Far Infrared Spectra of Crystalline Iodine Monobromide and Iodine Monochloride

Infrared spectra, in the range 20–400 cm−1, of thin polycrystalline films (at 80 °K) of iodine monobromide and the β-form of iodine monochloride have been recorded. For IBr, the intramolecular stretching region shows two peaks separated by 16 cm−1 and shifted appreciably from the gas phase value, indicating strong intermolecular interactions. Four lattice modes are also observed and assigned in accordance with group theoretical predictions. For β-ICl the spectrum of the stretching region confirms the presence of two distinct molecules with different bond lengths. The complexity of the ICl structure prevents an assignment of the ten observed lattice absorptions.