# Natural broadening and the electric field produced by a decaying atom?

To calculate the line profile due to natural broadening it is common to assume that a decaying atom produces an electric field of the form: $$E(t)=E_0 e^{-t\gamma}\cos(\omega_0t)$$ as stated e.g. here (link to Google Books).

Here is my problem, I thought that on decaying an atom would release a single photon not a continuous wave; so why can we assume this form of electric field and where does it come from?