Absorption Spectra of ${\mathrm{F}}_2^{-}$, ${\mathrm{Cl}}_2^{-}$, ${\mathrm{Br}}_2^{-}$, and ${\mathrm{I}}_2^{-}$ in the Alkali Halides

After x-ray irradiation at liquid nitrogen temperature, holes are trapped by forming ${\mathrm{F}}_2^{-}$ molecule ions in LiF; ${\mathrm{Br}}_2^{-}$ molecule ions in KBr containing one of the impurities ${\mathrm{Ag}}^{+}$, ${\mathrm{Tl}}^{+}$, ${\mathrm{Pb}}^{++}$, or N${\mathrm{O}}_2^{-}$; and ${\mathrm{I}}_2^{-}$ molecule ions in KI containing one of the impurities listed above. An investigation of the optical and paramagnetic resonance spectra of crystals containing ${\mathrm{F}}_2^{-}$, ${\mathrm{Br}}_2^{-}$, and ${\mathrm{I}}_2^{-}$ centers leads to an identification of the optical transitions of these centers. The polarizations and relative intensities of the optical absorptions in the ${\mathrm{X}}_2^{-}$ molecule ion series are discussed with reference to the energy level scheme to be expected for such a species.