Spectral line broadening

Spectral line broadening is the spreading of an emission or absorption line across a range of frequencies or wavelengths, caused by physical conditions or measurement limits.

Doppler broadening: Spectral-line broadening caused by the distribution of emitting or absorbing particles’ velocities along the line of sight. Thermal motion and bulk flows shift photon frequencies, spreading the observed line.
Spectral line: A narrow feature in a spectrum produced when matter emits or absorbs radiation at particular frequencies or wavelengths. Broadening describes how this spectral feature departs from an ideal sharp line.
Spectroscopy: The study of how matter interacts with electromagnetic radiation across frequencies or wavelengths. Spectroscopy uses line widths alongside line positions and strengths to infer physical properties.
Spectral line splitting: The division of one spectral line into multiple distinct components under an external influence or interaction. Splitting produces separate components; broadening widens a profile, though unresolved splitting can mimic it.
Natural broadening: Spectral-line broadening caused by the finite lifetime of an excited quantum state. Finite state lifetimes impose an intrinsic width even in motionless, isolated atoms.
Line profile: The distribution of spectral intensity or absorption across frequencies or wavelengths near a spectral line. Its shape distinguishes the physical processes that contribute to broadening.
Thermal Doppler broadening: Doppler broadening caused by the random thermal velocities of particles in an emitting or absorbing gas. The width can estimate gas temperature when the particle mass is known.
Spectral line shift: A change in a spectral line’s central frequency or wavelength relative to a reference value. A shift moves the line center, whereas broadening increases its width.
Pressure broadening: Spectral-line broadening caused by collisions and interactions among particles in a gas. Collisions perturb atomic energy levels and interrupt emission or absorption.
Full width at half maximum: The width of a peak measured between the points where its value is half its maximum. This common measure turns a broadened line profile into a single width.

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