Electron-Ejection Asymmetries by Molecular Orientation FollowingK-Shell Excitation of CO

We report the first observation of electron-ejection asymmetries due to molecular orientation following $K$-shell photoexcitation in a free molecule. The angular distributions of electrons emitted by the decay of $K$-shell vacancies were measured in the vicinity of the ${\pi }^{*}$ discrete and the ${\sigma }^{*}$ shape resonance of CO. We see a sharp drop of the angular distribution asymmetry parameter to $\beta =-1\mathrm{}$ at the ${\pi }^{*}$ resonance as predicted by theory, but no comparable effect across the ${\sigma }^{*}$ shape resonance. The results are discussed with respect to the intrinsic anisotropy of the $K$-shell vacancies.