Isotope shifts of double-excitation resonances in helium

Abstract

Isotopic effects on double-excitation resonances in $^{3}\mathrm{He}$ and $^{4}\mathrm{He}$ were observed in photoionization spectra using synchrotron radiation with high monochromator resolution. In $^{3}\mathrm{He}$, the resonances were found to be shifted to lower energies with respect to $^{4}\mathrm{He}$ by $\ensuremath{\Delta}E=3.06\ifmmode\pm\else\textpm\fi{}0.07\phantom{\rule{0.3em}{0ex}}\text{meV}$, in good agreement with theoretical expectations based on normal and specific mass shifts. From the experimental data, the resonance parameters ${E}_{r}$, $\ensuremath{\Gamma}$, and $q$ of the resonances $2,\ensuremath{-}{1}_{3},$ $2,{1}_{4},$ and $2,{0}_{7}$ were analyzed in detail using a double-convolution fit procedure that considers the different Doppler broadenings of the states in $^{3}\mathrm{He}$ and $^{4}\mathrm{He}$. 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.