Definition of ionization potential

Ionization potential is generally defined as the energy difference between the molecular orbit and the vacuum level. Since there is no single molecular orbit here, the ionization potential will differ for each orbit.
　If we consider charge transfer, it is the HOMO electrons that move between substances. Thus, ionization potential usually refers to HOMO ionization potential.

Standard definition: The difference in energy between the molecular orbit and vacuum level.
Extension: This usually refers to HOMO ionization potential. * HOMO: Highest occupied molecular orbital

　Electronic states differ respectively for metals, semiconductors, organic substances, and transparent conductive oxides (TCO).
　For metals, the conduction band and valence band overlap, and the Fermi level has the same energy as the valence band maximum energy.
　For semiconductors, there is no overlap between the conduction band and valence band. There are no electronic states with energy levels between the two. In other words, there are no electrons with these energy levels. This energy band is called the band gap. The Fermi level is located in the forbidden band.
　Organic substances are collections of the molecular orbits of the individual molecules. When considering charge transfer, it is important to consider the orbits in which charge transfers: the lowest unoccupied molecular orbit (LUMO) and the highest occupied molecular orbit (HOMO).
　Transparent conductive oxides (TCO) have a high-concentration donor levels in semiconductors with wide band gaps.
　For substances like ITO, the maximum level overlaps with conductors, lending them structures similar to metals—the occupied bands overlap with unoccupied bands, and the maximum occupied bands have the same energy as the Fermi level.
　
The electronic state diagram may be simplified as shown in Fig. 3.

　The extent of electron movement to a metal surface depends on the orientation of the metal crystals.

　Surfaces at the atomic level are not flat. Rather, they project where atoms are present and are indented where no atoms are present.
Electrons flow from protruding areas to indented areas to make the surface flat.
This is called the flattening effect.

　In metal monocrystals in which the atoms are arranged in an orderly pattern, a face may have small gaps between atoms (densely packed face) or large gaps between atoms (less densely packed face), depending on the face when a monocrystal is cut as shown in the figure.

　In the case of less densely packed faces with more gaps between atoms, electrons are drawn toward the metal by the flattening effect. On surfaces where the flattening effect is significant, the internal increase in positive charge due to electron movement will be reduced accordingly, resulting in a relatively smaller work function.

　This table compares the measured work function for various monocrystal surfaces.
　For metals with an fcc crystal lattice, such as Cu or Ag, the atomic density increases for surface orientations in the order 110, 100, 111.
　In this case, electron movement to the surface decreases and the work function increases in this order.
　For metals with a bcc crystal lattice, such as W, Mo, or K, the atomic density decreases for surface orientations in the order 110, 100, 111.
　In this case, electron movement to the surface increases and the work function decreases in this order.

　So far, we’ve explained variations in metal surface structure and work function in terms of the extent of electron movement to the surface.
The negative charge due to electron movement at a metal surface and the positive charge due to the remaining positive charge can be treated as a surface dipole.
　Fig. 7 shows the cross section of a metal sample. The top corresponds to the surface exterior. The bottom corresponds to the bulk interior.
Fig. 7(a) shows how the work function (WF) increases in the same way as when electrons move to the exterior.
　Variations in work function may also result from, for example, molecules adsorbed on the surface.
For example, if (a) in Fig. 7 represents Al with chlorine molecules that readily become negative adsorbed on the surface, the red area indicates the chlorine molecules, while the green and yellow areas are the Al bulk.
For Si with Ce atoms scattered on the surface, the Ce atoms become positive as shown in Fig. 7(b). In this case, the green area indicates the Ce atoms, while the red and yellow areas are the Si bulk.

　Fig. 8 shows the adsorbed molecules and atoms individually.

　While (a) shows the bulk alone before adsorption, (b) shows how the WF also varies due to dipole variations on the bulk surface caused by adsorption of a layer up to one molecule deep.
　However, this variation occurs only when the adsorbed molecules are in direct contact with the bulk surface and not more than one molecule deep.
If the adsorbed layer is two or more layers deep, the work function will be the same as when the first molecule layer is adsorbed.

　As shown in Fig. 10, the work function varies even when the adsorbed molecules themselves act as dipoles.