Experimental and theoretical studies on non-adiabatic ion dynamic contributions to the He I 4026 2³P-5³D, 5³F profile at low electron densities

The He I 4026 2 3 P-5 3 D, 5 3 F transition has been observed in a recombination plasma at low electron densities. The profile is in very poor agreement with all existing static ion theories and the discrepancies are different in form from those for the 2P-4D transitions studied previously. New theoretical predictions are also presented showing greatly improved agreement with the experiment, and emphasizing the importance of non-adiabatic ion dynamic broadening of the forbidden component. These results cast doubt on all existing criteria for the static approximation even when frequency separations are correctly measured from the centre of the forbidden component.