Photolysis of 2-NBA in the presence of BG Figure 1 shows the absorbance changes at 633 nm of a solution containing 2-NBA, and BG at several laser pulse energies. The reaction occurs on sub-µs time scale at all investigated laser pulse energies and its apparent rate increases with the laser pulse energy. It is clear, even at the lowest laser pulse energy, that protonation of more than 50 % of BGoccurs, indicating that the concentration of the photoreleased protons brings the pH well below the pK

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... a of the pH indicator. The apparent rate constant is determined by the concentration of photodetached protons, the concentration of the other reactants, essentially 2-nitrosobenzoate anion (ArCO 2 -) and BG -, as well as by the rate constants for the protonation reactions. The oscillations in curve C at times longer than ≈1 µs are due to acoustic effects induced by the absorption of the intense laser pulse.