Absolute cross section for near-threshold two-electron photoionization ofHe−

The first cross-section measurements have been made for the process $h\nu +{\mathrm{He}}^{-}\to {\mathrm{He}}^{+}+2\mathrm{}{e}^{-}$, from threshold (4.845 eV) to 0.41 eV above, where it attains a value of only 2×${10}^{-20\mathrm{}}$ ${\mathrm{cm}}^2$. Details of its energy dependence in this region were obscured by uncertainties due to a large background of ${\mathrm{He}}^{+}$ ions produced by the photodetachment-photoionization sequence, and the data do not permit a choice between existing theories. However, these are the first absolute cross sections for such a process, and provide the first determination of the branching ratio between the ejection of two electrons compared to a single electron in the threshold region for ionization.