Photoexcitation and Auger decay of the Renner-Teller split C 1s(-1) pi(*)(u) state in CO2

The broad (approximate to 700 meV) C 1s --> pi(u)* resonance in the absorption spectrum of CO2 has been decomposed into contributions from two Renner-Teller split con-excited states with bent and linear equilibrium geometries using resonant Auger spectroscopy. The C 1s(-1) pi* excited state was found to decay primarily via participator Auger transitions to the A (2)Pi(u) state of CO2+. Analysis of the vibrational structure in the high-resolution Auger spectra, measured at several photon energies across the broad C 1s-->pi(u)* resonance, was accomplished using calculated Franck-Condon factors for the electronic excitation and de-excitation processes. Estimations of the geometries of the Renner-Teller split core-excited states were obtained from a comparison of the calculations with the resonant Auger spectra. Transitions to the bent core-excited state were found to contribute to the absorption profile exclusively at the photon energies below the maximum of the C 1s --> pi(u)* resonance, whereas the linear con-excited state becomes accessible at higher photon energies. The symmetric stretch vibrational progression of the linear core-excited state was identified and assigned. The minimum of the potential energy surface of the C 1s(-1) pi* core-excited state at its linear configuration was estimated to be 290.4 eV above the ground vibrational level of the ground electronic state of CO2.