RENNER-TELLER SPLITTING IN THE C1S-]PI(ASTERISK) EXCITED-STATES OF CS2, OCS, AND CO2

Fragment ions energetically emitted following the perpendicular (Delta Lambda = +1) transitions of C 1s--> pi of CS2, OCS, and CO2 are obse rved not only in the perpendicular (90 degrees) direction but also in the parallel (0 degrees) to the linear polarization; that is, ions hav e a momentum orthogonal to the linear molecule. This arises in the Ren ner-Teller (RT) vibronic coupling of bending vibrations in the C 1s--> in-plane pi excited state with a bent equilibrium geometry, though t he RT splitting between the C 1s--> out-of-plane pi state with a line ar geometry and the C 1s--> in-plane pi state is not visible directly due to the lifetime broadening. The 0 degrees ion yield is relatively small in CS2 but is comparable to the 90 degrees yield in CO2; in the latter the peak maximum at 0 degrees is 0.06 eV lower than at 90 degr ees and the anisotropy parameter beta is heavily dependent on the phot on energy. In CO2 a great number of unresolved bending vibrations are coupled. The half-width at half-maximum on the lower energy side of th e pi 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-m aximum, 0.17, 0.39, and 0.64 eV. In CS2 the pi peak and beta value ar e sharp and symmetric, indicating that the zero-point vibrational leve ls are only involved. In OCS three fine structures observed with separ ations of 0.21 eV are assigned to the nu(3) mode, which is comparable to the stretching mode in CO. (C) 1997 American Institute of Physics.