Exciton energy and its pressure dependence in alkali halides

Three-photon excitation spectra of the lowest exciton of NaCl and KBr have been measured up to 6 kbar at low temperature. The experimental results on NaCl near 8.45 eV represent the deepest uv state studied to date under hydrostatic pressure. The exciton energy in both materials shows a linear blue shift with increasing pressure. In order to better understand the experimental results, the exciton energies were computed using the potential induced breathing model on the basis of the charge-transfer exciton model. We find evidence supporting the charge-transfer exciton model and show that the crystal-field stabilization of the anions and overlap energies which were neglected in previous models are quantitatively significant.