Fluorescence of low-lying doubly photoexcited states in helium

Abstract

In this work we investigate fluorescence decay of low-lying $(n<~8)$ resonances with odd parity and total angular momentum $J=1$ below the $N=2$ threshold of helium. After discussing the photoabsorption cross section for creation of such states, we estimate the probability for the primary fluorescence transition to $1snl$ states below the first ionization threshold. The subsequent fluorescence cascade decay brings helium into the ground state or into one of the two long lived $1s2s{}^{1,3}S$ states. We discuss the time scale of the cascade and combine all the calculated transition probabilities to determine branching ratios for singlet and triplet metastable production. Finally, we estimate the angular distribution of the emitted uv photons and perform a comparison with the recent experimental data of Penent et al. [Phys. Rev. Lett. 86, 2758 (2001)].