The contribution of bound - electron pair production to thedispersion relation for

In photon-atom scattering the customary partitioning of the elastic scattering amplitude into Rayleigh and Delbr uck amplitudes (as well as nuclear amplitudes) in the single-electron formalism involves summations over complete sets of intermediate electron states in the atomic potential. This leads to the Rayleigh amplitude which is usually compared with experiment. However another consequence of the partition is that the total cross section for bound-electron pair production into all bound states, regardless of occupation , is included in the optical theorem for the imaginary part of the forward Delbr uck amplitude. The corresponding real part of the forward Delbr uck amplitude can be obtained through the use of a dispersion relation. We show that for Z=92 the inclusion of the bound-electron pair production cross section leads to a contribution to the real part of forward Delbr uck amplitude which can be as much as 12% of the Born approximation result, for photon energies near the pair production threshold. This bound-electron pair production contribution is therefore comparable in magnitude to the corrections due to Coulomb and screening eeects in the ordinary pair production cross section (electron in continuum). The net correction to Born approximation is 1 small well below threshold, signiicant well above threshold.