Theory of alkali halide photofragmentation: Potential energy curves and transition dipole moments

Realistic potential energy curves are calculated for the ground and lowest five excited states of six alkali halide diatomic molecules (LiF, NaF, KF, LiCl, NaCl, and KCl). The curves are calculated using a covalent-ionic resonance model for the electronic structure of the molecules together with a semiempirical valence-bond method. The electronic wavefunctions obtained from this treatment are then used to calculate transition dipole moments as a function of internuclear separation for the five different possible photodissociating electronic transitions out of the ground electronic state of the molecules. At the equilibrium internuclear separation of the ground electronic state, transition dipole moments for both parallel and perpendicular transitions are of comparable magnitudes. The transition dipole moments show a considerable variation with internuclear separation. This indicates that it is advisable for these systems to avoid making the Franck-Condon approximation and to take proper account of the variation of the transition dipole moments with internuclear separation when calculating their absorption spectra and photofragmentation patterns. The simple model presented here for the electronic wavefunctions and transition dipole moments of alkali halide molecules provides a good foundation for the quantitative modeling of their experimentally observed absorption and photofragmentation spectra.