RETARDED INTERACTION ENERGIES BETWEEN LIKE ATOMS IN DIFFERENT ENERGY STATES

The long-range interaction energy between two like atoms in different energy states is discussed in general, through O(α2), using the Breit-Pauli approximation. Selection rules are derived for possible interactions where retardation or relativistic effects may be important. The S-P resonant interaction is considered as a specific example. The interaction energiesfor the resulting ∑ and ∏ molecular states are calculated in both the Breit-Pauli approximation and from the exact dipole-dipole results of McLone and Power, Stephen and others. The R-1 expansion of the exact result and the validity of the Breit-Pauli approximation are discussed in detail. The interaction energies are also compared with the linewidths for the various molecular states. It is shown that the Breit-Pauli approximation can give a good representation (especially for the ∑ states) of the interaction for quite large values of Rλ, where λ is the characteristic wavelength for the S-P transition.