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.

Kindly Read This Also :

Note from winnerscience: If you want the e-notes of all the laser  articles that will include the basics of lasers like stimulated absorption, difference between spontaneous and stimulated emission, Einstein  Coefficients, properties and applications of lasers, complete construction and working of lasers like Ruby laser, He-Ne laser, Carbon dioxide laser, Nd:YAG laser, dye laser, semiconductor laser, holography and additional articles of Q-switching and mode locking, then please contact winnerscience@gmail.com. You can post your queries also there.