Zero-kinetic-energy photoelectron spectroscopy was used to measure the Franck—Condon factors for the carbon and nitrogen K-shell ionized states in N2 and CO. Corresponding features in the two spectra showed nearly identical energy spacings, both below and above the ionization threshold, as predicted by the equivalent-core model. The equilibrium bond length, Re, for the CO C-1s−1 state was determined to be 1.077±0.005 Å, and Re for the N2 N-1s−1 state was found to be 1.077±0.010 Å, changes of −0.051 Å for CO and −0.020 Å for N2 from their ground-state equilibrium bond lengths. The two-electron states corresponding to 1s−1 val−1π* were examined in the binding energy region predicted by Green function calculations. Two-electron states absent in the X-ray photoemission spectrum are present in both molecules and show evidence of vibrational structure.
