Doppler broadening

Doppler broadening is the widening of a spectral line because emitting or absorbing particles have different velocities along the line of sight, shifting the radiation frequency by different amounts.

Doppler effect: The change in observed wave frequency caused by relative motion between a source and an observer. Each particle’s line-of-sight motion shifts its emitted or absorbed frequency through this effect.
Spectroscopy: The study of how matter interacts with electromagnetic radiation as a function of wavelength or frequency. Spectroscopy measures line profiles whose widths can encode particle velocities.
Natural broadening: Spectral-line broadening caused by the finite lifetime of an excited quantum state. It produces Lorentzian wings, unlike the Gaussian profile of thermal Doppler broadening.
Spectral line: A narrow feature in a spectrum associated with a transition between discrete energy states or a continuum process. Doppler broadening changes the width and shape of this spectral feature.
Maxwell–Boltzmann distribution: A probability distribution describing the speeds of particles in a classical gas at thermal equilibrium. Thermal particle velocities supply the spread of shifts that produces the line profile.
Astrophysical spectroscopy: The analysis of astronomical objects through their emitted, absorbed, or scattered spectra. Astronomers use Doppler widths to estimate temperatures and distinguish thermal from turbulent motion.
Pressure broadening: Spectral-line broadening caused by collisions and interactions among particles. Collisions broaden lines independently of the velocity-distribution mechanism.
Line-of-sight velocity: The component of an object's velocity directed toward or away from an observer. Only motion along the observation direction shifts a line through the classical Doppler effect.
Gaussian function: A symmetric function whose values fall exponentially with the square of distance from its center. A thermal Maxwellian velocity distribution produces a Gaussian spectral profile.
Plasma spectroscopy: The analysis of plasmas through radiation emitted, absorbed, or scattered by their particles. Doppler profiles help diagnose ion temperatures and velocity distributions in plasmas.

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