If a photon, the energy of which corresponds exactly to the energy difference between the current state and an energy level with lower energy, hits the excited atom, the atom can switch to the lower energy state and emit the energy difference as a further photon in addition to the incident photon . The two photons are coherent. However, this is generally only possible if the relevant transition is permitted by the selection rules . The figure on the right shows the stimulated emission using the example of a laser at the transition from to .
The newly generated photon has the same energy and wavelength as the incident photon. It moves in the same direction, has the same direction of polarization and also the same phase position , so that it behaves like a copy of the original photon, which is known as coherence .
The stimulated emission was postulated in 1916 by Albert Einstein in his essay on the quantum theory of radiation . In his hypotheses about the exchange of energy through radiation , he assumed that the reversal process of absorption also takes place under the influence of radiation. He summarized this process with absorption as “change of state due to radiation”. Einstein did not yet give this process a name. In 1928 Rudolf Ladenburg achieved the first experimental evidence of gas discharges .
Although the term induced emission is still used, the term stimulated emission has gained acceptance through adoption from English .
While the spontaneous emission takes place without prior irradiation of a photon, there are two possibilities, depending on the current state of a system, how an irradiated photon can interact with the system:
- If the system is in the energetically lower state of a transition triggered by the photon, it can be excited by absorption of the photon.
- if, on the other hand, the system is already in the excited state, the photon can cause a stimulated emission.
In the local thermodynamic equilibrium , i.e. in most situations that occur in nature, the excited state that allows stimulated emission is, according to the Boltzmann distribution, less likely than the state of lower energy that allows absorption. The relationship between the probabilities of these two processes and that of spontaneous emission is described by the Einstein coefficients . To operate a laser or maser, i.e. to multiply the irradiated photons, a prerequisite is that the higher-energy state is more frequent than the lower-energy state than the corresponding equilibrium for the given radiation density. Such a non-equilibrium state is referred to as population inversion and must be specially created if necessary. Stimulated emission is then more frequent than absorption, and the incident beam is amplified.
Since the ratio of stimulated to spontaneous emission is determined by the Boltzmann distribution, it also depends on the wavelength: in the microwave radiation range , stimulated emission is much more common than in the visible light or X-ray radiation range .
- A Einstein: Radiation emission and absorption according to the quantum theory . In: Negotiations of the German Physical Society . 18, 1916, pp. 318-323. bibcode : 1916DPhyG..18..318E .
- A Einstein: On the quantum theory of radiation . In: Physikalische Zeitschrift . 18, 1917, pp. 121-128. bibcode : 1917PhyZ ... 18..121E .