We have investigated the threshold of photodetachment of ${\mathrm{Li}}^{\mathrm{-}}$ leading to the formation of the residual Li atom in the 2${\mathit{p}}^2$P state. The excited residual atom was selectively photoionized via an intermediate Rydberg state and the resulting ${\mathrm{Li}}^{+}$ ion was detected. A collinear laser-ion beam geometry enabled both high resolution and sensitivity to be attained. We have demonstrated the potential of this state-selective photodetachment spectroscopic method by improving the accuracy of Li electron affinity measurements an order of magnitude. From a fit to the Wigner law in the threshold region, we obtained a Li electron affinity of 0.618 049(20) eV. © 1996 The American Physical Society.

• Received 6 November 1995

DOI:https://doi.org/10.1103/PhysRevA.53.4127