Fluorescence occurs when an excited electron relaxes to the ground state and combines with the hole.

For example, an electron may combine with a positron to form a positronium, an unstable atom.

Suddenly the electrons can combine with nuclei, to form atoms - and atoms don't interact strongly with photons.

When an electron and a positron combine, there is a mutual annihilation of the two particles.

At the same time, the cathode goes through a reduction reaction in which the cathode substance, ions and free electrons also combine to form compounds.

Meanwhile, the free electrons travel around the cell to power a given load and then combine with the oxygen and hydrogen at the anode to form water.

When the universe cooled enough, protons and electrons combined to form neutral atoms.

At one temperature, the electrons combine into Cooper pairs, but do not turn superconducting until the temperature is lowered further.

As the protons and electrons combine to form neutrons by means of electron capture, an electron neutrino is produced.

The electrons and protons have combined to make atoms, and the photons can move freely again!