  We have investigated the spin fluctuations at energy transfers up to ~110
meV, well above the resonance energy (33 meV) in the YBa2Cu3O6.5 ortho-II
superconductor using neutron time-of-flight and triple-axis techniques. The
spectrum at high energies differs from the low-energy incommensurate
modulations previously reported where the incommensurate wave vector is largely
independent of energy. Well above the resonance the peak of the spin response
lies at wave vectors that increase with energy. Within error the excitations at
all energies above the resonance are best described by a ring around the (pi,
pi) position. The isotropic wave-vector pattern differs from a recently
reported square pattern in different but related systems. The spin spectral
weight at high-energies is similar to that in the insulator but the
characteristic velocity is ~40% lower. We introduce a method of extracting the
acoustic and optic weights at all energies from time-of-flight data. We find
that the optic spectral weight extends to surprisingly low-energies of ~25 meV,
and infer that the bilayer spin correlations weaken with increase in hole
doping. When the low-energy optic excitations are taken into account we measure
the total integrated weight around (pi, pi), for energies below 120 meV, to
agree with that expected from the insulator. As a qualitative guide, we compare
spin-wave calculations for an ordered and a disordered stripe model and
describe the inadequacy of this and other stripe models for the high-energy
fluctuations.
