Raman spectroscopy

Raman spectroscopy relies on the inelastic scattering of monochromatic light, usually from a laser in the visible, near-infrared, or ultraviolet range. When photons interact with molecular vibrations, most are elastically scattered (Rayleigh scattering) with the same energy, but a tiny fraction undergo energy exchange, resulting in Stokes (energy loss) or anti-Stokes (energy gain) shifts. These shifts correspond to the vibrational energy levels of the molecule, producing a spectrum unique to the chemical bonds and symmetry.

The intensity of Raman scattering is inherently weak, often requiring sensitive detectors and long acquisition times. However, techniques such as surface-enhanced Raman spectroscopy (SERS) can amplify signals by factors up to 10¹⁴, enabling single-molecule detection. The selection rules differ from infrared spectroscopy: Raman is sensitive to changes in polarizability, while IR depends on dipole moment changes, making the two techniques complementary.

Modern Raman instruments typically consist of a laser source, a sample illumination and collection system, a spectrometer, and a detector such as a CCD. Dispersive Raman spectrometers use gratings to separate wavelengths, while Fourier-transform (FT) Raman systems employ interferometers, often with near-infrared lasers to reduce fluorescence interference.

Specialized variants extend the technique's capabilities: confocal Raman microscopy provides depth-resolved analysis with sub-micrometer spatial resolution; resonance Raman enhances signals from specific chromophores; and spatially offset Raman spectroscopy (SORS) allows non-invasive probing of subsurface layers. Portable handheld Raman devices have become common for field applications, including pharmaceutical verification and hazardous material identification.

Raman spectroscopy is widely used in chemistry and materials science to identify polymorphs, monitor reactions, and characterize carbon nanomaterials like graphene and carbon nanotubes. In pharmaceuticals, it is employed for quality control and counterfeit detection, while in geology it helps identify mineral inclusions and fluid inclusions in rocks.

In biology and medicine, Raman imaging maps biochemical distributions in cells and tissues, aiding in cancer diagnosis and surgical guidance. It is also used in art conservation to analyze pigments and in forensic science to identify trace evidence. Industrial applications include process monitoring in chemical plants and quality assessment in food production.

The Raman effect was predicted theoretically by Adolf Smekal in 1923, but it was C.V. Raman who experimentally demonstrated it in 1928, earning the Nobel Prize in Physics in 1930. The effect was initially observed using sunlight and a filter as the monochromatic source, predating lasers.

Raman spectroscopy has been used to analyze the composition of planetary surfaces, including Mars, via rover instruments like SuperCam. It also plays a role in the study of cultural heritage, revealing hidden layers in manuscripts. A niche application is the analysis of single-walled carbon nanotubes, where the radial breathing mode provides diameter information. Additionally, tip-enhanced Raman spectroscopy (TERS) combines scanning probe microscopy with Raman to achieve nanoscale spatial resolution, enabling the study of single molecules.

Stokes shift

The energy loss of scattered photons, corresponding to the excitation of a vibrational mode.

Anti-Stokes shift

The energy gain of scattered photons, occurring when the molecule starts in an excited vibrational state.

Rayleigh scattering

Elastic scattering of light with no energy change, dominating the scattered signal.

SERS

Surface-enhanced Raman spectroscopy, a technique that amplifies Raman signals using nanostructured metallic surfaces.

Confocal Raman microscopy

A Raman technique that uses a pinhole to reject out-of-focus light, enabling depth-resolved imaging.