Effect of a virtual state near ans-wave threshold: Absolute Li photodetachment cross sections near the Li (22P) threshold

Absolute cross sections for ${\mathrm{Li}}^{\mathrm{-}}$ photodetachment have been measured near the Li($2^2$P)+εs channel threshold. The energy dependence deviates rapidly from normal Wigner-cusp behavior on each side of threshold. From an analysis of the total cross sections in terms of a two-channel scattering theory that includes the effects of a virtual state, a parametric expression has been derived which accurately describes the energy dependence. A least-squares fit of the data yields the position of the virtual state (55±10 meV below $2^2$P), the phase shift at the Li($2^2$P)+εs threshold (1.4±0.1 rad), and the electron affinity of Li (617.3±0.7 meV).