The first complete experiment for the C $K$-shell of CO molecules in the region of the ${\sigma }^{*}$ shape resonance has been performed by detecting photoelectrons in coincidence with fragment ions. Four ratios of dipole matrix elements and four phase shift differences have been extracted from the experimental data. Their analyses show that, in the ${\sigma }^{*}$ shape resonance due to the $l$ mixing, $d$ and $f$ partial waves give the main contribution to the cross section, and $f$ and $g$ waves give the main contribution to a rapid increase of phase shift by $\pi$.

• Received 24 April 1997

DOI:https://doi.org/10.1103/PhysRevLett.80.1622