By means of energy supply, for example in the form of light, it is possible to bring the electrons of a molecule or a single atom to an energetically higher level.
The electron then is in an “excited state”, which however exists only for a trillionth of a second. Following physical laws, electrons always aim for the lower energy level. Dropping back to the basic state releases energy. This occurs in the form of electromagnetic radiation and also is called “spontaneous emission”.
Induced or stimulated emission
The released photon propagates in the respective medium (gas mixtures, solid matter, etc.), which leads to collisions with further electrons. When an electron exists in “excited state” and collides with a photon, this triggers an early discharge and releases two photons.
This process is called “induced or stimulated emission”, as the light emission was triggered by an external photon. The special nature of this process is that the released photons have the same wavelength and phase length.