Phonon Sidebands in the Emission Spectrum ofO2−in Alkali-Halide Crystals

The emission spectra of $\mathrm{O}_2^{}{}_{}{}^{-}$ ions dissolved in alkali-halide crystals consist of a series of zero-phonon lines each accompanied by multiphonon sidebands. We present high-resolution measurements of such emission spectra at 2 °K in the alkali halides NaCl, KCl, KBr, KI, RbCl, RbBr, and RbI. The zero-phonon lines are due to vibronic transitions within the $\mathrm{O}_2^{}{}_{}{}^{-}$ molecule, and the sidebands are produced by the interaction of even-parity lattice modes with the allowed ${\Pi }_g\leftarrow {\Pi }_u$ transition corresponding to the $\mathrm{O}_2^{}{}_{}{}^{-}$ emission. We show that most of the sideband structure can be explained by coupling with $A_{1g}$ lattice modes perturbed by relatively small changes in nearest-neighbor force constants. In two cases (KCl and RbCl) it is necessary to add $T_{2g}$ modes, or to consider changes in other force constants, to obtain better agreement between calculated and observed sidebands. A residual spectrum at very low frequencies cannot be explained by the above hypothesis; it is assigned to a second-order coupling to librations of the $\mathrm{O}_2^{}{}_{}{}^{-}$ ion. Some broad features in the emission spectra are interpreted in terms of transitions to short-lived low-lying electronic states.