Application Description

Raman spectroscopy is an analytical method that provides rich chemical information. It is particularly suitable for molecular fingerprinting of (bio)chemical systems by probing the sample’s vibrational states. It is also possible to gain information on the intermolecular interactions by controlling the environment of the sample through parameters such as its temperature. The simplicity of this method makes it possible to combine it with a microscopy technique allowing for sample imaging with chemical contrast.

Raman spectroscopy is based on inelastic scattering of light by an illuminated sample. The incident light puts the sample molecules in a virtual state and, upon decaying from this state, light is scattered. If the final state of the molecules is different from the initial one, the wavelength of the scattered light shifts. The shift could happen towards both longer or shorter wavelengths, which are referred to as Stokes shift or anti-Stokes shift. This difference in the wavelength is a direct measure of the energy difference between the states of the sample. When used in conjunction with complementary absorption and photoluminescence methods, Raman spectroscopy can provide the full picture of the sample's spectroscopic properties. It is also possible to monitor the strength of a specific Raman band to observe the dynamic process the sample experiences.

Measurement Strategies

Inelastically scattered light from the sample is several orders of magnitude less intense than the elastically scattered light. Detecting such a weak signal is the major challenge doing Raman spectroscopy. To overcome this issue, the samples are typically laser illuminated. Laser illumination also allows precise determination of the resulting wavelength shift and its strength. An optical filtering scheme is needed before detection to eliminate the dominant contribution from elastically scattered light (without a shift in wavelength) and to select the wavelength of the scattered light to be detected. Between the two stages of Raman spectroscopy - sample illumination and scattered light detection - Zurich Instruments lock-in amplifiers become the bridge.