## Wikipedia - Emissione spontanea (it) spontaneous emission

https://doi.org/10.1351/goldbook.S05886
That @[email protected] of @[email protected] which occurs even in the absence of a perturbing external electromagnetic field. The transition between states, $$n$$ and $$m$$, is governed by the @[email protected] @[email protected] of spontaneous emission, $$A_{nm}$$.