Near-threshold measurements ofK−two-electron photoionization cross sections

The absolute cross sections for two-electron photoionization have been measured for ${\mathrm{K}}^{\mathrm{-}}$ from 70 meV below to 250 meV above the threshold by measuring product ${\mathrm{K}}^{+}$ ions. To minimize complications from the photodetachment and field ionization, the electric field strength used to separate ${\mathrm{K}}^{+}$ ions was limited to 300 V/cm, which ionizes only Rydberg neutral atoms with n≥30. The data fit to σ∼$E^m$ (E is the total energy of the two electrons) yielded m=1.16±0.05 with a reduced ${\chi }^2$=0.69, in agreement with the Wannier theory m=1.127 [G. H. Wannier, Phys. Rev. 90, 817 (1953)]. However, a nearly equally satisfactory fit can also be made to the oppositional theory of A. Temkin [Phys. Rev. A 30, 2737 (1984)].