The electron orbitals of an atom have different binding energies. These orbital energies also differ depending on what element the sample atom is composed of.

The kinetic energy (K.E.), or speed of the electrons emitted from the sample is not the binding energy of the electron orbital the emitted electron came from, but is actually the incident energy (1486 eV from the X-ray) minus the binding energy. Because of this, if the kinetic energy of the emitted electrons can be measured, through a simple subtraction, the binding energy of the electron orbital can be calculated. And from this, the element of the parent (sample) atom can be determined.

By placing a negative charge on the outside plate to repel the electrons and a positive charge on the inner plate to attract the electrons, it is possible to put the photoelectrons into a small orbit.

But the potential difference between these two plates will only allow photoelectrons of a certain voltage to make it through the hemisphere. Too slow (low energy) and the electrons would crash into the inner plate, and too fast, they will crash into the outer plate.

Therefore it is possible to set a potential between the plates such that only electrons of a known voltage will produce a signal at the electron detector. By sweeping an energy range of different potentials, an ESCA spectrum is formed.

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