The frequency spectrum of electromagnetic radiation can be written as the Fourier transform of the first-order correlation function of the vector potential. If nuclei are coupled to the radiation field, the Heisenberg equations of motion of the field operators contain nuclear operators and vice versa. Under plausible assumptions the equations of motion for the nuclear operators can be integrated and hence, the equations of the field operators can be solved. The vector potential of the radiated field can then be expressed as a function of solely nuclear quantities. The first-order correlation function deduced from it contains only two-times and one-time averages of simple nuclear creation and annihilation operators. The theory can be used to explain homogeneous line broadening for long-lived nuclei submitted to small fluctuating interactions.