Internal mode (factor group) splitting in hydrogen halide crystals

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Abstract
A detailed theoretical study is undertaken of the influence of the relevant intermolecular interactions in the splitting of fundamental stretching (internal) modes in hydrogen halide crystals. The molecular exciton theory of Davydov has been used. Lattice sums have been carried out till good convergence is obtained. It is observed that the exchange interaction can be neglected in these crystals. The induction forces have a small contribution in the splitting, while the contribution from the dispersion forces is almost negligible. But the dipole-dipole interaction alone cannot account for the splitting of the internal mode and the quadrupole-quadrupole interaction must be taken into account. This is true since in these crystals the molecules possess a large value of the quadrupole moment and its first derivative with respect to the vibrational coordinate.

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