A photoelectron spectrum can show the relative energies of occupied MOs by ionization
Ionization energy is a direct measure of the energy required to remove the electron concerned from its initial level to the vacuum level (free electron)
Photoelectron spectroscopy measures the relative energies of the ground and excited positive ion states that are obtained by removal of single electrons from the neutral molecule
Ionization energies are directly related to the energies of molecular orbitals
