Resonances in Valence Photodetachment of Li- below the Opening of the K-shell

Calculations of the photodetachment cross sections for the valence 2s$^2$ shell of Li$^-$ have been performed using the Belfast R-matrix code (Berrington, et al. Computer Physics Communications, 1995). Both single-electron detachment and detachment-plus-excitation channels are included; specifically, channels leaving the Li atom in the 1s$^2$2s and the 1s$^2$2p states are included. A single strong resonance is predicted just below the opening of the K-shell threshold and is a 1s$\rightarrow$2p resonance leading to a 1s2s$^2$2p resonance state. This resonance decays to both the 1s$^2$2s and the 1s$^2$2p states of neutral Li, and it is found that it appears most strongly (by and order of magnitude or so) in the detachment-plus-excitation channel leaving the atom in the excited 1s$^2$2p of Li. This is explained in terms of the structure of the 1s2s$^2$2p resonance state in which the interaction of the two 2s electrons is much larger than the interaction between the 2s and 2p electrons.