  Energy- and time-resolved spectroscopy reveals a photoinduced softening of
the charge-transfer gap in the insulating copper oxide Sr2CuO2Cl2 that
indicates rapid and efficient photoproduction of optical phonons. By relating
the pump-probe signal amplitude to the thermal difference spectrum, we estimate
that eleven to twenty optical phonons are created for every one 3 eV photon.
Assuming relaxation to the optical absorption edge at 1.5 eV, this corresponds
to 70-130 meV per boson. While the lower limit is consistent with relaxation
exclusively through optical phonons, the upper limit suggests a significant
role for magnetic excitations. We observe a photoinduced bleaching of the gap
excitation that we associate with phase space filling, and estimate the
excluded area of the photoexcited state to be about nine copper oxide
plaquettes. The temporal decay of the pump-probe signal is consistent with
anharmonic phonon decay.
