Two photon resonant, four photon ionization of CO via the A 1Pi state with photoelectron energy analysis

We report measurements of photoelectron spectra from resonantly enhanced four photon ionization of CO via a two photon transition to the R-branch heads of the A 1Pi, v = 1--3 reverse arrow X 1Σ+, v = 0 bands. In addition, we present preliminary data on the rotational state dependence of the photoelectron spectra. The results are compared with our earlier measurements of the photoelectron spectra obtained in the six photon ionization of CO via a three photon resonant transition to the same intermediate levels. The observed vibrational branching ratios in the photoelectron spectra differ both from those observed in the earlier study and from the pattern predicted by the Franck--Condon overlap between the intermediate A 1Pi state and the ionization continua. It is shown that accidental resonances with the J 1Σ+ state at the three photon energy account for the observed photoelectron spectra obtained at the wavelengths of the A 1Pi, v = 2,3 R-branch bandheads. Finally, we present conclusive evidence for the production of electronically excited CO+ A 2Pi and discuss possible mechanisms for its formation