Radiative cascades from doubly excited He states

Resonance profiles of spectroscopically resolved radiative transitions from ${(2pnd)}^{1}{P}^{o},$ ${(sp,2n+)}^{1}{P}^{o},$ and $(sp,2n\ensuremath{-}{)}^{1}{P}^{o}$ doubly excited He states with $n=3,\dots{},7$ were investigated. The ${(2p3d)}^{1}{P}^{o}--{(1s3d)}^{1}{D}^{e}--{(1s2p)}^{1}{P}^{o}--{1s}^{2}{}^{1}{S}^{e}$ cascade transitions were observed using photon induced fluorescence spectroscopy in the vacuum ultraviolet and the visible spectral range. The $(2pnd)$ levels populate predominantly ${(1snd)}^{1}D$ levels, while for the $(sp,2n+or\ensuremath{-})$ levels, ${n}^{1}S$ levels are populated as well. The experiments demonstrate that further quantitave branching ratio measurements are needed in order to test matrix elements for radiative transitions from ${\mathrm{He}}^{**}.$