## Wikipedia - Emissione spontanea (it) spontaneous emission

https://doi.org/10.1351/goldbook.S05886
That @M03958@ of @E02056@ which occurs even in the absence of a perturbing external electromagnetic field. The transition between states, $$n$$ and $$m$$, is governed by the @E01914@ @C01124@ of spontaneous emission, $$A_{nm}$$.