Absorption Spectra of F - 2 , Cl - 2 , Br - 2 , and I - 2 in the Alkali Halides

After x-ray irradiation at liquid nitrogen temperature, holes are trapped by forming ${\mathrm{F}}_{2}^{\ensuremath{-}}$ molecule ions in LiF; ${\mathrm{Br}}_{2}^{\ensuremath{-}}$ molecule ions in KBr containing one of the impurities ${\mathrm{Ag}}^{+}$, ${\mathrm{Tl}}^{+}$, ${\mathrm{Pb}}^{++}$, or N${\mathrm{O}}_{2}^{\ensuremath{-}}$; and ${\mathrm{I}}_{2}^{\ensuremath{-}}$ molecule ions in KI containing one of the impurities listed above. An investigation of the optical and paramagnetic resonance spectra of crystals containing ${\mathrm{F}}_{2}^{\ensuremath{-}}$, ${\mathrm{Br}}_{2}^{\ensuremath{-}}$, and ${\mathrm{I}}_{2}^{\ensuremath{-}}$ centers leads to an identification of the optical transitions of these centers. The polarizations and relative intensities of the optical absorptions in the ${\mathrm{X}}_{2}^{\ensuremath{-}}$ molecule ion series are discussed with reference to the energy level scheme to be expected for such a species.