PHD → Physical Chemistry → Spectroscopy and molecular structure ↓
Raman Spectroscopy
Raman spectroscopy is a powerful analytical tool in the field of physical chemistry used to study molecular structure. It is based on the inelastic scattering of photons, known as Raman scattering, which provides extensive details about molecular vibrations and chemical structure.
Basic principle of Raman spectroscopy
When light interacts with a molecule, most of the light is scattered elastically, which is called Rayleigh scattering. However, a small portion of the light is scattered inelastically, which occurs at frequencies different from the incident light. This inelastic scattering is called Raman scattering.
Raman spectroscopy measures the shift in the frequency of inelastically scattered light, giving information about the vibration modes of molecules. If the polarizability of the molecule changes during vibration, Raman scattering can be observed.
Incident photon + molecule --> scattered photon + vibration energy of molecule
Raman Effect
The Raman effect is the basis of Raman spectroscopy. When light scatters inelastically, the scattered photon either gains or loses energy compared to the incident photon, causing a shift known as the Raman shift.
Raman shift (in wave number) = (1/λ incident ) - (1/λ scattered )
Here, λ incident
and λ scattered
refer to the wavelengths of the incident and scattered light.
Process of Raman scattering
When a photon interacts with a molecule, it can be absorbed, causing the molecule to move into a virtual energy state. From this state, the molecule relaxes back into a different vibrational energy state, causing the emission of a photon of different energy. This process can have two outcomes:
- Stokes scattering: The energy of the emitted photon is lower than that of the incident photon, resulting in vibrational energy in the molecule.
- Anti-Stokes scattering: The energy of the emitted photon is higher than that of the incident photon, because the molecule was initially in an excited vibrational state.
In Raman spectroscopy, Stokes scattering is typically measured because it is stronger than anti-Stokes scattering.
Selection rules for Raman spectroscopy
Raman activity depends on the change in the polarizability tensor of the molecule during vibration. For a vibration to be Raman active there must be a change in the polarizability tensor of the molecule.
The selection rules are determined by the symmetry of the molecular vibrations. In general, vibrations that are symmetric with respect to the rotation axis of the molecule are Raman active.
Comparison with infrared spectroscopy
Raman spectroscopy and infrared spectroscopy are complementary techniques used for vibrational analysis.
Aspect | Raman Spectroscopy | Infrared Spectroscopy |
---|---|---|
Principle | Inelastic scattering of light (Raman effect) | Absorption of light (IR effect) |
Sample preparation | Minimum - Single Measurement | Sample preparation may be required |
Limit of detection | Visible range | IR Range |
Selection Rules | Changes in polarization | Change in Dipole Moment |
Applications of Raman Spectroscopy
Raman spectroscopy is used in a variety of fields because of its ability to provide detailed information about molecular vibrations and material structure. Some of its applications are as follows:
- Chemical analysis: Identifying chemical bonds and functional groups.
- Materials science: characterization of materials, especially carbon-based materials.
- Biological studies: Non-destructive analysis of biological materials.
- Forensics: Identifying pigments, explosives, and other substances.
In conclusion, Raman spectroscopy is a versatile tool that provides extensive insight into the molecular structure and composition of substances through the analysis of vibrational modes. Its non-destructive nature and minimal sample preparation make it an invaluable technique in physical chemistry and beyond.