Controlling Activated Processes

We will discuss activated escape from a metastable state and nucleation in driven systems. The formulation applies for an arbitrary noise spectrum. We will show that the escape probabilities can be changed very strongly even by a comparatively weak force. In a broad parameter range, the activation energy of escape depends linearly on the force amplitude, even for a high-frequency field. This dependence is described by the logarithmic susceptibility, which has interesting analytical properties. A closed-form explicit expression for the escape rate of an overdamped periodically driven Brownian particle will be presented. It has been quantitatively confirmed by simulations. We will also describe some recent experiments on driven systems and the onset of directed diffusion in spatially periodic non-sinusoidally driven systems.