Raman Spectra of Solid Chlorine and Bromine

Librational lattice modes have been observed at 15°K in solid Cl₂ at 83, 100, and 118 cm⁻¹, and at 55, 74, 86, and 101 cm⁻¹ in solid Br₂. Their intensities relative to one another can be qualitatively accounted for by the oriented gas model. The stretching frequencies $\text{[ν(1–0)]}$ of Cl₂ and Br₂ are shifted −13 and −20 cm⁻¹, respectively, from their gas phase values and show structure due to isotope splittings and inter‐molecular coupling. The spectra of the lattice modes, the Br₂ stretching motion, and the Cl₂ stretching motion indicate, respectively, strong, intermediate, and weak intermolecular coupling relative to the isotope splittings. Both lattice and internal frequencies indicate stronger intermolecular forces in solid Br₂ than in solid Cl₂.