Double photoionization of atomic oxygen: Feshbach resonances in the two-electron continuum

We describe a joint experimental and theoretical investigation on oxygen double photoionization — the emission of two electrons from atomic oxygen followings ingle photon absorption. High-resolution experimental measurements were performed at the Advanced Light Source, revealing sharp resonance structure superimposed on the more familiar Wannier-like, nearly-linear background.These resonancefeatures are attributed to ionization-plus-excitation Feshbach-resonances embedded in the double ionization continuum, doubly-excited states that lie above the double;-ionization threshold. Such features are absent in the double photoionization crosssection of He, or other quasi-two-electron systems, for which the doubly-ionized atomic core remains cannot inert. For a corresponding theoretical analysis, the R-matrix with pseudostates (RMPS) method was invoked by calculating final-state,two-electron resonances-plus-continua wavefunctions and corresponding single-photon absorption cross sections. Overall agreement is found in the direct, background double photoionization cross section. However, the RMPS method, using a small basis due topractical computational limitations, was unable to reproduce quantitatively the smooth background or the sharper resonance features observed in the measurements, showing instead large-scale oscillations about the experimental background, and characteristic pseudoresonance jitter, associated with an insufficient convergence of the pseudostate representation to the true two-electron infinite series of Feshbach resonances embedded in the two-electron continuum. The prominent resonance structure observed highlights the need to consider multiple excitation processes in atoms more complex than He or quasi-two-electron systems.