Vibronic Couplings in the C 1s → nsσg Rydberg Excited States of CO2

Fragment ion yields in the C 1s → Rydberg excitation region of CO₂ were measured in the 90° and 0° directions relative to the electric vector of the linearly polarized light. The C 1s → ns (n = 3, 4), npπ and npσ (n = 3?7), and nd (n = 3, 4) Rydberg transitions are clearly observed and show some vibrational structures. The dipole-forbidden C 1s(σ_g) → 3sσ_g Rydberg transition is the strongest of all the Rydberg transitions, and the ion yield in the 90° direction is dominant. This indicates that the bending vibration is predominantly coupled with the 3sσ_g Rydberg state and the intensity-lending dipole-allowed state is a very strong π* resonance, only 2 eV lower than the 3sσ_g state. On the other hand, in the 4sσ_g Rydberg state the vibronic coupling through the antisymmetric stretching mode is strongly observed in the 0° direction. This is probably because the 4sσ_g state approaches another intensity-lending state with Σ_u⁺ symmetry and goes away from the π* resonance. The angle-resolved ion-yield technique is very powerful for elucidating the vibronic coupling mechanism.