Molecular symmetries and vibrations in high-resolution inner-shell spectroscopy

Abstract

Fragment ion yields in the 90° and 0° directions are measured in the C 1s→Rydberg excitation region of CO 2 and CH 4. In CO 2 the dipole-forbidden C 1sσ g→3sσ g Rydberg transition is the strongest of all the Rydberg transitions. The ion yields in the 90° direction are dominant, indicating 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 nearly located. On the other hand, in the 4sσ g Rydberg state the vibronic coupling through the antisymmetric stretching mode is clearly observed in the 0° direction. Even in CH 4 with a highly symmetrical geometry, the C 1s a 1→3p t 2 and 3d t 2 Rydberg transitions show anisotropic angular distributions of the fragment ions. This result indicates that the vibronic coupling with the two-fold degenerate bending vibration causes the Jahn-Teller distortion to the C 3v geometry and enhances mixing with t 2 ∗ valence state of antibonding character. Thus, in inner-shell excitations of polyatomic molecules the axial recoil mechanism fails when bending vibrations are highly excited by the Renner-Teller or Jahn-Teller effects and vibronic couplings, though it will still be valid even in polyatomic molecules if only stretching (symmetric and antisymmetric) vibrations are excited.