It was proposed that extensive ro-vibronic state mixing is responsible for the spectral congestion. This gives rise to frequent local perturbations and lends intensity to the many dark states that are nearly degenerate with the bright states that carry the oscillator strength. Supporting evidence for this interpretation was provided by observations associated with sequential two-color, two-photon ionization . Pulsed field ionization -zero kinetic energy (PFI-ZEKE) photoelectron spectra for UO2 + were recorded using resonant excitation of an intermediate excited state of UO2. The bond lengths for the UO2 ground state, the intermediate state and ground state UO2 + were all closely similar. Despite this circumstance, long progressions of the bending and stretching vibrational modes were observed in the PFI-ZEKE spectrum. The intensity pattern was a dramatic violation of the Franck-Condon principle, but intelligible if the intermediate excited state of the neutral molecule was heavily mixed with neighboring vibronic states.