Infrared spectra of thin films of α-crystalline hexafluoroethane: a manifestation of resonant dipole–dipole interaction in the range of fundamental vibrational modes ν5 and ν10

Abstract

Infrared reflection-absorption spectra of thin films of α-crystalline hexafluoroethane deposited on a gold-plated copper mirror are measured at temperatures of 70 and 80 K. The bands corresponding to strong in the dipole absorption vibrations ν₅ and ν₁₀ have complex contours, the shape of which is explained in terms of the resonant dipole–dipole interaction between identical spectrally active molecules of the crystal. Splittings of the complex ν5 and ν10 bands are explained taking into account two effects: the Davydov splitting and the LO–TO splitting of the strong modes. Bands of the asymmetric ¹³С¹²СF₆ isotopologue in the absorption spectrum of the crystal exhibit an anomalously large isotope shift as compared with the shift in the spectrum of free molecules. This anomaly is explained by intermolecular resonant dipole–dipole interaction of asymmetric ¹³С¹²СF₆ isotopologue with molecules of the environment, consisting of the most abundant ¹²C₂F₆ isotopologue. The correctness of the given interpretation is confirmed calculating these three effects in the model of resonant dipole–dipole interaction.