Single and double electron photodetachment from the oxygen anion at 41.7 nm

Single and double photodetachment of the oxygen anion O${}^{\ensuremath{-}}$ have been investigated at 41.7 nm (29.8 eV) in a crossed beams experiment using intense photon pulses from a free-electron laser. The ratio of single (O${}^{0}+{e}^{\ensuremath{-}}$) and double (O${}^{+}+2{e}^{\ensuremath{-}}$) detachment was determined to be ${\ensuremath{\sigma}}_{{\mathrm{O}}^{0}}/{\ensuremath{\sigma}}_{{\mathrm{O}}^{+}}=4.12\ifmmode\pm\else\textpm\fi{}$ 0.17 as identified directly from the yield of O${}^{0}$ and O${}^{+}$ fragments after irradiation. The absolute cross section for the dominating single detachment channel was measured to ${\ensuremath{\sigma}}_{{\mathrm{O}}^{0}}=(2.1\ifmmode\pm\else\textpm\fi{}0.6)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}19}$ cm${}^{2}$. Analysis of photoelectrons detected in coincidence with neutral fragments (O${}^{0}$) suggests that single photodetachment primarily happens via the ground (${}^{3}\phantom{\rule{-0.16em}{0ex}}P$) or lowest excited (${}^{1}\phantom{\rule{-0.16em}{0ex}}D$) state of oxygen. The results demonstrate the feasibility and advantage of crossed beams experiments for complete studies of photodetachment reactions.