Manifestation of interaction of the transition dipole moments in IR spectra of low-temperature liquids and solutions in liquefied noble gases Original Research Article

The infrared absorption spectra in the overtone range are studied for liquid SF6, CF4, NF3, and OCS and their solutions in liquid Ar (T=93 K), Kr (T=143 K), and Xe (T=175 K) at concentrations C=(10−5–1) mole fractions (m.f.). In the spectra of low-temperature liquids, the band shapes of transitions to combined vibrational states in which the strongly IR active ν3 mode is excited are shown to be completely determined by the interaction of transition dipole (TD) moments. The formula for the second spectral moment of these bands is derived accounting for the n-particle dipole coupling within the first coordination sphere. The approach developed is confirmed by the observed linear M(2) dependence on concentration at C>0.1 m.f. For highly symmetric (Td or Oh) species, the dimer spectra can be reliably separated from the band shapes observed at low concentrations; we thus found RSS=5.22(2) Å for (SF6)2. In the spectra of NF3 and OCS recorded at the number densities N=(0.3–3)×1021 molecule/cm3, the TD–TD interaction causes band broadening which is linear in N, with the self-broadening coefficient independent on solvent and temperature.