Vibrationally ResolvedK-shell Photoionization of CO with Circularly Polarized Light

Diffraction of a low energy ($l4\text{ }\text{ }\mathrm{e}\mathrm{V}$) carbon-$K$-photoelectron wave that is created inside a CO molecule by absorption of a circularly polarized photon is investigated. The measurements resolve the vibrational states of the $K$-shell ionized $\mathrm{C}{\mathrm{O}}^{\mathrm{+}}$ molecule and display the photoelectron diffraction patterns in the molecular frame. These show significant variation for the different vibrational states. This effect is stronger than predicted by state-of-the-art theory. As this study is performed close to C-$K$-threshold and, therefore, far below the molecule's $\ensuremath{\sigma}$-shape resonance, this surprisingly strong effect is not related to that resonance phenomenon.