First principle mechanism of ionization

The photoelectric effect of ionization involves the complete absorption of the photon energy during the process of knocking an electron out of orbit. This process primarily occurs with low energy photons ranging in energy between 10 Kev and less than 500 Kev.

Notice in the above illustration that an ion pair is created in the interaction between the radiation photon and the atom. During this process, when the photon liberates the electron, all of the photon’s energy is transferred to create the ion pair and total absorption has occurred. Remember, there is a binding force that the holds the electron in its orbital shell. The amount of energy required to create the ion pair must be at least equal to this binding force.

If during the ionization process, only part of the photons energy is needed to liberate the electron, the rest of the energy is transferred to the electron in the form of speed (velocity). Now that all of the photon's energy is accounted for, the photon ceases to exist and total absorption has occurred. Remember that a photon is not a particle, but acts like one. When the energy of the photon is used, there is nothing left to cause further ionization. Keep in mind that electrons orbit in various shells of the atom and not all electrons have the same binding energy. This binding energy is dependent upon the elements (Z) number and the position of the electron in the atom. Those electrons nearer the nucleus possess greater binding energy and will require greater photon energy to remove them than will electrons in the outer shells.


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