The anomalous diffusion-limited reaction kinetics of a phototrapping reaction

A focused laser beam acts as both a ''phototrap'', bleaching fluorophore molecules which diffuse into the beam path, and as a confocal probe, detecting the excited, unbleached fluorophore molecules still present in the trap. With this focused laser beam, we observe anomalous asymptotic rate laws similar to those predicted for a diffusion-controlled elementary trapping reaction, A + T --> T, in one and two dimensions. One dimensional diffusion-limited trapping kinetics are approached in capillaries with 10 mu m diameters while two dimensional diffusion limited trapping kinetics are observed with unstirred samples having a quasi 2-D geometry. In the presence of stirring, the 2-D samples exhibit the classical, constant trapping rate over time.