Jun-ichi ADACHI, Nobuhiro KOSUGI, Eiji SHIGEMASA* and Akira YAGISHITA* (*KEK-PF)
[J. Phys. Chem. 100, 10783 (1996)]
Fragment ion yields in the C 1s -> Rydberg excitation region of CO2 were measured in the 90º and 0º directions relative to the electric vector of the linearly polarized light. The C 1 -> ns (n = 3 and 4), np(sigma) and np(pi) (n = 3 - 7), and nd (n = 3 and 4) Rydberg transitions are clearly observed and show some vibrational structures. The dipole-forbidden C 1s ((sigma)g) -> 3s(sigma)g Rydberg transition is the strongest of all the Rydberg transitions, and the ion yield in the 90º direction is dominant. This indicates that the bending vibration is predominantly coupled with the 3s(sigma)g Rydberg state and the intensity-lending dipole allowed state is a very strong (pi)* resonance, 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 antisymmetric stretching mode is strongly observed in the 0º direction. This is probably because the 4s(sigma)g state approaches another intensity-lending state with (SIGMA)u+ symmetry and goes away from the (pi)* resonance. The angle-resolved ion-yield technique is very powerful for elucidating the vibronic coupling mechanism.