Vibronic couplings in the C 1s-Rydberg and valence excitations of C2H2, revealed by angle-resolved photoion yield spectroscopy

High resolution angle-resolved ion-yield spectra are reported for the C1s→Rydberg excitations of acetylene. Vibronic coupling features are found in the energy regions of 3sσg/3σu∗, 3pσu, and near threshold. By increasing retarding potentials for ion detectors to select more energetic fragmentation channels, the feature observed in the 90° direction is assigned to the C1s→3σu∗ valence state coupled with the C1s→1πg∗ excited state via cis bending (πu) vibrational mode.