Knowledge Bank

A molecular orbital calculation has been applied to the impurities $OH-$ and $SH-$ and their perturbing alkali halide lattice environments. The impurities were described by free state molecular wave functions of the Roothan LCAO-MO form, and expressions for the various lattice interactions derived using first order perturbation theory. In one case, this led to a one-electron, three-center integral, for which a method was developed to conveniently obtain the solution. Computer programs were written for evaluation of the lattice interactions. The equilibrium energies, internuclear separations, and vibrational frequencies were computed for several possible impurity positions in the lattices. Single crystals of several alkali halides were grown doped with OH- and SH-, and infrared absorption spectra obtained at temperatures from $30\circ K$ to $300\circ K$. Using the calculated energy values and a comparison of the observed and calculated frequency shifts from lattice to lattice, it was shown that the $OH-$ and $SH-$ are aligned along the 100 symmetry axis of the face centered cubic crystals, with the center of mass displaced. The displacement of the center of mass increases with the lattice constant. The importance of a molecular representation, compared to the classical point representation, will be discussed.