Immediately after the excitation, a broad positive transient absorption band was observed at 640 nm (Fig. ). The transient species decayed with a time constant of 0.90 μs (Fig. ). Moreover, the signal increased nonlinearly with increase in the excitation intensity (Fig. ). The spectral shape, lifetime, and nonlinear dependence indicated that the short-lived transient species originated from a hydrated electron generated by Auger processes. Because the photon flux density was insufficient for a simultaneous two-photon process, the hydrated electron was produced by the stepwise two-photon process. The quantum yield for hydrated electron generation was estimated to be 1.5 × 10 -2 when the excitation intensity was 4.0 mJ pulse -1 (details can be found in Supplementary Note 1) The transient signal associated with the hydrated electron decayed faster (0.46 μs) upon the addition of PFOS to the solution (~6.2 × 10 -4 M, Fig. ). This result shows that PFOS reacted with the generated hydrated electron.