A study of the vibronic structure in the HeI excited photoelectron spectrum of CO2 involving the X 2Πg and A 2Πu ionic states

The HeI excited photoelectron spectrum of the CO2 molecule covering the X 2Πg and A 2Πu 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 2Πg state, the ν2 and ν3 modes are observed to be excited in both an odd and even numbers of quanta in addition to the ν1 mode, whereas for the A 2Πu state the spectrum is dominated by excitations of the ν1 mode alone and in combinations with excitations of the ν2 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.