Optical excitation of polaronic impurities inLa2CuO4+y

The temperature dependence of the infrared reflectivity of lightly oxygen-doped ${\mathrm{La}}_2$ ${\mathrm{CuO}}_{4+\mathit{y}}$ shows that spectral weight is transferred between free-carrier conductivity and a photoionization band of impurity-bound carriers. The large difference between the optical 0.13-eV and thermal 0.035-eV ionization energy indicates that the impurity states are stabilized by a lattice distortion. It is shown that the line shape as well as the temperature dependence is consistent with absorption from a polaronic impurity state.