Spectral-Line Profiles for a Fast-Ion-Beam Source*

Abstract

When fast, foil-excited ions are used as a light source, the resulting spectral lines are Doppler broadened because the radiating particles are moving and the spectrograph has a finite acceptance angle. An exact expression has been obtained for the line profile produced by a stigmatic spectrograph without vignetting. For β2 < <1, the irradiance is I(λ) ~ {¼θ2-[(λ0-λ)/βλ0]2}½ when the square root is real, and zero otherwise. Theta is the spectrograph’s acceptance angle. The line width at half maximum is Δλ~0.866 θβλ0. Vignetting causes the line shape to deviate from the theoretical profile. For a Meinel-type spectrograph, this effect can give doubled line images.