Threshold spectrum of CO2

A high-resolution threshold electron impact spectrometer is used to obtain a threshold spectrum of CO 2 in the energy range from 3 to 20 eV. The dominant feature of the spectrum is the strong excitation of the unoccupied 2 pi u valence orbital from both 1 pi g and other deeper lying filled orbitals, giving rise to strong overlapping continua. A new, long vibrational progression in the bending mode is found, between 10 and 11 eV. The most prominent Rydberg series start above 12 eV and are associated with optically forbidden components of the transition to nd orbitals, not detected previously. Isoenergetic decay of the 2 Pi u resonance, populating the ground-state vibrational levels, is observed with an intensity comparable with the strongest electronic transitions. Strong resonant near-threshold excitation is found at 11.02 eV.