Probing confinement resonances by photoionizing Xe inside a C$_{60}^+$ molecular cage

Double photoionization accompanied by loss of $n$ C atoms ($n = 0, 2, 4, 6$) was investigated by merging beams of Xe@C$_{60}^+$ ions and synchrotron radiation and measuring the yields of product ions. The giant $4d$ dipole resonance of the caged Xe atom has a prominent signature in the cross section for these product channels, which together account for $6.2 \pm 1.4$ of the total Xe $4d$ oscillator strength of 10. Compared to that for a free Xe atom, the oscillator strength is redistributed in photon energy due to multipath interference of outgoing Xe $4d$ photoelectron waves that may be transmitted or reflected by the spherical C$_{60}^+$ molecular cage, yielding so-called confinement resonances. The data are compared with an earlier measurement and with theoretical predictions for this single-molecule photoelectron interferometer system. Relativistic R-matrix calculations for the Xe atom in a spherical potential shell representing the fullerene cage show the sensitivity of the interference pattern to the molecular geometry.