Molecular frame photoelectron angular distributions in inner valence photoionisation of CO

The velocity imaging photoionisation coincidence (VIPCO) technique is applied to the photoionisation of CO into the inner-valence 4 ²Σ⁺ state of CO⁺, and the molecular frame photoelectron angular distributions are measured on the high energy side of the known σ* shape resonance. The angular distributions at 40.8 and 48.4 eV show that partial waves with the angular momentum l ≤ 3 provide the dominant contribution to both parallel and perpendicular transitions, and the contributions from the l ≥ 4 partial waves are relatively negligible. By restricting the partial wave expansion to l ≤ 3, eight and four sets of dipole matrix elements and phase shift difference are extracted from the measured angular distributions for parallel and perpendicular transitions, respectively. Although it is not possible to choose unambiguously among these sets, combined enhancement among the l ≤ 3 partial waves for the parallel transition is obvious at the shape resonance.