Figure 16: Structural isosbestic point and fluence dependence. The Bragg spot profiles of the 0010 spot are shown at different delay times for five laser fluences (2.8, 7.0, 11.3, 14.3, and 20.6 mJ/cm2) at room temperature. The amplitude of the profiles is normalized by the height of the highest curve at each intensity, and curves at different fluences are displaced vertically for clarity. No laser-induced change can be observed at the lowest fluence (2.8 mJ/cm2). Above this fluence, transitions into a higher axis constant state can be observed as evidenced by the existence of structural isosbestic points. The change in the center position of the Bragg spot ( ) increases linearly with increasing intensity. (a) Phase transition region showing the times 233, 33, 4, 8, 20, 32, 44, 56, 68, 80, 92, and 112 ps. (b) Relaxation region showing the times 112, 147, 217, 317, 617, 917, and 1217 ps. Here, no crossing point is observed at any fluence; all the peaks relax back to equilibrium by continuously shifting their center positions.