What is the Difference between IR and Raman Spectroscopy?

What is IR Spectroscopy?

Infrared (IR) spectroscopy is a class of vibrational spectroscopy, which relies on the transmittance, absorbance, or reflectance of infrared light. By means of IR spectroscopy, light is absorbed in diverse amounts in a sample at different frequencies, which resemble the vibrational frequencies of bonds in the sample.

What is Raman Spectroscopy?

Raman spectroscopy is an occurence of inelastic scattering, that probes molecular vibrations to provide a molecular footprint of materials. 

The following article discusses the differences between IR spectroscopy and Raman spectroscopy.

Difference between IR Spectroscopy and Raman Spectroscopy

• Raman spectroscopy reckons on an amendment in polarizability of the molecule, whereas IR spectroscopy is reliant upon the change in dipole moment.
• Raman spectroscopy calculates relative frequencies in which the sample scatters radiation. IR spectroscopy evaluates absolute frequencies in which a sample absorbs radiation.
• Raman spectroscopy is sensitive to homo-nuclear molecular bonds. It is able to distinguish between single, double, and triple bonds between carbon atoms. IR spectroscopy has responsiveness with regard to hetero-nuclear functional group vibrations and polar bonds. An example is OH stretching in water.
• In sample preparation, Raman spectra analysis has no requirement. But IR spectroscopy has constraints of a thickness of sample, uniformity, and dilution for avoiding saturation. 
• Fluorescence will interfere with the Raman imaging, but it is not an issue in taking the IR spectrum.
• Both the methods have some limitations when taken separately. But, when used combinedly, they become a powerful tool in material characterization. Both can be used with microscopic techniques.
• Raman spectroscopy is a weaker technique, while IR spectroscopy is a stronger technique. 
• Molecules with functional groups having strong dipoles display strong peaks in IR. Molecules with functional groups possessing weak dipoles readily alter in polarizability, manifesting strong peaks in the Raman spectrum.
• Raman spectroscopy furnishes data about intramolecular vibration and intermolecular vibration. Intramolecular vibration displays specific vibrations of atoms in a molecule and thus, helps in identifying a substance. Intermolecular vibration provides information about lower frequency modes, which reflects crystal lattice structure and polymorphism.
• IR spectroscopy probes the fingerprint region of the region, Intramolecular vibrations are highly characteristic of bonding of atoms as they are well defined,
• In the investigation of crystallization processes, Raman spectroscopy analyses solid crystal forms in solution, whereas IR spectroscopy is utilized for solution-phase supersaturation studies.
• Raman spectrometers have laser light sources operating in the near IR or visible region in the electromagnetic spectrum. IR spectrophotometers possess black body radiators providing energy in the mid-IR region.
• Raman spectrometers have holographic grating with CCD detectors. IR spectrometers use interferometers providing modulated radiation, which is detected by photonic detectors or pyroelectric detectors.

• The Raman spectrum results from the scattering of light by vibrating molecules. The IR spectrum results from the absorption of light by vibrating molecules.
• Raman activity occurs due to the change of polarizability of a molecule. IR activity occurs due to changing dipole moments.
• In Raman spectroscopy imaging, monochromatic light of high-intensity laser beams can be used in IR, UV, or visible regions. In IR spectroscopy, the light range is limited to IR frequency.
• In Raman spectroscopy instrumentation, scattered light is observed at right angles to the incident beam. Regarding the IR spectroscopy, the absorption signal is estimated in the same direction as the incident beam.
• The technique of Raman spectroscopy is non-destructive. Sample measurement can be done directly in a glass container. In the case of pharmaceutical samples, they can be measured directly in sachets.IR spectroscopy needs sample preparation through the KBr pellet for observation of gels, films, and liquids.
• Raman spectrum is a result of Raman spectroscopy, where a laser is used to analyze the sample due to the inelastic scattering of photons in the sample. IR spectrum is a result of IR spectroscopy, where IR radiation is utilized to analyze the sample.
• Raman spectrum is based on light scattering, whereas Raman scattering is based on light absorption. 
• Vibrational modes are active in Raman spectroscopy if they cause a change in polarizability. Vibrational modes are active in IR spectroscopy if they cause changes in dipole moments.
• Raman analysis can use water as a solvent, whereas IR spectroscopy cannot use water as a solvent.

• Raman spectroscopy is a highly expensive method, while IR spectroscopy is relatively inexpensive.
• In Raman spectroscopy, laser sources are highly intensive. It facilitates focusing the beam on a small sample area with small volumes. When the quantity of samples is limited, this facility serves as a primary advantage. A higher degree of amplification of weak Stoke signals is a requisite in the presence of intense Rayleigh light scattering. This results in a higher cost for the analysis when compared to IR spectroscopy. However, the higher cost can be easily justified against the benefits offered by the technique.
• In Raman spectroscopy, bands present below 400 cm^-1 can be measured. As scattering effects are usually weaker, Raman spectroscopy exhibits lower sensitivity at times. Sometimes, visible light excitation, fluorescence, and contamination can overlap with the signal.
• Observation of Raman spectra will be decided by the polarizability of the molecule. For observation of IR spectra, the criterion is presence of dipole moment in a molecule.
• The Raman spectrum can be recorded at a single exposure, whereas two separate runs are required with different prisms to cover the whole region of IR.
• Water is used as a solvent for Raman spectroscopy, while water cannot be used as a solvent for IR spectroscopy as it is opaque to IR
• Raman spectroscopy is very accurate, but not that much sensitive. IR spectroscopy is both accurate and sensitive.
• In Raman spectroscopy,optical systems are made up of quartz or glass, while in IR spectroscopy, optical system is composed of special crystals like NaBr and CaF₂ 
• Occasionally, photochemical reactions occur in the regions of Raman lines and create difficulties for the Raman spectrum. In IR spectroscopy, photochemical reactions do not occur.
• In Raman spectroscopy, substances to be investigated should be colorless and pure. 
• Since Raman lines are weaker in intensity, concentrated solution should be used in increasing the intensity of Raman lines. In IR spectroscopy, dilute solutions are preferred for measurement
• In Raman spectroscopy, vibrational frequencies of larger molecules can be measured. In the case of IR spectroscopy, vibrational frequencies of larger molecules cannot be measured.
• Homonuclear diatomic molecules are found to be active in Raman spectroscopy. In IR spectroscopy, homonuclear diatomic molecules are not active.

Conclusion

We have learned the differences exhibited by IR and Raman spectroscopy. Raman Spectroscopy is a simple process. But at the same time, it is costlier than IR spectroscopy. Both the processes, Raman spectroscopy, and IR spectroscopy are complementary to each other. Hence, for highly demanding experiments, it is wise to choose Raman spectroscopy. For not-so-important experiments, IR spectroscopy can be exploited.