The HeI excited photoelectron spectrum of the CO 2 molecule covering the X 2 ⌸ g and A 2 ⌸ u ionic states has been recorded at a resolution of better than 5 meV. Complex vibrational structures are resolved in both photoelectron bands. In the X 2 ⌸ g state, the 2 and 3 modes are observed to be excited in both an odd and even numbers of quanta in addition to the 1 mode, whereas for the A 2 ⌸ u state the spectrum is dominated by excitations of the 1 mode alone and in combinations with excitations of the 2 mode in two quanta involving strong Fermi resonance. The observed spectrum has been assigned by comparison with optical spectra and with calculations of the vibrational fine structure including vibronic and spin-orbit coupling.

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