Radiative properties of an atom in the vicinity of a mirror

The decay rate of an excited atom is described using Fermi's golden rule. Welton's interpretation of the Lamb shift is extended by introducing a damping term in the Heisenberg equation of motion associated with the fluctuation in the position of the electron. These expressions an related to the imaginary part of the vector potential Green function through the fluctuation dissipation theorem and Kubo's formula. The results are applied to the calculation of the radiative properties of an atom in the vicinity of a perfect mirror.