Collision-Induced Effects in Allowed Infrared and Raman Spectra of Molecular Fluids
It has been often assumed that collision-induced effects do not influence the infrared and Raman vibrational spectra. However, recent experimental work and molecular dynamics simulations strongly suggest that, in the liquid state, the intermolecular translational dynamics may play a role in several spectral properties as the molecular intensities and the bandshapes. This paper reports some experimental evidence of induced effects on the spectral profiles and discusses the various methods already proposed to separate the molecular and the collision-induced components from the total observed spectrum.
Infrared absorption and Raman scattering techniques are currently used to study the dynamics of the molecules in a liquid sample. Owing to technological improvements of spectrometers and their coupling with small computers it is possible today to obtain spectra on a broad frequency range with a satisfactory noise ratio.
Numerous experiments allowed for a better knowledge of the orientational dynamics of the molecules (Bailey, 1974; Perrot and Lascombe, 1978) and for the last few years, the vibrational dynamics was also widely studied (Clarke, 1978; Vincent-Geisse, 1980). Nevertheless, for the condensed state, intermolecular induced effects also may influence the spectral profiles.
We have already known for a long time that for liquids composed of atoms or spherical molecules, collisional dynamics gives rise to purely induced spectra in various spectral regions. More recently it has been observed that induced profiles of the same collisional origin are frequently superimposed on the allowed orientational spectra of anisotropic molecules in the liquid state. This is observed in depolarized Rayleigh scattering and in far-infrared absorption (Tabisz , 1979).
For the infrared and Raman vibration-rotation spectra, it has been traditionally assumed that collision-induced effects were negligible relative to the allowed profiles. But, recent experimental results (Sch roeder et al., 1978) along with molecular dynamics simu-lations (Frenkel and McTague, 1980; Ladanyi, 1983) seem to indicate that collision-induced effects do exist on the vibrational infared and Raman profiles of allowed transitions for some molecular fluids.
So the aim of this paper is to try to present published experimental results related to vibrational spectra which exibit evidence of collision-induced effects superimposed on allowed spectra and to discuss the methods already proposed in order to separate from these profiles, the allowed and the induced contributions. We will also try to give some direction for future experimental work in this field.
KeywordsMolar Intensity Spectral Profile Rotational Dynamic Vibrational Relaxation Translational Dynamic
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