Autoionization and isotope effect in the threshold photoelectron spectrum of 12CO2 and 13CO2

Threshold photoelectron spectra of 12CO2 and 13CO2 have been obtained from the onset at 900 Å (13.75 eV) to 620 Å (20 eV) at a 9 meV resolution using dispersed synchrotron radiation with a 0.3 Å bandpass. A number of forbidden and/or previously unobserved transitions have been assigned for the X̃, Ã, B̃, and C̃ states of CO+2. Photoelectron spectra by electron time of flight at selected excitation wavelengths in the Franck–Condon gap region below the à state, reveal the autoionization of neutral valence states as well as Rydberg series converging to the à 2Πu and B̃ 2Σ+u states. The photoelectron energy distribution obtained upon excitation of the Rydberg states is consistent with that given by the Bardsley–Smith model for autoionization, while the superexcited valence states decay via a resonance autoionization process proposed previously by Guyon, Baer, and Nenner. These valence states are also responsible for the strong enhancement of vibrationally forbidden transitions, such as (000)→(010). Finally, several threshold electron peaks in the region between the à and C̃ states, whose origin can be attributed to autoionization, are shown to be highly sensitive to isotopic substitution.