Figure 4. Phase-averaged interferometric two-pulse correlation scans for polycrystalline Mo (blue triangles), bare reduced TiO2 (red diamonds), and the latter surface after exposure to 0.7 (green circles) and 1.6 L (orange squares) of H2O. The Mo data represent the laser pulse autocorrelation. The autocorrelation is simulated by assuming a Gaussian shape for the main pulse with subpulse at 35 fs delay (solid lines). The data for TiO2 surfaces are fit with thus obtained laser pulse assuming single exponential decay kinetics for the intermediate state using a three-level optical Bloch equation model. The lifetime for the bare surface is immeasurable, while for the 0.7 and 1.6 L H2O/TiO2 surfaces it is 10±1 and 14±1 fs, respectively.