The emission-absorption principle

The three different mechanisms are shown below (Figure 2):

1. Absorption: An atom in a lower level absorbs a photon of frequency hν and moves to an upper level.

2. Spontaneous emission: An atom in an upper level can decay spontaneously to the lower level and emit a photon of frequency hν if the transition between E2 and E1 is radiative. This photon has a random direction and phase.

3. Stimulated emission: An incident photon causes an upper level atom to decay, emitting a “stimulated” photon whose properties are identical to those of the incident photon. The term “stimulated” underlines the fact that this kind of radiation only occurs if an incident photon is present. The amplification arises due to the similarities between the incident and emitted photons.