  The a-axis optical properties of a detwinned single crystal of
YBa_2Cu_3O_6.50 in the ortho II phase (Ortho II Y123, T_c= 59 K) were
determined from reflectance data over a wide frequency range (70 - 42 000
cm^-1) for nine temperature values between 28 and 295 K. Above 200 K the
spectra are dominated by a broad background of scattering that extends to 1 eV.
Below 200 K a shoulder in the reflectance appears and signals the onset of
scattering at 400 cm^-1. In this temperature range we also observe a peak in
the optical conductivity at 177 cm^-1. Below 59 K, the superconducting
transition temperature, the spectra change dramatically with the appearance of
the superconducting condensate. Its spectral weight is consistent, to within
experimental error, with the Ferrell-Glover-Tinkham (FGT) sum rule. We also
compare our data with magnetic neutron scattering on samples from the same
source that show a strong resonance at 31 meV. We find that the scattering
rates can be modeled as the combined effect of the neutron resonance and a
bosonic background in the presence of a density of states with a pseudogap. The
model shows that the decreasing amplitude of the neutron resonance with
temperature is compensated for by an increasing of the bosonic background
yielding a net temperature independent scattering rate at high frequencies.
This is in agreement with the experiments.
