Raman spectroscopy relies on inelastic scattering of monochromatic light, usually from a laser source. The laser light interacts with molecular vibrations, phonons, or other excitations, resulting in the energy of the laser photons being shifted up or down. The shift in energy provides information about the vibrational modes in the molecular system under study.
A more concise yet formal definition would be:
Raman spectroscopy is a non-destructive and non-invasive spectroscopic technique used to analyze vibrational, rotational, and other low-frequency modes in a molecular system.
A Brief History of Raman Spectroscopy
Before delving further in, a bit of the history of Raman Spectroscopy as well as the Raman scattering phenomenon will provide better context.
- The Raman scattering phenomenon was first predicted theoretically by Adolf Smekal in 1923.
- The effect was experimentally observed for the first time by C.V. Raman and K.S. Krishnan in 1928 using sunlight as the excitation source.
- In 1930, Raman won the Nobel Prize in Physics for this discovery.
- Early studies used mercury lamps or solar radiation as excitation sources. The advent of lasers revolutionized the field starting in the 1960s by providing monochromatic, high-intensity excitation. This helped Raman spectroscopy become a practical analytical technique.
- Instrumentation, detection, and data processing advances since the 1970s have improved the sensitivity, resolution, and speed of Raman systems.
The Working Principle of Raman Spectroscopy
Raman Spectroscopy is based on the simple phenomenon known as Raman Scattering
A Brief Rundown on Raman Scattering
The process of Raman scattering can be described in simple terms. When light hits a molecule, the electrons around the bonded atoms can be considered to form an oscillating dipole that emits radiation of specific wavelengths.
The majority of the scattered light is at the same wavelength as the excitation source, this is known as Rayleigh or elastic scattering. However, a very small fraction of light (approximately 1 in 107 photons) is inelastically scattered such that the emitted photon has a different wavelength than the incident photon.
When a molecule relaxes from an excited vibrational energy state, the emitted photon has lower energy (longer wavelength) than the excitation photon. This is known as Stokes scattering. If the molecule was already in an excited vibrational state before excitation, the emitted photon will have higher energy than the incident photon, which is called anti-Stokes scattering.
Raman spectroscopy can characterize chemical structure, crystalline phase, polymorphs, and intermolecular interactions in a non-invasive way. Unlike other techniques that require extensive sample prep or destroy the sample, Raman analysis can be done rapidly on the sample in its native state. This makes it a versatile tool for detailed chemical and structural characterization across many fields.
By measuring the wavelength shift between the incident and scattered light, the energy of molecular vibrations can be determined. This provides a Raman spectrum that acts like a molecular fingerprint, allowing identification and characterization of the molecule.
Raman Spectrum of Benzonitrile
An example of a Benzonitrile compound is shown below for a better understanding of Raman Spectroscopy.
The distinct peaks correspond to different vibrational normal modes in the benzonitrile molecule, providing a spectroscopic fingerprint. The strong band at 2226 cm−1 represents the CN triple bond stretching vibration. The peaks between 1000-1700 cm−1 arise from C=C and C=N vibrations. The lower wavenumber bands correspond to ring vibrations and C-C stretches. This detailed vibrational information allows unambiguous identification of benzonitrile and differentiation from similar molecules.
Notable Advantages of Raman Spectroscopy
Raman spectroscopy has several notable advantages that make it a highly useful analytical technique.
- Firstly, it requires little to no sample preparation since water has a weak Raman scattering cross-section. Therefore aqueous solutions and biological samples can be analyzed without interference. The scattering process is weak and non-destructive, allowing repeated analysis of precious samples.
- Raman also provides chemical fingerprinting through the vibrational spectra, allowing the identification of molecular structures.
- A major advantage is the versatility of Raman spectroscopy. It can analyze solid, liquid, and gas phase samples across diverse disciplines in physics, chemistry, biology, geology, and more.
- Raman is also compatible with fiber optics, allowing remote sensing capabilities. Significant improvements in spatial resolution are possible using confocal laser excitation and microscopy, enabling analysis at the micron scale. With carefully engineered substrates, even single-molecule sensitivity can be achieved.
Applications of Raman Spectroscopy
Due to its multitude of advantages, Raman spectroscopy has become an indispensable analytical technique across many scientific and industrial fields.
