The hydrogen anion is the dominant bound-free opacity source at visible and near-infrared wavelengths in the atmospheres of stars like the Sun and cooler; its importance was first noted in the 1930s. The ion absorbs photons with energies in the range 0.75–4.0 eV, which ranges from the infrared into the visible spectrum. Most of the electrons in these negative ions come from the ionization of metals with low first ionization potentials, including the alkali metals and alkaline earths. The process which ejects the electron from the ion is properly called photodetachment rather than photoionization because the result is a neutral atom (rather than an ion) and a free electron.