Role of density in Raman scattering of iodine

Raman-scattering measurements have been performed under nonresonance conditions on liquid iodine at elevated temperatures up to the critical point. Substantial variations are observed in the peak of the intramolecular vibrational band and its half-width for the depolarized and polarized components as a function of density. These changes are attributed to increased molecular character and a reduction of intermolecular charge transfer processes as the density is reduced. At moderate to higher densities the liquid Raman spectra suggest a phononlike description, whereas at lower densities the liquid spectra suggest a transition to a more gaslike multiple-collsion regime. A comparison of the liquid and gaseous spectra to that obtained by Shimomura, Takemura, and Aoki in crystalline iodine at high pressures suggests that a competition between charge transfer and interatomic repulsive interactions determines the transition from a semiconducting to semi- metallic state. The possible relation of the liquid iodine results to the nonmetal-to-metal transition in other diatomic-based halogen systems and to solid hydrogen is discussed.