  We consider the van der Waals interaction of an excited atom and a ground
state atom across a vacuum-medium interface under the circumstances of the
resonant coupling of the excited atom to a surface polariton mode of the
system. We demonstrate that the nonretarded interaction potential between the
atoms is in this case the same as in an effective medium described by the
average dielectric function of the media in contact except for the different
local-field correction factor. The estimate performed for the vacuum-sapphire
interface shows that the nonretarded van der Waals (atom*-atom) force can be at
the surface mode resonance enhanced by almost three orders of magnitude in
comparison with its free-space value. Owing to the local-field effect in the
present configuration this enhancement factor is larger then previously
estimated for atoms in front of the same medium by (almost) an order of
magnitude.
