Dispersion and Polarizability and the van der Waals Potential in the Alkali Halides

It is shown that the ultraviolet absorption of NaCl, KCl and KI, which one may estimate from recent experimental work, is in agreement with the dispersion of these salts. With these assumed absorption curves, the dipole-dipole potential constant for the van der Waals attraction between negative ions can be calculated with considerable accuracy. The constants can be estimated for the other alkali halides. The quadrupole-dipole constant is also approximated for all alkali halides. The resulting van der Waals potential is much greater than that previously calculated and accounts for the stability of CsCl type lattice for CsCl, CsBr and CsI. The polarizability of a given ion depends on the crystal. It is here assumed that the polarizability varies inversely as the ``main frequency'' in the crystal. This is shown to be in approximate agreement with experiment.