Observed discrepancies in the central region of the Hβ profile at low electron densities

Profiles of the H beta transition have been obtained at low electron densities (n c =10 15 cm -3 ) in both a pure hydrogen plasma and also in a 99.99% helium plasma containing 0.01% hydrogen. The theoretically predicted central dip is entirely absent at this density for the hydrogen plasma, and is very much smaller than theory in the helium plasma.