Vibrational structure of PE bands

Here AFyib is equal to the difference in vibrational energy between the ion and the original molecule, measured in each case from the ground vibrational level. As most of the molecules will be in or near the ground state before ionization, AFyib is rarely negative but could be positive or zero, depending on whether the ion is vibrationally excited or not. There is no simple selection rule for Av in this case, and in some cases 

Photoelectron spectrum and energy-level diagram for H2O. Adapted from [5]. 

In case (a), loss of an anti-bonding electron has strengthened the bond, so the bond is shortened and the vibrational levels have become more widely separated. Excitation can be to several different vibrational levels of the ion, so there is a series of resolved lines in the photoelectron spectrum, with a relatively wide 

Energy-level diagrams for the valence-shell molecular orbitals of (a) O2 and (b) N2. 

Ionization of a diatomic molecule by removal of an electron from (a) an anti-bonding orbital, (b) a bonding orbital, and (c) a non-bonding orbital. 

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