Rami Arieli: "The Laser Adventure"

Rami Arieli: "The Laser Adventure" Chapter 2.7 page 3

Possible Processes Between Photons and Atoms
Figure 2.4 summarizes the three possible processes between photons and atoms: absorption, spontaneous emission, and stimulated emission.

Photon Absorption: A photon with frequency n₁₂ hits an atom at rest (left), and excites it to higher energy level (E₂) while the photon is absorbed.

Spontaneous emission of a photon: An atom in an excited state (left) emits a photon with frequency n₁₂ and goes to a lower energy level (E₁).

Stimulated emission of a photon: A photon with frequency n₁₂ hit an excited atom (left), and cause emission of two photons with frequency n₁₂ while the atom goes to a lower energy level (E₁).