Introduction to Raman Spectroscopy Fundamentals

What is Raman spectroscopy?

Raman spectroscopy is a type of scattering spectroscopy that is based on the interaction between light and materials.

Definition of Raman scattering: When intense incident light from a laser source is scattered by molecules, most of the scattered light has the same wavelength (color) as the incident laser, and this scattering is called Rayleigh scattering. However, there is a very small portion (about 1/10^9) of scattered light whose wavelength (color) is different from the incident light. The change in wavelength is determined by the chemical structure of the test sample (the so-called scattering substance), and this portion of scattered light is called Raman scattering.

Figure 1:Schematic Diagram of Raman Spectroscopy Principle

What is Raman spectroscopy analysis?

Raman spectroscopy analysis is a method based on the Raman scattering effect discovered by Indian scientist C.V. Raman, which analyzes the scattered spectra with different frequencies from the incident light to obtain information about molecular vibrations, rotations, and is applied to the study of molecular structures.

What are the significant characteristics of Raman spectroscopy?

1. Although the wavenumbers of Raman scattering lines differ with the wavenumbers of the incident light, for the same sample, the shift of the same Raman line is independent of the wavelength of the incident light. It is only related to the vibration and rotation energy levels of the sample, and the Raman shifts of different substances are different. (This is also the basis for using Raman spectroscopy for qualitative analysis of sample structures.)

Figure 2:Illustration of Stokes Scattering and Anti-Stokes Scattering

2. In Raman spectra plotted with wavenumber as the variable, Stokes lines and anti-Stokes lines are symmetrically distributed on both sides of the Rayleigh scattering line. This is because in the two cases mentioned above, they respectively correspond to gaining or losing the energy of a vibrational quantum.

Note: In practical use, Raman shift (Δν) is usually plotted on the x-axis, and Raman intensity is plotted on the y-axis.

3. Generally, Stokes lines are stronger than anti-Stokes lines. This is due to the Boltzmann distribution, where the number of particles in the vibrational ground state is much larger than the number of particles in the vibrational excited state.

What does a Raman spectrum typically consist of? What are its characteristics?

A Raman spectrum typically consists of a number of Raman peaks, each representing a specific Raman shift and intensity. Each peak corresponds to a specific molecular vibrational mode, including individual chemical bonds such as C-C, C=C, N-O, C-H, as well as vibrations of groups composed of several chemical bonds, such as the breathing mode of benzene rings, vibrations of polymer chains, and lattice vibrations.

Raman spectroscopy can provide detailed information about sample chemical structures, phases, morphologies, crystallinity, and molecular interactions.

Figure 3: (a) Wide scan XPS spectra of (a) GO, and (b) Rh@rGO hybrid. The high‐resolution XPS spectra of (b) C1s of GO, (c) C1s of Rh@rGO hybrid, and (d) Rh 3d of Rh@rGO hybrid

Is Raman spectroscopy used for qualitative or quantitative analysis?

Raman spectroscopy is typically used for qualitative analysis but can also be used for quantitative analysis under specific conditions. Typically, Raman spectra (including peak positions and relative intensities) provide unique chemical fingerprints of substances, which can be used to identify and differentiate them from other substances. Actual Raman spectra obtained from testing are often complex, and determining the identity of unknown substances through peak assignment can be quite complicated. However, searching Raman spectral databases for matching results can quickly discriminate unknown substances.

Under constant conditions, the intensity of the spectrum is proportional to the sample concentration. By using samples of known concentrations to establish the relationship between peak intensity and concentration (standard curve), quantitative analysis can be performed. For mixtures, relative peak intensities can provide information about the relative concentrations of various components, while absolute peak intensities can reflect absolute concentration information (with reference to standard concentration calibration).

Significant advantages of Raman spectroscopy technology

1. Wide Analytical Range: Almost all substances containing real molecular bonds can be analyzed by Raman spectroscopy, including solids, powders, pastes, liquids, colloids, and gases. Raman spectra can cover a range of 50-4000 wavenumbers simultaneously, allowing analysis of organic and inorganic materials, and even biological materials (while changing gratings, beam splitters, filters, and detectors would be required to cover the same range with infrared spectroscopy). Raman spectroscopy can analyze solutions, solid mixtures, and pure substances.Raman Spectra of Mixed Material Samples: The Raman spectrum obtained from a sample contains information from all molecules within the test volume (the volume illuminated by the laser). Therefore, the Raman spectrum of a mixture contains Raman signals representing all different molecules within the test volume. If the various components of the mixture are known, then the relative peak intensities can measure the relative concentrations of the mixture components.Gas Sample Raman Spectra: Although gas samples can also be analyzed by Raman spectroscopy, measuring Raman spectra of gases is relatively challenging due to the extremely low molecular density of gases. Typically, high-power lasers and sample cells with longer path lengths are required for gas Raman spectroscopy.

2. Minimal Sample Damage: Raman spectroscopy is a non-destructive analytical technique (thus widely used in archaeology, artifact identification, etc.).

3. Fast, Simple, and Reproducible: No sample preparation is needed, and samples can be measured directly through fiber optic probes or through glass, quartz, and optical fibers.The acquisition time for Raman spectra is determined by a series of factors, including the properties of the sample itself, the quality requirements of the spectrum, and the Raman spectrometer used. Modern Raman spectrometers can acquire high-quality Raman spectra in a matter of seconds.

4. Very suitable for analyzing samples containing water, including solutions, biological tissues, and cells, etc. The Raman scattering cross-section of water molecules is very small, so the Raman scattering intensity is much weaker than that of other molecules. In addition, the Raman spectrum of water molecules is very simple, with only a few Raman peaks, and there is minimal interference from Raman peaks of dissolved substances. In most cases, even though water molecules are present in large quantities, the Raman peak intensities of solutes are much greater than those of water. Therefore, analyzing solutes in aqueous solutions is straightforward.

5. Raman spectral peaks are clear and sharp, making them more suitable for quantitative studies, database searches, and qualitative studies using difference analysis. In chemical structure analysis, the intensity of independent Raman bands can be correlated with the number of functional groups.

6. Raman testing requires small sample amounts and small test areas: Because the diameter of the laser beam is typically only 0.2-2 millimeters at its focal point, conventional Raman spectroscopy requires only a small amount of sample. This is a major advantage of Raman spectroscopy over conventional infrared spectroscopy. Moreover, the objective lens of a Raman microscope can further focus the laser beam to 20 microns or even smaller, allowing analysis of smaller sample areas. Of course, in fact, modern Raman systems can also analyze macroscopic large samples.

7. Resonance Raman Effect (Surface-Enhanced Raman Scattering, SERS, etc.): The resonance Raman effect can selectively enhance the vibrations of large biomolecule-specific chromophores, whose Raman signal intensities can be selectively enhanced by 1000 to 10,000 times.

8. Raman spectroscopy has various advanced applications and can be combined with various characterizations, such as Micro-Raman spectroscopy (Raman spectroscopy imaging, etc.), automated Raman spectroscopy detection and high-throughput screening, in-situ Raman technology (real-time analysis of the relationship between catalyst structure and performance), Raman-Atomic Force Microscopy, Raman-Photoluminescence, Raman-Scanning Electron Microscopy-Cathodoluminescence, etc.