Infrared spectroscopy

Infrared spectroscopy studies substances by measuring their absorption, emission, or reflection of infrared radiation. Molecular vibrations produce characteristic spectral features that reveal chemical bonds and environments.

Molecular vibration: Periodic motion of atoms in a molecule around their equilibrium positions. Infrared absorption occurs when radiation couples to a vibration that changes the molecule’s dipole moment.
Electromagnetic spectrum: The full range of electromagnetic radiation frequencies and wavelengths. Infrared spectroscopy uses a defined region between visible light and microwaves.
Fourier-transform infrared spectroscopy: An infrared spectroscopy method that uses an interferometer and Fourier transformation to obtain a spectrum. It is the dominant modern approach for collecting broadband infrared spectra.
Raman spectroscopy: A vibrational spectroscopy method based on inelastic scattering of monochromatic light. Its selection rules complement infrared activity, so the methods often reveal different vibrations.
William Herschel: An astronomer who discovered infrared radiation in 1800 by measuring heating beyond the red end of sunlight’s spectrum. His discovery established the radiation that infrared spectroscopy later measured.
Harmonic oscillator: An idealized system whose restoring force is proportional to displacement from equilibrium. It approximates molecular bond vibrations and predicts their fundamental frequencies.
Wavenumber: The number of wave cycles per unit distance, commonly reported in reciprocal centimeters. Infrared spectra conventionally place wavenumber on the horizontal axis.
Attenuated total reflectance: A sampling method that measures absorption from an evanescent field at a crystal–sample interface. It enables rapid measurements of solids and liquids with little sample preparation.
Near-infrared spectroscopy: Spectroscopy using near-infrared radiation, where signals are dominated by overtones and combination bands. It is useful for rapid bulk analysis but often requires statistical calibration.
William Coblentz: An American physicist who measured infrared spectra of many compounds in the early twentieth century. His spectral measurements helped establish infrared spectroscopy as a tool for chemical identification.

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