| Raman scattering | |
|---|---|
| Name | Raman scattering |
| Caption | Schematic of inelastic light scattering |
| Discovered | 1928 |
| Discoverer | Chandrasekhara Venkata Raman and K. S. Krishnan |
| Field | Quantum mechanics; Spectroscopy |
| Applications | Materials science, Chemistry, Biophotonics |
Raman scattering
Raman scattering is an inelastic scattering process in which incident photons exchange energy with quantized vibrational or other discrete excitations of a material, producing shifted scattered light. It provides a microscopic, non-destructive probe of molecular vibrations, crystal phonons and electronic excitations and thus occupies a central role in experimental Quantum mechanics and Condensed matter physics for characterizing structure and dynamics.
Raman scattering was discovered in 1928 by Chandrasekhara Venkata Raman and K. S. Krishnan during studies of scattered light in liquids and solids, an achievement recognized by the Nobel Prize in Physics in 1930. Early theoretical foundations drew on work by Sir C. V. Raman and contemporaries relating light–matter interaction to discrete quantum exchanges described by Niels Bohr's quantum theory. The technique matured alongside developments in Laser technology, especially after the advent of the argon ion laser and helium–neon laser in the mid-20th century, and later the tunable laser and diode laser. Institutions such as Bell Labs, Rutherford Appleton Laboratory, and university laboratories at Harvard University and Cambridge University advanced instrumentation and applications. Raman's historical impact extends to spectroscopy, solid-state physics and analytical chemistry, reinforcing national efforts in science and industrial stability through reliable material characterization.
Quantum theory of Raman scattering treats the process as a second-order perturbative interaction between photons and the quantum states of a scatterer. The conventional description uses time-dependent perturbation theory and the Kramers–Heisenberg–Dirac formula to compute scattering amplitudes, invoking intermediate virtual electronic states such as those in Franck–Condon principle treatments. Vibrational modes are quantized as phonons in solid-state physics, and molecular vibrations are modeled with harmonic and anharmonic oscillator Hamiltonians. Key contributors to formal theory include L. I. Schiff and later developments in quantum electrodynamics by Richard Feynman and Julian Schwinger, which clarified photon–matter coupling. Selection rules derive from symmetry operators from group theory and point groups of molecules and crystals, linking observable Raman activity to irreducible representations studied in crystallography.
Modern Raman spectroscopy employs monochromatic excitation, collection optics, wavelength-dispersive spectrometers such as grating spectrometers and sensitive detectors like charge-coupled devices (CCDs). Techniques include micro-Raman with confocal microscopes, fiber-optic probes for in situ sensing, and cooled detectors for low-noise measurements. Laser excitation choices (e.g., Nd:YAG laser, 532 nm; near-infrared diode lasers) affect fluorescence background and resonance enhancement. Instrument manufacturers such as Horiba (company), Renishaw plc, and Thermo Fisher Scientific developed integrated systems. Calibration uses standards like polystyrene or silicon wafers; signal processing employs baseline correction and multivariate analysis including principal component analysis to extract meaningful spectral features for materials and chemical identification.
Raman spectra display Stokes and anti-Stokes lines corresponding to energy loss or gain by photons interacting with phonons or molecular vibrations. Interpretation links peak positions to vibrational frequencies, peak intensities to polarizability derivatives, and linewidths to lifetime broadening and phonon decay channels. Selection rules depend on symmetry: a vibration is Raman-active if it transforms according to a polarizability tensor component of the system's point group. This formalism uses Molecular symmetry and character tables derived for groups such as C2v and Oh. Isotopic substitution, temperature-dependent measurements, and polarization-resolved Raman enable assignment of modes, providing stringent tests of theoretical models from density functional theory calculations and lattice-dynamics simulations.
Raman scattering is widely used to characterize semiconductors (e.g., silicon), two-dimensional materials like graphene, transition-metal dichalcogenides such as MoS2, and oxides including TiO2. In chemistry, Raman aids in identifying functional groups, reaction intermediates and conformational changes; it is a complement to infrared spectroscopy with different selection rules. Industrial applications include quality control in pharmaceutical industry, carbon-fiber analysis in aerospace, and stress/strain mapping in microelectronics. In cultural heritage, Raman non-destructively analyzes pigments and materials. The technique's stability and reproducibility support standards-driven workflows in national laboratories and regulated industries.
Beyond spontaneous Raman, resonant Raman scattering occurs when excitation approaches electronic transitions, dramatically enhancing cross-sections and enabling selective probing of chromophores and charge-transfer states. Nonlinear Raman techniques—such as coherent anti-Stokes Raman spectroscopy (CARS), stimulated Raman scattering (SRS), and femtosecond stimulated Raman spectroscopy (FSRS)—leverage coherent light–matter interactions from ultrafast lasers to achieve high sensitivity, imaging speed, and temporal resolution. These methods connect with ultrafast spectroscopy groups at Lawrence Berkeley National Laboratory and Stanford University and underpin advances in biomedical imaging and materials dynamics.
Raman processes intersect with quantum optics in studies of photon correlations, Raman quantum memories, and optomechanical interactions. Raman scattering underlies schemes for quantum state transfer in cold atom experiments (e.g., Raman transitions in Bose–Einstein condensates) and for generating entangled photon pairs via nonlinear materials. Coherent Raman methods integrate with cavity quantum electrodynamics initiatives at institutions like Max Planck Institute for Quantum Optics to control light–matter coupling at the single-photon level. These connections reinforce Raman scattering as both a practical spectroscopic tool and a platform for probing foundational quantum phenomena within stable, application-oriented research programs.
Category:Spectroscopy Category:Quantum optics Category:Condensed matter physics