If the atom is in the excited state E2 and a photon of energy exactly equal to E2 – E1 = hυ is incident on it, then the incident photon interacts with the atom in the excited state and then it stimulates or induces the atom to come down to the ground state E1. A fresh photon is emitted in this process. Therefore, when an atom ejects a photon due to its interaction with a photon incident on it, the process is called stimulated emission (or induced emission).
Characteristics of stimulated emission:
a) For each incident photon, there are two outgoing photons moving in the same direction.
b) As the emitted photon has exactly the same energy, phase and direction as the incident photon, we will achieve an amplified and unidirectional coherent beam.
Note: The laser is based on the principle of stimulated emission.
If the atom in the excited state automatically decays to the ground state by emitting a photon of energy (E2 – E1 = hυ), then this process is called spontaneous emission. Generally an atom/electron in excited state can stay for
10-9 – 10-8 seconds.
Characteristics of spontaneous emission:
a) The emitted photon of energy E2 – E1 = hυ can move in any random direction.
b) There will not be any phase relationship between the photons emitted from various atoms.
Hence, the radiations coming out due to spontaneous emissions are incoherent.
Reference: The content of all the articles of lasers posted here is referred from my book: Optics and Lasers by Amit Sarin and Sameer Kalia, published by kalyani publishers.
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When a photon of light having energy E2 – E1 = hυ is incident on an atom in the ground state, the atom in the ground state E1 may absorb the photon and jump to higher energy state E2. This process is called stimulated absorption or induced absorption. This is called so because the incident photon has stimulated the atom to absorb the energy.
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