Photoabsorption cross section and ion-yield spectra of helium double-excitation resonances

The photoabsorption cross section and ion-yield spectra of He have been measured in the region of the double-excitation resonances below the N = 2 threshold. For resonances decaying predominantly by autoionization [the 2,0(n) series, low values of n; notation of Herrick and Sinanoilu, Phys. Rev. A 11, 97 (1975)], the two kinds of spectra yield the same information. For resonances where substantial decay by fluorescence occurs (2,1(n) and 2,- 1(n) series), the ion-yield spectrum differs significantly from the cross section. The ratios of peak intensity of one series to the other are clearly different and the Fano line shape parameter (q) of the 2,1(n) series also changes as a function of n, in qualitative agreement with the trends expected when the fluorescence channel is taken into account. These direct measurements confirm the influence of fluorescence on the cross section and agree with previous indirect measurements.