Photoinduced Changes of Reflectivity in Single Crystals of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{O}_{6.5}$ (Ortho II)

We report measurements of the photoinduced change in reflectivity of an untwinned single crystal of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.5}$ in the ortho II structure. The decay rate of the transient change in reflectivity is found to decrease rapidly with decreasing temperature and, below $T_c$, with decreasing laser intensity. We interpret the decay as a process of thermalization of antinodal quasiparticles, with a rate determined by inelastic scattering of quasiparticle pairs.