# Photoelectric effect

Why the photoelectric effect is observed only for metals and not for non-metals? Isn't it possible for the photon to release an electron from a non-metal surface?

Conduction electrons in metals lie in the conduction band, they have a very low binding energy compared to electrons in an insulator, for instance. It thus requires a low amount of energy to excite them and have a photoelectric effect.

You can see on this article showing original data from Milikan, that the minimum amount of energy per photon required to have a photoelectric effect with sodium is of order of $\Delta E_{\text{conductor}} \simeq 1,8$ eV.

In insulators, the highest-energy electrons lie in the valence band and you need a huge amount of energy to excite them : it is harder to have a photoelectric effect in this case. In the case of insulators, this energy is of order of magnitude of the first ionization energy of the atom or molecule which the solid is made of. You can see here a table giving in eV the first ionization energy of each atom. (It's in french, couldn't find the same table on English wikipedia). You see that the energy required is of order of $\Delta E_{\text{insulator}} \simeq 10$ eV for most insulating pure compounds. It is thus possible to have a photoelectric effect, but would require photons with ten times more energy than for insulators.

• What is the difference between a conduction and a valence band? – hello there Dec 18 '15 at 11:08
• Conduction and valence bands is what creates the difference in behavior between most insulators and conductors. If you look at the energy levels of electrons in a solid, they are gathered in what we call "bands" with an energy gap between them. In insulators, the highest energy band is completely filled, wherehas in conductors it is only partially filled : we call this band the conduction band, and the lower-energy bands the valence band. You have a full explanation on this on wikipedia : en.wikipedia.org/wiki/Valence_and_conduction_bands – Dimitri Dec 18 '15 at 11:14
• I think this is the right answer, but there's also a practical difficulty. As electrons leave the insulator it acquires a positive charge. One has to figure out a way to replenish the supply of electrons. Easy in a metal. – garyp Dec 18 '15 at 16:59

The outer electrons in insulators tend to have higher binding energies, so it requires more photon energy to free the electron. So, the photoelectric effect is still possible in insulators, it's just harder.

• kotozna, could you make your answer more quantitative by comparing the energy needed for the case of metals with that in the case of insulators — to see how "harder" it is? Please also mention if experiments are performed for some insulators. – AlQuemist Dec 18 '15 at 11:00