Ionization Energy - Vertical and Adiabatic Ionization Energy in Molecules

Vertical and Adiabatic Ionization Energy in Molecules

Ionization of molecules often leads to changes in molecular geometry, and two types of (first) ionization energy are defined – adiabatic and vertical. The adiabatic ionization energy of a molecule is the difference between the energies of the neutral molecule and its positive ion, each in its vibrational ground state, and possibly with different equilibrium geometries.

The vertical ionization energy is the energy required to form a positive ion with the same geometry as the neutral molecule, corresponding to a vertical transition on a diagram of potential energy as a function of geometry. Such ionization is often accompanied by vibrational excitation. According to the Franck-Condon principle, vertical ionization is the most probable, since electrons move much more rapidly than nuclei and the ionized electron departs before the nuclei have time to adjust their position.

For a diatomic molecule the geometry is defined by one bond length. 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 curve is for the neutral molecule as a function of bond length. The horizontal lines are vibrational levels with their associated vibrational wave functions. The upper curve is the ionized state, with its minimum energy further to the right since the bond length in the ion is greater. The adiabatic ionization is the diagonal transition to the vibrational ground state of the ion. Vertical ionization involves vibrational excitation of the ionic state and therefore requires greater energy.