  By applying a unitary transformation method, we have derived the
leading-order corrections on the effective Hamiltonian of a dynamical model
developed in Phys. Rev. C54, 2660 (1996) for electromagnetic pion production
reactions. The resulting energy-independent one-loop corrections on the baryon
masses and the gamma N -> Delta vertex interaction are associated with the
structure of the nucleon and Delta and have been calculated within a
constituent quark model. We find that the one-loop corrections on the magnetic
M1 transition of the gamma N -> Delta are very small, while their contributions
to the electric E2 and Coulomb C2 transitions are found to be in opposite signs
of that due to pion cloud effects associated with the scattering states. Our
results further indicate that the determination of the nonspherical L=2
components of the constituent quark wavefunctions of N and Delta from the
extracted empirical E2 and C2 form factors requires a rigorous and complete
calculation of meson cloud effects. We also find that the one-loop corrections
on the non-resonant pion production operator can resolve the difficulty in
describing the near threshold gamma p -> pi^0 p reaction. Possible future
developments are discussed.
