Far infrared multipole induced absorption in compressed gaseous C2F6

Abstract

The very weak far infrared collision induced absorption band of hexafluoroethane (C₂F₆) has been detected using a polarising interferometer and a Rollin [He(1) cooled] In/Sb crystal detector. The absorption, centred at ė 16 cm^–1(296 K) and markedly asymmetric with a high frequency shoulder, is treated in terms of a multipole induced dipole that gives rise to sets of rotational ΔJ= 2 (quadrupolar) and ΔJ= 4 (hexadecapolar) lines. These were dynamically broadened using the j-diffusion model of Gordon into bands the relative intensities of which were estimated by a self consistent field molecular orbital calculation of the non-zero elements of the quadrupole and hexadecapole moments.

This analysis reproduces the experimental data fairly well, but the calculated cross section is too small. The local charges on each atom of the C₂F₆ molecule estimated with m.o. theory were used to represent its electrostatic field as an alternative to the multipole expansion which tends to diverge at the very short intermolecular distance (R) needed for the R^–10 hexadecapole field to become effective.