Retardation effects in atom-wall interactions

The onset of retardation effects in atom-wall interactions is studied. It is shown that the transition range from the 1/z3 short-range (van der Waals) interaction to the 1/z4 long-range (Casimir) retarded interaction critically depends on the atomic properties and on the dielectric function of the material. For simple non-alkali-metal atoms (e.g., ground-state hydrogen and ground-state helium) interacting with typical dielectric materials such as intrinsic silicon, the transition to the retarded regime is shown to proceed at a distance of about 10 nm (200 Bohr radii). This is much shorter than typical characteristic absorption wavelengths of solids. Larger transition regimes are obtained for atoms with a large static polarizability such as metastable helium. We present a simple estimate for the critical distance, zcr = 137 root alpha(0)/Z atomic units, where alpha(0) is the static polarizability (expressed in atomic units) and Z is the number of electrons of the atom.