In photon-atom scattering the customary partitioning of the
elastic scattering amplitude into Rayleigh and Delbrück
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 Delbrück
amplitude. The corresponding real part of the forward
Delbrück 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 Delbrück
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
effects in the ordinary pair production cross section
(electron in continuum). The net correction to Born
approximation is small well below threshold, significant
well above threshold.
