Isotope shifts of double-excitation resonances in helium

Isotopic effects on double-excitation resonances in 3He and 4He were observed in photoionization spectra using synchrotron radiation with high monochromator resolution. In 3He, the resonances were found to be shifted to lower energies with respect to 4He by ΔE=3.06±0.07 meV, in good agreement with theoretical expectations based on normal and specific mass shifts. From the experimental data, the resonance parameters Er, Γ, and q of the resonances 2,-13, 2,14, and 2,07 were analyzed in detail using a double-convolution fit procedure that considers the different Doppler broadenings of the states in 3He and 4He. In this way, the resolution function of the monochromator could be derived, which can be accounted for in a quantitative way on the basis of the properties of undulator radiation and the optical design of the monochromator