Local-field correction to the spontaneous decay rate of atoms embedded in bodies of finite size

The influence of the size and shape of a dispersing and absorbing dielectric body on the local-field-corrected spontaneous decay of an excited atom embedded in a body is studied on the basis of the real-cavity model. By means of a Born expansion of the Green tensor of the system it is shown that to linear order in the susceptibility of the body the decay rate exactly follows Tomas's formula found for the special case of an atom at the center of a homogeneous dielectric sphere [Phys. Rev. A 63, 053811 (2001)]. It is further shown that for an atom situated at the interior of an arbitrary dielectric body this formula remains valid beyond linear order. The case of an atom embedded in a weakly polarizable sphere is discussed in detail.