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 following single 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 resonance features are attributed to ionization-plus-excitation Feshbach-resonances embedded in the double ionization continuum.  Such features are absent in the double photoionization cross section of He, or other quasi-two-electron systems, for which the doubly-ionized, inert atomic core that remains cannot capture a Feshbach resonance.  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 to practical 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.