Figure 1.

(a) Schematic of the electronic and vibrational transitions in a fluorophore during pump-probe excitation (S₀ - electronic ground state, S₁ - first excited electronic state). (b) Illustration of the changes in the PA signal amplitude measured in a fluorophore solution in a cuvette using pump-probe excitation. By changing the time delay between the pump and probe pulses, the PA signal amplitude is modulated, yielding a difference signal (τ_f - fluorescence lifetime).