Linewidth

Definition

The expression linewidth designates the intrinsic width of a spectral line expressed in units of wavelength (Å, nm, μm, etc.) or frequency (Hz). Generally, it is the full width at half maximum (FWHM) that is used. The linewidth can refer to a line in absorption or emission. The width of a line results in general from the contributions of several phenomena: for instance, the natural linewidth is linked to the lifetime of the transition, while the Doppler broadening is due to chaotic movements (turbulence, thermal motion) in the emitting medium. Note that the spectrograph used to measure a spectral line must have a spectral resolution better than the linewidth to derive a proper value. Figure 1 illustrates the linewidth of a strong hydrogen line (Hα) seen in the solar spectrum.

Linewidth. Figure 1

The linewidth of the hydrogen Hα line seen in absorption in the spectrum of the Sun at λ = 656.6 nm. The arrowsmark the width adopted here (FWHM). One notes here that the pedestal...