Renner–Teller splitting in the C 1s→π* excited states of CS2, OCS, and CO2

Abstract

Fragment ions energetically emitted following the perpendicular (ΔΛ=+1) transitions of C 1s→π* of CS2, OCS, and CO2 are observed not only in the perpendicular (90°) direction but also in the parallel (0°) to the linear polarization; that is, ions have a momentum orthogonal to the linear molecule. This arises in the Renner–Teller (RT) vibronic coupling of bending vibrations in the C 1s→ in-plane π* excited state with a bent equilibrium geometry, though the RT splitting between the C 1s→ out-of-plane π* state with a linear geometry and the C 1s→ in-plane π* state is not visible directly due to the lifetime broadening. The 0° ion yield is relatively small in CS2 but is comparable to the 90° yield in CO2; in the latter the peak maximum at 0° is 0.06 eV lower than at 90° and the anisotropy parameter β is heavily dependent on the photon energy. In CO2 a great number of unresolved bending vibrations are coupled. The half-width at half-maximum on the lower energy side of the π* peak is much more sensitive to the RT splitting; 0.08, 0.11, and 0.29 eV for CS2, OCS, and CO2, compared with the full-width at half-maximum, 0.17, 0.39, and 0.64 eV. In CS2 the π* peak and β value are sharp and symmetric, indicating that the zero-point vibrational levels are only involved. In OCS three fine structures observed with separations of 0.21 eV are assigned to the ν3 mode, which is comparable to the stretching mode in CO.