Moving out of the solid now, we have the vacuum level, E_VAC, which is the energy level of an electron removed from the solid and at rest. The term is slightly vague in that it refers to an electronic state at rest (zero kinetic energy, E_k) at a distance where it may still experience the surface dipole, usually defined as a ‘few nanometers’. A second term, E_VAC(∞) described the electron at a distance far removed from the effects of the surface.

Next, we describe the Fermi level, E_F, which is defined as the highest energy level of the electronic states in a solid when at absolute zero.

The work function, WF, may be simple characterized as the energy to take an electron from E_F to E_VAC. A more technical description is that the work function is the difference betweeen the electrochemical potential (μ¯ ) of electrons in the bulk and the electrostatic potential energy (−eΦ_vac) of an electron in the vacuum just outside the surface WF=−eΦ_vac−μ¯.⁽³⁾

Finally we have our ionization energy (IE) and electron affinity (EA). While ionization energy should be familiar to most readers (the minimum energy to remove an electron to E_VAC) electron affinity has a slightly different definition than some will be familiar with. When dealing with solid state chemistry (or physics), the electron affinity is defined as the energy released when an electron moves from E_VAC to the lowest energy state in the conduction band (CBM/LUMO), aka EA = IE – E_G.