  Vector mesons may be photoproduced in relativistic heavy-ion collisions when
a virtual photon emitted by one nucleus scatters from the other nucleus,
emerging as a vector meson. The STAR Collaboration has previously presented
measurements of coherent $\rho^0$ photoproduction at center of mass energies of
130 GeV and 200 GeV in AuAu collisions. Here, we present a measurement of the
cross section at 62.4 GeV; we find that the cross section for coherent $\rho^0$
photoproduction with nuclear breakup is $10.5\pm1.5\pm 1.6$ mb at 62.4 GeV. The
cross-section ratio between 200 GeV and 62.4 GeV is $2.8\pm0.6$, less than is
predicted by most theoretical models. It is, however, proportionally much
larger than the previously observed $15\pm 55$% increase between 130 GeV and
200 GeV.
