Introduction to Raman Scattering

Abstract

The Raman effect was predicted by Smekal in 1923 but was first observed by Raman in 1928. The first experiments were carried out using focussed sunlight and filters and relied on visual observation of colour changes in the scattered light. Later Raman recorded spectra of several liquids including benzene and carbontetrachloride using a mercury lamp and a spectrograph. The experiments used 600 ml of sample and required a 24 hour exposure to obtain measurable spectra. The Raman effect is an inherently weak effect, typically 10⁻⁸ of the intensity of the incident exciting radiation and for many years source stability and intensity made Raman spectroscopy extremely difficult particularly in comparison with the faster and less expensive infrared spectrometers that were developed. In 1952, a major improvement occurred with the introduction of the Toronto arc source. This comprised a four turn helix of pyrex glass capable of radiating as much as 50 watt in the 435.8 nm emission line of mercury, though only a fraction of this power could be used practically to excite a spectrum.