A theoretical simulation of symmetry-resolved spectra for excitation to the C 1s-12πu Renner-Teller states in CO2

Symmetry-resolved spectra for excitation to the C1s(-1) 2pi(u) Renner-Teller pair states A(1) and B-1 in CO2 are simulated theoretically. Reliable ab initio potential energy surfaces of the core excited states are calculated at the level of multi-configuration self-consistent-field theory. The problem of two-dimensional symmetric stretching and bending vibrational motion in the excited states is solved numerically and then the vibrationally resolved excitation spectra are analysed in detail. The calculated spectrum for excitation to the B-1 state is in excellent agreement with a recent experimental result. The vibrational structures in the spectrum are found to originate from the symmetric stretching motion in the B-1 state. On the other hand, the present calculation reproduces global features of the experimental spectrum for excitation to the A(1) state except for the vibrational structures in the higher-energy region. The absence of vibrational structures in the present simulation is ascribed to no account being taken of a vibronic coupling between the A(1) and B-1 adiabatic states. A very simple analysis taking account of the vibronic coupling demonstrates that the vibrational structures in the A(1) spectrum also originate from the symmetric stretching motion in the coupled B-1 state.