Single atom inside or outside a dielectric or metallic bubble

The dispersive van der Waals energy of a ground-state atom encapsulated in, or placed in the vicinity of, a spherical dielectric ol metallic shell is calculated. The treatment, the validity of which is restricted to the intermediate distance range, makes use of the random phase approximation and the propagator method. It is shown that the dielectric medium appears as a special case in the general nonlocal treatment used for a metal. Examples are presented and discussed. The interaction between an argon atom and a fullerene molecule, averaged over the molecule orientation and including a short-distance repulsive contribution, is calculated. It is then used to evaluate elastic differential cross sections at low energies (26 and 52 meV)-both of them exhibiting clear rainbow structures-and the total cross section in the range 10-100 meV.