Photoelectron studies of resonant multiphoton ionization of CO via the A 1Π state

We report measurements of multiphoton ionization of CO involving a three photon resonance to the v=1–3 levels of the CO A 1Π state followed by the absorption of either two (v=3) or three (v=1,2) additional photons to reach the ionization continua. In one measurement, the CO+ ion intensity is measured as a function of wavelength, yielding the rotational structure of the intermediate resonant state. In a second measurement, the laser frequency is set at the v=1–3 bandheads and the kinetic energy spectra of the ejected electrons are measured. The observed vibrational branching ratios in these photoelectron spectra do not follow the pattern predicted by the Franck–Condon overlap between the intermediate A 1Π state and the ionization continua. Several possible causes for this (not unexpected) deviation from Franck–Condon behavior are discussed.