A Heitler-London treatment is applied to the transfer mechanism which has been traditionally associated with the fundamental absorption of alkali halide crystals. In particular, detailed calculations are made for NaCl using the free atom Na $3s$ Hartree-Fock solution for the one-electron transfer function and the free ion Hartree-Fock solutions for the remaining one-electron functions. The energy and oscillator strength so determined are irreconcilable with experiment showing that the free atom Na $3s$ function cannot be used in conjunction with the transfer model if the model is to yield sensible results. In a very semiquantitative way it is shown that an electron-hole overlap integral of about 0.1 coupled with Overhauser's exciton model leads to a doublet structure which is in agreement with experimental results for many of the chlorides, bromides, and iodides. These findings do not distinguish the transfer model from the "excitation model."

• Received 16 July 1959

DOI:https://doi.org/10.1103/PhysRev.119.525