In chemistry, it is routinely used for identifying unknown materials, monitoring chemical reactions in situ, and analyzing pharmaceutical compounds.
Physics researchers employ Raman spectroscopy to characterize low-dimensional materials like graphene and carbon nanotubes.
Biology and Medicinal Research
The technique is also widely used in biology and medicine for DNA/RNA characterization, cancer diagnostics, and live tissue measurements. Raman’s compatibility with water and fiber optics makes it ideal for in vivo studies.
In geology, Raman serves as a useful tool for analyzing the mineral composition of rocks and soils. The same principles are being used in planetary science experiments to study the composition of planetary bodies.
Raman spectroscopy also has diverse industrial applications ranging from quality control of manufactured products to process monitoring and optimization. The automotive and semiconductor industries rely heavily on Raman for rapid, non-destructive analysis.
There are also niche applications in fields like art, archaeology, and forensics where Raman can identify pigments, analyze artifacts, or detect traces of explosives.
Current Developments Regarding Raman Spectroscopy
Since its discovery in 1928, Raman spectroscopy has come a long way, enabled by numerous technological advances. The advent of lasers revolutionized the field in the 1960s by providing high-intensity monochromatic sources. Holographic notch filters and multichannel detectors improved signal-to-noise ratios. Fiber optic probes have enabled remote, in situ sampling. In the past decade, major improvements in CCD detectors, laser, and microscope optics have increased the sensitivity and spatial resolution of Raman techniques.
Current trends focus on enhancing resolution, multiplexing capability, and developing novel contrast mechanisms. Tip-enhanced Raman spectroscopy (TERS) uses an atomic force microscope tip to overcome the diffraction limit and provide nanoscale resolution.
Multiplex stimulated Raman techniques like Coherent Anti-Stokes Raman Scattering (CARS) enable video-rate hyperspectral imaging.
Surface and nonlinear enhancement techniques like SERS (Surface Enhanced Raman Spectroscopy) provide sensitivity down to single molecule levels. Machine learning methods are being utilized for spectral analysis and image processing. Portable, low-cost Raman systems are also being developed. These current advances are expanding the possibilities and applications of Raman spectroscopy.
Looking ahead, the incorporation of Raman techniques with emerging lab-on-a-chip technologies could provide rapid spectral diagnosis capability. New nanoscale contrast agents and multiplexing methods may enable sensing biochemical processes in live cells. Machine learning has significant potential to improve the analysis of complex Raman datasets. Portable Raman devices could find widespread point-of-care clinical use. There are also possibilities for novel endoscopic probes and in vivo surgical guidance. Raman spectroscopy has come a long way since its discovery, but the future looks even brighter in terms of capabilities and impactful applications across a diverse range of fields.
Frequently Asked Questions
What type of radiation is used in Raman spectroscopy?
Monochromatic radiation, usually from a laser in the visible, near-infrared, or near ultraviolet range is used. Lasers allow an intense source of photons to interact with the sample. Common wavelengths include 532 nm, 633 nm, 785 nm, and 1064 nm.
How does Raman spectroscopy differ from IR spectroscopy?
Both techniques probe molecular vibrations. IR spectroscopy measures the absorption of light by the vibrational modes. Raman spectroscopy measures the inelastic scattering of light by the vibrational modes. IR is subject to specific selection rules whereas Raman can analyze symmetric vibrations in non-polar molecules. Water also strongly absorbs IR but has a weak Raman scattering cross-section, so Raman is better for biological samples.
What does a Raman spectrum look like?
A Raman spectrum plots Raman intensity vs Raman shift in wavenumbers (cm-1). The wavenumber shift from the excitation wavelength corresponds to the energy of different vibrational modes. The spectrum appears as a series of peaks or bands at different wavenumbers that form a molecular fingerprint.
What types of samples can be analyzed by Raman spectroscopy?
Raman can analyze solid, liquid, gaseous, and biological samples with little preparation. It is suitable for samples enclosed in glass or aqueous solutions. For surface analysis, samples should be reflective or scattering. Very luminescent or fluorescent samples can pose challenges.
How long does a Raman experiment take?
A single Raman spectrum can take seconds to minutes to acquire depending on the sample and desired signal-to-noise ratio. Mapping a larger sample area with hyperspectral Raman imaging takes longer but provides valuable spatial information.