Figure 17: Fluence dependence of the expansion. (a) The change in the -axis constant () is plotted as a function of time for each laser fluence (2.8 (cyan), 7.0 (blue), 11.3 (green), 14.3 (red), and 20.6 mJ/cm2 (black)). The c-axis lattice constant at each time delay was obtained by fitting the profiles in Figure 16 to a single Gaussian form. The inset shows the same curves normalized. relaxes back to ground state with a time constant of about 300 ps independent of the laser fluence. Even after 1 ns, the system does not completely relax back to equilibrium; a small residual expansion is present and has a much longer time constant. (b) (130 ps) around the dotted line shown in (a) was measured as a function of laser fluence at both 20 K and 300 K. Below a threshold intensity of about 5 mJ/cm2, no change can be observed. Above this threshold, grows linearly with the laser fluence. The top horizontal scale shows the number of photons absorbed per copper site calculated using the absorption coefficient (see text). The threshold fluence corresponds to 0.1 photons absorbed per copper site. The red line is a linear fit to room temperature data above the threshold value.