Spontaneous and stimulated emission of radiation
When an isolated atom is excited into a high-energy state, it generally remains in the excited state for a short time before emitting a photon and making a transition to a lower energy state. This fundamental process is called spontaneous emission. The emission of a photon is a highly probabilistic event; that is, the likelihood of its occurrence is described by a probability per unit time. Emission refers to the emission of energy in photons when an electron is transitioning between two different energy levels. Characteristically, atoms, molecules and other quantum systems are made up of many energy levels surrounding the core.
This is the other method in which emission takes place when an electron transitions from a higher energy level to a lower energy level or to the ground state. However, as the name suggests, this time emission takes place under the influence of external stimuli such as an external electromagnetic field. When an electron moves from one energy state to another, it does so through a transition state which possesses a dipole field and acts like a small dipole. Therefore, when under the influence of an external electromagnetic field the probability of the electron to enter the transition state is increased. Stimulated emission does require external electromagnetic stimuli to release energy.
By properly matching the energy gaps and incident frequencies, stimulated emission can be used to greatly amplify the incident radiation beam; whereas this is not possible when spontaneous emission takes place.