Orientational order in the molecular solids N₂, CO, and O₂ investigated via combined excitations with FTIR

We present results of studies of very weak absorption bands in the mid infrared (IR), lying in the overtone region, of solid N₂, CO, O₂, and a N₂-CO mixture. The IR activity is induced in a molecule when two neighboring molecules interact and vibrate out of phase simultaneously: (0-1)(0-1) excitation. From the temperature dependence of band intensities and bandwidths, we were able to evaluate quantitative measures of order/disorder phenomena, i.e., long range orientational ordering and/or short range correlation between molecules. The long range order parameter for ordered phases, which we determined by optical spectroscopy, agrees with values η(T) found by x-ray Bragg scattering or nuclear quadrupole resonance data. We were able to model the temperature dependence of the order parameter by assuming a specific interaction of neighboring molecules (dipole-dipole, quadrupole-quadrupole, magnetic) and/or correlation between molecules.