Hyper‐Raman spectra and polariton‐vibration resonances in liquid C2Cl4 and PCl3

The polarized hyper‐Raman spectra and the polaritons were studied in liquid PCl3 and C2Cl4. The TO‐LO splitting of the dipole modes was found in PCl3 [480 (TO)‐515 (LO) cm−1 and 494 (LO)‐500 (TO) cm−1]. The LO and TO mechanical modes vanished in the back‐scattering and forward‐scattering hyper‐Raman spectra, respectively. The possibility of observation of the splitting due to polariton‐vibration resonance is analysed. It is shown that the observation at ω≲300 cm−1 is possible if the oscillator strength of the vibration exceeds 0.1. Using the value of the splitting, the oscillator strengths were estimated for a number of dipole modes in PCl3 and C2Cl4. The trend of the polariton branches and the observation of the resonances showed that all dipole modes of a liquid have the same symmetry, although the symmetry of these modes was different when considered in terms of the 'molecular' selection rules. To analyse the selection rules of the vibrational spectra of liquids it is necessary to invoke its macroscopic symmetry.