Onset of Casimir-Polder retardation in a long-range molecular quantum state.

Two 4He atoms form a diatomic molecule with a significant vibrational wave function amplitude at interatomic separations R>100 Å, where the retardation switches the London R(-6) decay of the potential to the Casimir-Polder R(-7) form. It has been assumed that this effect of retardation on the long-range part of the potential is responsible for the 2 Å (4%) increase of the bond length of 4He2. We show that is, unexpectedly, insensitive to the potential at R>20 Å and its increase is due to quantum electrodynamics effects computed by us from expressions valid at short R--beyond the validity range of Casimir-Polder theory--that seamlessly extend this theory to distances relevant for properties of long molecules.