Excited-state absorption spectroscopy of self-trapped excitons in alkali halides

Optical transitions between the lowest metastable triplet state and higher states of the self-trapped exciton are reported for nine alkali halides, in the energy range from 0.6 to 5.5 eV. Measurements were made on pure crystals using techniques of time-resolved spectroscopy and electron-pulse irradiation. Variation of the transition energies with lattice constant provides additional evidence for the classification of the spectra in terms of two basic categories: hole transitions localized on the $X_2^{}{}_{}{}^{-}$ core, and electron transitions largely decoupled from the influence of the core ions. Optical binding energies of the self-trapped exciton are estimated from the spectra, and are found to be 2-4 times greater than the binding energies of corresponding unrelaxed excitons. The relaxation of the lattice around the exciton is discussed in terms of a two-dimensional configuration-coordinate model. Hole self-trapping and the Stokes shift of emission occur principally through the axial mode, as commonly accepted. The increase of exciton binding energy upon relaxation to the equilibrium configuration is attributed to a nonaxial mode of relaxation which may be viewed as analogous to the breathing mode that broadens and shifts $M$-center transitions.