Time-resolved spectroscopy of the charge-transfer gap in Sr$_2$CuO$_2$Cl$_2$

We present energy- and time-resolved pump-probe spectroscopy near the charge-transfer gap in the undoped cuprate compound Sr$_2$CuO$_2$Cl$_2$. Upon photoexcitation, an increase in absorption is observed for energies below 1.95~eV, whereas a decrease occurs above 1.95~eV. Overall, the spectral weight is not conserved over the probe range of 1.6-2.3 eV. No hole-burning is observable at the pump energy $E_{\mathrm{pump}} = 2.1$~eV. The transient spectral changes appear as one spectral unit instantaneously after the excitation, and they decay, again as one spectral unit, on a picosecond time scale. The photoinduced response relates simply to the thermal response, indicating a common boson-mediated origin. These results support a theoretical model that places the gap energy near 1.5 eV, well below the peak in the charge-transfer absorption spectrum.\footnote{K. M. Shen \textit{et al.}, Phys.~Rev.~B \textbf{75}, 075115 (2007).} In this model, the photoexcited state decays rapidly to the gap energy via phonon emission, and the presence of the additional phonons then has the same effect on the charge-transfer absorption as an increase in the equilibrium lattice temperature.