One- and two-photon detachment of O−

Cross sections for one- and two-photon detachment of ${\mathrm{O}}^{\ensuremath{-}}(1{s}^{2}2{s}^{2}2{p}^{5}\phantom{\rule{0.16em}{0ex}}^{2}\mathrm{P}^{\mathrm{o}})$ have been determined in a joint experimental and theoretical study. The absolute measurement is based on the animated-crossed-beam technique, which is extended to the case of pulsed lasers, pulsed ion beams, and multiphoton detachment. The ab initio calculations employ $R$-matrix Floquet theory, with simple descriptions of the initial bound state and the residual oxygen atom which reproduce well the electron affinity and ground-state polarizability. For one-photon detachment, the measured and computed cross sections are in good mutual agreement, departing significantly from previous reference experiments and calculations. The generalized two-photon detachment cross section, measured at the Nd:YAG laser wavelength, is in good agreement with the $R$-matrix Floquet calculations. Long-standing discrepancies between theory and experiment are thus resolved.