Widths and Profiles of Spectral Lines

Abstract

Spectral lines in discrete absorption or emission spectra are never strictly monochromatic. Even with the very high resolution of interferometers, one observes a spectral distribution I(ν) of the absorbed or emitted intensity around the central frequency ν₀ = (E _i − E _k )/h corresponding to a molecular transition with the energy difference ΔE = E _i − E _k between upper and lower levels. The function I(ν) in the vicinity of ν₀ is called the line profile (Fig. 3.1). The frequency interval δν = |ν₂−ν₁| between the two frequencies ν₁ and ν₂ for which I(ν₁) = I(ν₂) = I(ν₀)/2 is the full-width at half-maximum of the line (FWHM), often shortened to the linewidth or halfwidth of the spectral line.