Influence of the radiative decay on the cross section for double excitations in helium

The fluorescence yield spectrum of the double excitations in helium has bee n measured. All three Rydberg series converging to the N = 2 threshold are resolved, and the intensity of the individual lines is determined. The inte nsity variation through each series indicates that the radiative decay chan nel cannot be neglected when discussing the double excitations in helium. T his calls for a revision of the interpretation of the absorption spectrum, and sets the limit for using the ion yield method to monitor the absorption probability.