VIBRONIC COUPLINGS IN THE C1S-]NS-SIGMA(G) RYDBERG EXCITED-STATES OF CO2

Fragment ion yields in the C 1s --> Rydberg excitation region of CO2 w ere measured in the 90 degrees and 0 degrees directions relative to th e electric vector of the Linearly polarized light. The C 1s --> ns (n = 3, 4), np pi and np sigma (n = 3-7), and nd (n = 3, 4) Rydberg trans itions are clearly observed and show some vibrational structures. The dipole-forbidden C 1s(sigma(g) -- 3s sigma(g) Rydberg transition is th e strongest of all the Rydberg transitions, and the ion yield in the 9 0 degrees direction is dominant. This indicates that the bending vibra tion is predominantly coupled with the 3s sigma(g) Rydberg state and t he intensity-lending dipole-allowed state is a very strong pi resonan ce, only 2 eV lower than the 3s sigma(g) state. On the other hand, in the 4s sigma(g) Rydberg state the vibronic coupling through the antisy mmetric stretching mode is strongly observed in the 0 degrees directio n. This is probably because the 4s sigma(g) state approaches another i ntensity-lending state with Sigma(u)(+) symmetry and goes away from th e pi resonance. The angle-resolved ion-yield technique is very powerf ul for elucidating the vibronic coupling mechanism.