Diffusion influenced reactions at short times: breakdown of the Debye-Smoluchowski description

Fluorescence quenching studies were used to explore the validity of the Debye–Smoluchowski equation on the 0.5–200 ps time scale, using fluorescence upconversion. Complementary studies extending to 3.5 ns were carried out using time correlated single photon counting. We find, for the system rhodamine B quenched by ferrocyanide, that the fluorescence decay (i. e., reaction rate) is much faster at short times than implied by fits to the lower time resolution data. Possible reasons for the breakdown of the Debye–Smoluchowski equation at short times are briefly discussed.