Spectroscopy

Spectroscopy is the study of interactions between matter and electromagnetic radiation, analyzed as a function of wavelength, frequency, or energy. Originally concerned with visible light separated into colors, it now encompasses measurements across the electromagnetic spectrum and techniques that detect emitted particles, such as electrons. Spectra provide information about chemical identity, concentration, molecular structure, and physical conditions. The field connects physics, chemistry, and astronomy, and supplies many tools used in analytical chemistry. (edu.rsc.org)

Physical principles

A spectrum records a measured quantity, such as transmitted intensity or emitted power, against a spectral coordinate. Wavelength and frequency describe the same radiation in different ways: in vacuum, $c=\lambda\nu$, where $c$ is the speed of light. A photon carries energy $E=h\nu$, with $h$ the Planck constant. Spectral measurements therefore connect the properties of radiation to energy differences within matter. (nist.gov)

In quantum mechanics, bound systems possess discrete energy levels. For a transition involving absorption or emission of one photon,

This relationship makes spectroscopy a direct probe of energy quantization. An atom absorbs radiation when the photon energy matches an accessible transition to a higher level; an excited atom can emit radiation when it moves to a lower level. Different electronic structures produce different patterns of spectral lines. (science.nasa.gov)

A molecule additionally has rotational and vibrational states. Its spectrum can therefore contain groups of closely spaced transitions forming bands rather than isolated lines. Electronic, vibrational, and rotational contributions provide different kinds of structural information. Spectral patterns are often described as fingerprints, although identifying a substance requires comparison with appropriate reference data and measurement conditions. (science.nasa.gov)

Types of interaction

Absorption spectroscopy measures the reduction of radiation passing through a sample. Frequencies absorbed by the sample appear as reduced transmission or increased absorbance. Emission spectroscopy measures radiation produced by excited matter, including gases, flames, plasmas, and astronomical objects. Corresponding transitions have the same characteristic frequencies in absorption and emission, although their observed strengths depend on the populations of the relevant states. (science.nasa.gov)

Scattering methods examine radiation redirected by matter. In Raman spectroscopy, a small fraction of incident light exchanges energy with molecular vibrations or other excitations. The resulting frequency shifts reveal information complementary to absorption measurements. Fluorescence spectroscopy instead observes emission following excitation; fluorescence lifetimes characterize how long excited states persist. (ccmr.cornell.edu)

Major techniques

Ultraviolet–visible spectroscopy principally probes electronic transitions. It can measure the concentration of absorbing substances and characterize optical properties, including the band gaps of semiconductors. Infrared spectroscopy commonly probes molecular vibrations, helping identify functional groups and distinguish compounds through their absorption patterns. Vibrations depend on atomic masses and the properties of chemical bonds. (mri.psu.edu)

Nuclear magnetic resonance spectroscopy probes nuclear spin transitions in a magnetic field. Differences in resonance behavior reveal local chemical environments; proton NMR can distinguish hydrogen environments within an organic molecule. It is therefore a structural method rather than a technique restricted to visible or infrared light. (edu.rsc.org)

X-ray emission spectroscopy identifies elements through characteristic photon energies. Photoelectron spectroscopy measures electrons ejected after irradiation rather than the transmitted or emitted light itself. Electron energies can reveal elemental composition, chemical bonding, and oxidation states. These techniques illustrate why spectroscopy extends beyond simply separating visible light into colors. (ccmr.cornell.edu)

Instruments and measurement

A typical optical spectrometer combines a radiation source, a sample arrangement, wavelength-selecting or encoding optics, and a detector. Sources may include lamps or a laser. A diffraction grating or prism separates wavelengths; entrance and exit slits control the admitted light and influence resolution. Absorption instruments compare radiation transmitted through the sample with a reference or background measurement. (sites.science.oregonstate.edu)

Fourier-transform instruments encode spectral information differently. In Fourier-transform infrared spectroscopy, an interferometer varies the optical path difference between two beams, producing an interferogram. A Fourier transform converts this signal into a spectrum. Combining spectroscopy with a microscope permits spatially resolved measurements of composition and structure. (sites.science.oregonstate.edu)

Quantitative absorption measurements often use the Beer–Lambert law:

where $I_0$ and $I$ are incident and transmitted spectral intensities, $\varepsilon$ is the molar absorption coefficient, $c$ is concentration, and $l$ is path length. The relation assumes suitable conditions, including a homogeneous medium and radiation bandwidth narrow relative to the absorption feature. (goldbook.iupac.org)

Historical development

In the seventeenth century, Isaac Newton demonstrated that a prism separates sunlight into a continuous sequence of colors. Nineteenth-century investigations established that spectra also contain distinctive bright and dark lines. Matching laboratory spectra with astronomical observations transformed spectroscopy from an investigation of color into a means of determining the composition of distant objects. Spectral lines subsequently became important evidence for atomic energy-level structure. (science.nasa.gov)

Applications and interpretation

Astronomers use spectra to identify substances in stars and nebulae, estimate temperatures and densities, and investigate motion through spectral shifts. Atmospheric measurements identify pollutants and quantify greenhouse gases. In materials research, infrared and Raman measurements characterize polymers, molecular structure, and material defects. (science.nasa.gov)

Interpretation uses line positions, intensities, widths, and overall patterns—not merely the presence of a peak. Instrumental resolution can merge nearby features, while background absorption can introduce signals unrelated to the sample. Reliable measurements consequently depend on calibration, reference spectra, background correction, and an understanding of the physical processes producing each feature. (nist.gov)