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 accounts for lattice polarization to calculate adiabatic potential energy surfaces of the ground and excited states of the self-trapped exciton and to model its decomposition into Frenkel defect pairs in KBr. The characteristic optical 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 evolution 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 prior to the holes relaxing into their most stable $V_K$ center state. Contrary to these results, the femtosecond spectroscopy of the $V_K$-center formation in NaBr demonstrates that this is an extremely fast (< 1 ps) process. Theoretical modeling is used to show that the fast process of the F center (electron 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 ground electronic state of the exciton. This process is driven by the interaction of the relaxing hole with electron. We conclude that the speed of the hole vibrational relaxation prior to recombination with an electron and formation of an exciton is an important factor that determines the speed and effectiveness of exciton decomposition into Frenkel defects in alkali halides.