Analysis of the linear and non-linear optics of crystals of CHI3, S8 and their 1:3 complex Original Research Article

The molecules CHI3 (iodoform) and S8 (octasulfur) form not only their own homomolecular crystals but also a complex of formula CHI3·3S8. Molecular polarizabilities are deduced for the molecules in their homomolecular crystals from the crystal linear susceptibilities. These polarizabilities, treated as transferable between crystal environments, are then used to calculate the refractive indices of the complex, but the results do not agree well with experiment. Approximate local fields deduced at the different molecules in the complex using the crystal susceptibilities directly yield better agreement. These local fields also indicate that to reproduce the experimental refractive indices of the complex requires an iodoform polarizability increased perpendicular to the molecular axis, which could occur through charge transfer from octasulfur. Theoretical hyperpolarizabilities for the two molecules combined with the same local fields are used to calculate the quadratic non-linear optical susceptibility χ(2) of the complex, yielding only moderate agreement with experiment. Distortions of the octasulfur molecular framework in the crystal structure of the complex are important because they induce sizeable hyperpolarizability components, absent in the free molecule, which determine the sign of the major components of χ(2).