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Raman Spectroscopy


Raman spectroscopy is a spectroscopic technique used to observe vibrational, rotational, and other low-frequency modes in a system. It relies on inelastic scattering, or Raman scattering, of monochromatic light, usually from a laser in the visible, near infrared, or near ultraviolet range. The laser light interacts with molecular vibrations, phonons or other excitations in the system, resulting in the energy of the laser photons being shifted up or down. The shift in energy gives information about the vibrational modes in the system. Infrared spectroscopy yields similar, but complementary, information. Raman spectroscopy is commonly used in chemistry to provide a fingerprint by which molecules can be identified.

Typically, a sample is illuminated with a laser beam. Electromagnetic radiation from the illuminated spot is collected with a lens and sent through a monochromator. Elastic scattered radiation at the wavelength corresponding to the laser line (Rayleigh scattering) is filtered out, while the rest of the collected light is dispersed onto a detector by either a notch filter or a band pass filter.

Spontaneous Raman scattering is typically very weak, and as a result the main difficulty of Raman spectroscopy is separating the weak inelastically scattered light from the intense Rayleigh scattered laser light.

The Raman effect occurs when electromagnetic radiation impinges on a molecule and interacts with the polarizable electron density and the bonds of the molecule in the phase (solid, liquid or gaseous) and environment in which the molecule finds itself. For the spontaneous Raman effect, which is a form of inelastic light scattering, a photon excites the molecule in either the ground rovibronic state or an excited rovibronic state.

When photons are scattered from an atom or molecule, most photons are elastically scattered (Rayleigh scattering), such that the scattered photons have the same energy (frequency and wavelength) as the incident photons. A small fraction of the scattered photons (approximately 1 in 10 million) are scattered by an excitation, with the scattered photons having a frequency different from, and usually lower than, that of the incident photons.

Raman spectra

With the self-built spectrometer described in one of the previous posts, with the software Theremino Spectrometer and with the setup used for the fluorescence measurements we tried to acquire some Raman spectra. The results are only qualitative because the resolution and sensitivity of the instrument are not adequate, however, the acquired spectra  are enough to get an idea of the phenomenon.

Spectrum of Raman scattering of 95% ethanol – Excitation 532nm laser . The peak on the left is the diffusion Raileigh , while the peaks on the right are due to Raman scattering
Spectrum of Raman scattering of chlorophyll in 95% ethanol – Excitation 532nm laser . The peak on the left is the diffusion Raileigh , while the peaks on the right are due to Raman scattering

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