Spontaneous vs Stimulated Emission
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. Electrons reside in these electron levels and often transit between levels by the absorption and emission of energy. When absorption takes place, electrons move to a higher energy state called an ‘excited state’, and the energy gap between the two levels equals the amount of energy absorbed. Likewise, electrons in the excited states will not reside in there forever. Therefore, they come down to a lower excited state or to the ground level by emitting the amount of energy that matches the energy gap between the two states of transition. It is believed that these energies are absorbed and released in quanta or packets of discrete energy.
Spontaneous Emission
This is one 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. Absorption is more frequent than emission as the ground level is generally more populated than the excited states. Therefore, more electrons tend to absorb energy and excite themselves. But after this process of excitation, as mentioned above, electrons cannot be in the excited states forever as any system favours being in a lower energy stable state rather than being in a high energy unstable state. Therefore, excited electrons tend to release their energy and return back to the ground levels. In a spontaneous emission, this emission process happens without the presence of an external stimulus/ magnetic field; hence the name spontaneous. It is solely a measure of bringing the system to a more stable state.
When a spontaneous emission occurs, as the electron transitions between the two energy states, an energy packet to match the energy gap between the two states is being released as a wave. Therefore, a spontaneous emission can be projected in two main steps; 1) Electron in an excited state comes down to a lower excited state or ground state 2) The simultaneous release of an energy wave carrying energy that matches the energy gap between the two transitioning states. Fluorescence and thermal energy are released this way.
Stimulated Emission
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.
This is true for both absorption and emission alike. When an electromagnetic stimulus such as an incident wave, is passed through the system, electrons in the ground level can readily oscillate and come to the transition dipole state whereby the transition to a higher energy level could take place. Likewise, when an incident wave is passed through the system, electrons that are already in excited states waiting to come down could easily enter the transition dipole state in response to the external electromagnetic wave and would release its excess energy to come down to a lower excited state or ground state. When this happens, since the incident beam is not absorbed in this case, it will also come out of the system with the newly released energy quanta due to the transition of the electron to a lower energy level releasing an energy packet to match the energy of the gap between the respective states. Therefore, stimulated emission can be projected in three main steps; 1) Entering of the incident wave 2) Electron in an excited state comes down to a lower excited state or ground state 3) The simultaneous release of an energy wave carrying energy that matches the energy gap between the two transitioning states along with the transmission of the incident beam. The principle of stimulated emission is used in the amplification of light. E.g. LASER technology.
What is the difference between Spontaneous Emission and Stimulated Emission?
• Spontaneous emission does not require an external electromagnetic stimulus to release energy, whereas stimulated emission does require external electromagnetic stimuli to release energy.
• During spontaneous emission, only one energy wave is released, but during stimulated emission two energy waves are released.
• The probability of stimulated emission to take place is higher than the probability for spontaneous emission to take place as external electromagnetic stimuli increases the probability of attaining the dipole transition state.
• 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.