Editing Ionization energy (section)

===Vertical ionization energy===
Due to the possible changes in molecular geometry that may result from ionization, additional transitions may exist between the vibrational ground state of the neutral species and vibrational [[excited state]]s of the positive ion. In other words, ionization is accompanied by [[vibrational spectroscopy|vibrational excitation]]. The intensity of such transitions is explained by the [[Franck–Condon principle]], which predicts that the most probable and intense transition corresponds to the vibrationally excited state of the positive ion that has the same geometry as the neutral molecule. This transition is referred to as the "vertical" ionization energy since it is represented by a completely vertical line on a potential energy diagram (see Figure).

For a diatomic molecule, the geometry is defined by the length of a single [[bond length|bond]]. The removal of an electron from a bonding [[molecular orbital]] weakens the bond and increases the bond length. In Figure 1, the lower [[Potential energy surface|potential energy curve]] is for the neutral molecule and the upper surface is for the positive ion. Both curves plot the potential energy as a function of bond length. The horizontal lines correspond to [[Molecular vibration|vibrational levels]] with their associated [[Quantum harmonic oscillator|vibrational wave functions]]. Since the ion has a weaker bond, it will have a longer bond length. This effect is represented by shifting the minimum of the potential energy curve to the right of the neutral species. The adiabatic ionization is the diagonal transition to the vibrational ground state of the ion. Vertical ionization may involve vibrational excitation of the ionic state and therefore requires greater energy.

In many circumstances, the adiabatic ionization energy is often a more interesting physical quantity since it describes the difference in energy between the two potential energy surfaces. However, due to experimental limitations, the adiabatic ionization energy is often difficult to determine, whereas the vertical detachment energy is easily identifiable and measurable.