The a-axis optical conductivity of detwinned Ortho-II YBa$_{2}$Cu$_{3}$O$_{6.50}$

The a-axis optical properties of a detwinned single crystal of YBa$_{2}$Cu$_{3}$O$_{6.50}$ in the Ortho II phase have been determined from reflectance data over wide frequency range (70 - 42 000 cm$^{-1}$). At high temperature the spectra are dominated by a broad background of scattering extending to 1 eV. Below 200 K a shoulder in the reflectance signals the opening of a scattering channel with an onset frequency of 400 cm. In this temperature range we also observe a peak in the optical conductivity at 177 cm. The shoulder spectrum is consistent with a scattering model with a bosonic mode at 33 meV whose amplitude {\it decreases} with temperature in parallel with the magnetic resonance measured on crystals from the same source by Stock {\it et al.} This temperature dependence enables us to set an upper limit to any phonon contribution to the scattering. Below $T_c=59$ K, the spectra change dramatically as the superconducting condensate appears. Its spectral weight is consistent, to within experimental error, with the FGT sum rule.