Spectral line broadening

The finite width of a spectral line can arise from finite optical coherence or a distribution of resonance frequencies. lifetime broadening and impact-regime pressure broadening often give Lorentzian line shapes; thermal Doppler broadening gives a Gaussian component. The instrument response must also be included when interpreting a measured spectrum.

Finite interaction time with a spatially confined optical beam gives a finite spectral response width. Its detailed shape depends on the spatial beam profile, particle trajectory and interaction envelope; a duration alone does not imply a universal Lorentzian profile.

An ensemble-wide distribution of resonance frequencies, such as velocity-dependent Doppler shifts or spatially varying static-field shifts. The observed profile averages the individual responses. A velocity class need not have the full width of the ensemble's thermal Doppler broadening.

Broadening shared by otherwise identical emitters, often described by a common optical coherence time $T_2$. Exponential field-coherence decay produces a Lorentzian line shape with full angular-frequency width $2/T_2$. Population decay, dephasing and collisions can all contribute, with their time constants distinguished.