The magnetic quantum number selection rule implies that radiation that is linearly polarized perpendicular to the electric field will not excite Δm=0 transitions. This phenomenon in Stark spectra has been used to determine the electric field vector in a positive column dc discharge in helium. The technique is applicable to both linear and nonlinear Stark effects.

• Received 20 May 1985

DOI:https://doi.org/10.1103/PhysRevA.32.2544