Raman spectroscopy

Raman spectroscopy is a powerful analytical technique used to study vibrational, rotational, and other low-frequency modes in a system. It relies on the inelastic scattering of monochromatic light, typically from a laser source, to provide a molecular fingerprint of the material under investigation. The technique is named after C. V. Raman, who, with K. S. Krishnan, first observed the effect in liquids. It has become a cornerstone tool in fields ranging from chemistry and pharmacology to materials science and art conservation.

Principles of Raman scattering

The fundamental principle involves the interaction of photons with the vibrational or rotational energy states of molecules. When monochromatic light from a source like an argon-ion laser interacts with a sample, most photons are elastically scattered. A tiny fraction, however, undergoes inelastic scattering, where the photon exchanges energy with the molecule. This energy shift, known as the Raman shift, is measured relative to the incident laser frequency and corresponds to specific molecular vibrations. The process can result in Stokes scattering, where the scattered photon has less energy, or anti-Stokes scattering, where it gains energy. The resulting spectrum provides detailed information about molecular symmetry, chemical bonds, and crystal structure, complementary to data obtained from infrared spectroscopy.

Instrumentation

A typical system consists of several key components. A high-intensity, single-wavelength laser, such as those from Coherent or Spectra-Physics, serves as the excitation source. The light is focused onto the sample via lenses or a microscope objective, with systems from companies like Olympus or Nikon enabling micro-Raman spectroscopy. The scattered light is collected and passed through a filter, like a notch filter or edge filter, to block the intense Rayleigh line. The Raman signal is then dispersed by a high-resolution spectrograph, often using a diffraction grating from manufacturers like Newport, and detected by a sensitive device such as a charge-coupled device developed by companies like Teledyne or a photomultiplier tube. Modern systems are often controlled by software from LabVIEW or similar platforms.

Applications

The technique finds extensive use across numerous scientific and industrial disciplines. In pharmaceutical analysis, it is used for polymorph screening and tablet quality control, as endorsed by the Food and Drug Administration. Within materials science, it characterizes carbon nanotubes, graphene layers, and semiconductor stresses. Geologists employ it for in-situ mineral identification, while in art conservation, it helps identify pigments without damaging priceless works, such as those in the Louvre or the British Museum. It is also pivotal in life sciences for studying cellular components and in security screening for detecting explosive materials at airports like Heathrow Airport.

Variations and related techniques

Several advanced variants have been developed to enhance sensitivity or specificity. Surface-enhanced Raman spectroscopy utilizes nanostructured surfaces of gold or silver to dramatically amplify the signal from adsorbed molecules. Resonance Raman spectroscopy involves tuning the laser wavelength to match an electronic transition of the analyte, greatly increasing the signal for specific chromophores. Other notable techniques include tip-enhanced Raman spectroscopy, which combines the method with atomic force microscopy, and coherent anti-Stokes Raman spectroscopy, a nonlinear process used for rapid imaging. These methods are often discussed at conferences organized by societies like the Society for Applied Spectroscopy.

History and development

The effect was theoretically predicted by Adolf Smekal in 1923 and first experimentally observed in 1928 by C. V. Raman and his collaborator K. S. Krishnan at the University of Calcutta, using sunlight and a telescope as a collector. For this discovery, C. V. Raman was awarded the Nobel Prize in Physics in 1930. Initial progress was slow due to the weakness of the signal, but the advent of the laser in the 1960s, following work by Theodore Maiman and others at Hughes Aircraft Company, revolutionized the field. Subsequent developments in detectors, such as the charge-coupled device, and filters transformed it into a routine analytical tool. Landmark applications include its use on the Viking program landers to analyze the Martian soil.

Category:Spectroscopy Category:Indian inventions