When coherent electromagnetic radiation resonantly couples two levels of a nucleus, the system can emit photons by stimulated and by spontaneous emission. The spontaneous emission spectrum is defined as the Fourier transform of the first-order correlation function of the magnetic or electric field operator. This function can be expressed in terms of one-time and two-time averages of nuclear operators. The one-time averages can be calculated by solving the Bloch-type equations for the matrix elements of the nuclear density operator. Application of the quantum regression theorem permits to calculate two-time averages with the aid of one-time averages and hence the radiated spectrum.