  Spontaneous decay of an excited atom near a dispersing and absorbing
microsphere of given complex permittivity that satisfies the Kramers-Kronig
relations is studied, with special emphasis on a Drude-Lorentz permittivity.
Both the whispering gallery field resonances below the band gap (for a
dielectric sphere) and the surface-guided field resonances inside the gap (for
a dielectric or a metallic sphere) are considered. Since the decay rate mimics
the spectral density of the sphere-assisted ground-state fluctuation of the
radiation field, the strengths and widths of the field resonances essentially
determine the feasible enhancement of spontaneous decay. In particular, strong
enhancement can be observed for transition frequencies within the interval in
which the surface-guided field resonances strongly overlap. When material
absorption becomes significant, then the highly structured emission pattern
that can be observed when radiative losses dominate reduces to that of a
strongly absorbing mirror. Accordingly, nonradiative decay becomes dominant. In
particular, if the distance between the atom and the surface of the microsphere
is small enough, the decay becomes purely nonradiative.
