Vibronic coupling and valence mixing in the 1s -> Rydberg excited states of C2H2 in comparison with N-2 and CO

Angle-resolved ion-yield spectra are reported for the Cls --> Rydberg excit ations of linear acetylene C2H2 in comparison with N-2 and CO. The 3 sigma( u)* valence state is observed in the 3s sigma(g) Rydberg region with no mix ing. The 3p sigma(u) state is found in the same region as the Cls sigma(g) --> 3p pi(u) state, which shows only totally symmetric vibrations. This is the first to observe that the Rydberg state in C2H2 keeps gerade and ungera de symmetries without vibronic coupling through antisymmetric stretching vi brations related to core-hole localization. On the other hand, the lowest 1 pi(g)* valence state induces vibronic coupling through bending vibrations in the 3 sigma(u)* and 3s sigma(g) states. (C) 1999 Elsevier Science B.V. A ll rights reserved.