Values of the optical matrix elements are estimated from experimental values of the dielectric constant and the energy of the optical reflectivity peak. The matrix elements of the momentum operator are found independent of polarity, as in studies by Lawaetz and by Cohen. This is found to be consistent with the expectations of the Penn model of the dielectric constant and with linear-combination-of-atomic-orbitals models. It is also found to differ from but not nullify the assumptions of the Phillips-Van Vechten ionicity model. The matrix elements of momentum also vary significantly with bond length (or metallicity), but in such a way that both the bond-orbital and the pseudopotential descriptions simultaneously retain essential validity. Significant corrections to the $\gamma$ values of the bond-orbital model for the heavier elements are suggested by the study.

• Received 15 March 1976

DOI:https://doi.org/10.1103/PhysRevB.14.691