Intermolecular potentials and lattice dynamics of Cl₂ and Br₂ crystals

Abstract

The Raman spectra of chlorine and bromine crystals have been observed at low temperature. Calculations of external frequencies show that the atom-atom interaction is inadequate to explain the spectra of halogen crystals. A charge-transfer term was included in the intermolecular potential to take account of a specific interaction, and the external frequencies calculated with this modified potential agree well with the spectroscopic data. The effect of temperature on lattice frequencies has been estimated, showing disagreement between experimental and theoretical values at high temperature. This disagreement is explained as an anharmonic effect. Dispersion curves of external and internal modes of vibration using the modified potential are also included.