Lattice-type Vibrations in Associated Liquids and the Origin of Anomalous Rayleigh Scattering

Abstract

WHEN a sample is irradiated with monochromatic light of frequency ν₀ and the scattered light at right angles to this incident beam examined by means of a spectroscope, a line spectrum is observed containing besides the Rayleigh component ν₀, the Raman lines ν₀ ± ΔE_i/h, where ΔE_i are the energy differences between stationary states of the sample. Although an arc having a relatively isolated line is normally used as the source of monochromatic light, all arcs do give rise to some continuum and this also appears in the spectrum as Rayleigh scattering. This continuum can be reduced by a carefully designed arc (such as the Toronto type¹ mercury arc), while the Rayleigh scattering is minimized by preparation of the sample for investigation in an optically clean condition. However, even under such favourable conditions some continuous scattering does occur, but the fact that it appears to depend on the nature of the scattering medium, extending less than 100 cm.⁻¹ from the exciting line in the case of unassociated liquids^2–4 and their corresponding solids and many hundreds of cm.⁻¹ in the case of highly associated liquids⁵ and solids, would suggest that it is not Rayleigh scattering as is often thought. Its origin is, therefore, of interest, and it is the purpose of the present communication to discuss the possible sources of these anomalous Rayleigh ‘wings’.