We present Hubble Space Telescope UV/optical spectrophotometry and ground-b
ased optical spectrophotometry of the brightest knots in the emission-line
regions of the Seyfert galaxies NGC 5643 and NGC 5728. These objects have i
onization cones, radio and soft X-ray emission, and evidence of nuclear out
flows and winds, suggesting that shocks may be associated with the emission
-line gas. Comparison of the UV/optical data with grids of ionization-bound
ed and mixed ionization- and matter-bounded photoionization models, and sho
ck models with and without ionized precursors, are used to determine whethe
r radiative shocks that photoionize the shock precursor material ("autoioni
zing shocks") are a significant excitation mechanism for the emission-line
gas in these objects. Although the UV/optical spectra studied here are gene
rally compatible with both photoionization and shock + precursor models, an
unambiguous shock + precursor signature is not present. This suggests that
mechanical energy input may not play the same dominant role in exciting th
ese objects with weak radio jets as appears to be the case in strong radio
jet sources such as M87.