Laser-induced reactions in crystals: Femtosecond pump-probe spectroscopy and ab initio calculations of self-trapped excitons and holes in KBr

We used an embedded cluster Hartree-Fock method that self-consistently acco unts for lattice polarization to calculate adiabatic potential energy surfa ces of the ground and excited states of the self-trapped exciton and to mod el its decomposition into Frenkel defect pairs in KBr. The characteristic o ptical excitation and luminescence energies of the self-trapped exciton and basic Frenkel defects are calculated. We present the experimental results of femtosecond pump-probe spectroscopy, which demonstrate the time evolutio n of the optical absorption of the KBr crystal excited by an 8-eV pulse at 80 K. These results reveal that Frenkel-defect pairs are formed in KBr prio r to the holes relaxing into their most stable V-K center state. Contrary t o these results, the femtosecond spectroscopy of the V-K-center formation i n NaBr demonstrates that this is an extremely fast (< 1 ps) process. Theore tical modeling is used to show that the fast process of the F center (elect ron trapped by a halogen vacancy) and H center (interstitial halogen atom) pair formation in KBr prior to the hole self-trapping can happen in the gro und electronic state of the exciton. This process is driven by the interact ion of the relaxing hole with electron. We conclude that the speed of the h ole vibrational relaxation prior to recombination with an electron and form ation of an exciton is an important factor that determines the speed and ef fectiveness of exciton decomposition into Frenkel defects in alkali halides .