Infrared and Raman Spectroscopy

Infrared and Raman spectroscopy probe molecular vibrations to identify functional groups and characterize chemical structure.

Definition

Infrared and Raman spectroscopy are vibrational spectroscopic methods that characterize molecules from the energies of their bond vibrations, measured respectively through infrared absorption and inelastic light scattering.

Scope

This topic covers the two complementary vibrational techniques: infrared absorption—dominated today by Fourier-transform instruments using sampling modes such as attenuated total reflectance—and Raman scattering. It treats the selection rules that determine which vibrations are infrared- or Raman-active, the interferometer and detector technologies, and the use of vibrational fingerprints for qualitative identification and, increasingly, quantitative analysis.

Core questions

• Which molecular vibrations are infrared-active versus Raman-active, and why?
• How does Fourier-transform infrared spectrometry achieve its speed and sensitivity advantages?
• How is the fingerprint region used to identify and distinguish compounds?
• When are infrared and Raman complementary rather than redundant?

Key theories

Vibrational selection rules

A vibration absorbs infrared radiation only if it changes the molecular dipole moment, whereas it scatters Raman radiation only if it changes the polarizability; this complementarity means symmetric vibrations weak in the infrared are often strong in Raman, and vice versa.

Raman scattering

A small fraction of light scattered by a molecule is shifted in energy by the amount of a vibrational quantum, producing Stokes and anti-Stokes lines whose shifts identify vibrational modes independent of the excitation wavelength.

Mechanisms

In infrared spectroscopy, broadband radiation passes through or reflects from a sample and vibrations that modulate the dipole moment absorb at their characteristic frequencies; a Fourier-transform instrument encodes all frequencies simultaneously through an interferometer and recovers the spectrum mathematically. In Raman spectroscopy, a monochromatic laser illuminates the sample and the small inelastically scattered fraction is dispersed and detected, its frequency shifts reporting the same vibrational modes.

Clinical relevance

Vibrational spectroscopy is widely used for material and polymer identification, pharmaceutical raw-material verification and polymorph screening, forensic trace analysis, and process monitoring, valued for requiring little or no sample preparation.

History

Infrared absorption was used analytically from the early 20th century, with Fourier-transform instruments becoming dominant after the 1960s thanks to faster computing and the multiplex advantage. The Raman effect was reported by C. V. Raman and K. S. Krishnan in 1928, and laser sources later turned Raman scattering into a practical analytical tool.

Key figures

• C. V. Raman
• K. S. Krishnan
• Peter Fellgett

Seminal works

• raman1928
• skoog2017
• harris2020

Frequently asked questions

Why are infrared and Raman spectroscopy considered complementary?

They obey different selection rules: infrared detects vibrations that change the dipole moment, Raman detects those that change polarizability, so a vibration weak in one technique is often strong in the other, and together they give a fuller vibrational picture.

What is the advantage of Fourier-transform infrared over dispersive instruments?

An interferometer measures all frequencies at once rather than scanning them one at a time, giving faster acquisition, higher throughput, and better signal-to-noise—the multiplex and throughput advantages.