Vibronic coupling and valence mixing in the 1s→Rydberg excited states of C2H2 in comparison with N2 and CO

Angle-resolved ion-yield spectra are reported for the C1s→Rydberg excitations of linear acetylene C2H2 in comparison with N2 and CO. The 3σu* valence state is observed in the 3sσg Rydberg region with no mixing. The 3pσu state is found in the same region as the C1sσg→3pπu state, which shows only totally symmetric vibrations. This is the first to observe that the Rydberg state in C2H2 keeps gerade and ungerade symmetries without vibronic coupling through antisymmetric stretching vibrations related to core–hole localization. On the other hand, the lowest 1πg* valence state induces vibronic coupling through bending vibrations in the 3σu* and 3sσg states.