Nonclassical dimension-dependent kinetics of a photobleaching reaction in a focused laser beam “phototrap”

A focused laser beam acts as both a “phototrap,” bleaching fluorophore molecules that 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 $A+T\to T$ in both one and two dimensions. One-dimensional diffusion-limited trapping kinetics are observed in capillaries with $10$ $\mu$m diameters, while two-dimensional (2D) diffusion limited trapping kinetics are observed with unstirred samples having a quasi-2D geometry. However, in the presence of stirring, the 2D samples exhibit the classical, constant trapping rate.