What Is Spectroscopy?

Spectroscopy is a branch of physical science concerned with the study of how matter interacts with electromagnetic radiation as a function of wavelength or frequency. When radiation illuminates a substance, some wavelengths are absorbed, emitted, or scattered in patterns determined by the substance's atomic and molecular structure, producing a spectrum that serves as a quantitative fingerprint. The discipline encompasses a broad range of techniques operating from X-ray and ultraviolet wavelengths through visible light, infrared, and into microwave and radio frequency regimes, each probing different aspects of matter's internal energy structure.

Spectroscopy draws its foundations from quantum mechanics and classical electromagnetism. Atomic and molecular energy levels are quantized, meaning transitions between states occur only at specific photon energies corresponding to specific wavelengths. This physical basis gives spectral measurements their chemical specificity: no two elemental species produce identical line patterns, and the positions and relative intensities of spectral features encode information about composition, temperature, pressure, and local electromagnetic environment.

Atomic Spectroscopy

Atomic spectroscopy characterizes the electronic structure of atoms by recording the wavelengths of light they absorb or emit when electrons transition between energy levels. The NIST Atomic Spectra Database maintains the most comprehensive publicly available collection of evaluated atomic spectroscopic data, containing wavelengths, energy levels, and transition probabilities for more than 1,000 atomic spectra spanning from hard X-ray to infrared wavelengths. The NIST Atomic Spectroscopy Group uses spectrometers operating over this full range to compile reference data that underpin analytical techniques in industrial chemistry, plasma physics, astrophysics, and atomic clocks. Laser-induced breakdown spectroscopy (LIBS) is one modern applied form, using a pulsed laser to ablate a small volume of material and recording the emission spectrum to determine elemental composition, a technique applied to geological samples on planetary missions.

Infrared Spectroscopy

Infrared spectroscopy probes molecular vibrations and rotations rather than electronic transitions, making it sensitive to the bonds present in a molecule rather than only its elemental composition. When infrared radiation passes through a sample, bonds absorb radiation at frequencies matching their characteristic vibrational modes, producing an absorption spectrum that identifies functional groups and molecular structure. Fourier transform infrared (FTIR) spectroscopy, the dominant modern implementation, uses a Michelson interferometer to record an interferogram and applies the Fourier transform to recover the spectral domain representation. IEEE research on lamellar grating interferometer FTIR spectroscopy examines advanced interferometer designs that improve throughput and miniaturization. A review in PMC describes FTIR spectroscopic techniques for inorganic material analysis, covering identification of mineral phases, ceramic characterization, and corrosion product analysis where infrared spectral bands reveal structural bonding environments. Thermoreflectance imaging, an adjacent technique, measures wavelength-dependent reflectance changes induced by thermal gradients in semiconductor devices and uses spectral data from these measurements to map surface temperatures.

Optical Emission and Absorption Spectroscopy

Optical emission spectroscopy collects light emitted from excited samples, typically plasmas or flames, and resolves it into constituent wavelengths. Absorption spectroscopy instead measures the attenuation of a broadband source as it passes through a sample. Both approaches exploit the same atomic and molecular line catalog maintained in databases such as NIST's, and both can achieve parts-per-billion sensitivity for trace element detection when combined with laser sources or wavelength-modulated detection schemes.

Applications

Spectroscopy has applications in a wide range of disciplines, including:

• Chemical analysis and quality control in pharmaceutical manufacturing
• Astrophysics and the determination of stellar composition and velocity
• Environmental monitoring of atmospheric gases and pollutants
• Semiconductor device characterization and materials science
• Medical diagnostics including tissue analysis and breath testing
• Food safety testing and agricultural product authentication