coefficient, which combines Compton and photoelectric effects (μ = σ + τ). lines) that differ in wavelengthDistance from one peak of a wave to the next. Wavelength is measured in units of distance. The wavelengths of visible light correspond to ~400-650 nm. Wavelength is an important way to characterize a wave. For light, the shorter the wavelength, the higher the energy of the light wave. from those produced by neutral atoms. Photons are also emitted when an ionAtom with a net electrical charge because it has lost, or gained, one or more electrons relative to the number possessed by a neutral atom of the same element. A positively charged ion (cation) has fewer electrons than a neutral atom; a negatively charged ion (anion) has more. captures, or recaptures, an electron (this process is called “recombination”).