Symmetry-Resolved Vibrational Spectroscopy for the C $1{\mathit{s}}^{-1\mathrm{}}{2\pi }_{\mathit{u}}$ Renner-Teller Pair States in ${\mathrm{CO}}_2$

Symmetry-resolved excitation spectra have been measured for the Renner-Teller pair states $A_1$ and $B_1$ split from the core-excited C ${1s}^{-1\mathrm{}}2{\pi }_u$ state in ${\mathrm{CO}}_2$. A vibrational progression with the spacings of $\sim 145\mathrm{meV}$ is found in both the $A_1$ and $B_1$ spectra at different energies and assigned to the symmetric stretching mode caused in the $B_1$ linear state, with the help of ab initio calculations. Appearance of the vibrations in the $A_1$ spectrum is interpreted as due to nonadiabatic coupling between the $A_1$ and $B_1$ states via the bending motion.