Ion implantation followed by rapid thermal processing is widely employed to reduce implantation damage and to accomplish electrical activation. Current annealing technology uses rapid heating by incandescent lamps, high-intensity flash lamps or lasers. There has long been suspicion that the strong lamp illumination required for this procedure may nonthermally influence the diffusion of dopants. The present work employs low-intensity illumination (without significant photo-induced heating) to demonstrate such effects for boron and arsenic diffusion and activation. Depending on annealing temperature and time, the diffusion can be either enhanced or inhibited. Dopant activation is similarly affected. The effects arise from photostimulated changes in the average charge state of key point defects that mediate diffusion and activation, such as silicon interstitial atoms. A quantitative rate-based model has been formulated to quantify such effects. Furthermore, simulations using continuum equations for the reaction and diffusion of defects show that the changes in average charge state alter the electric fields in the implanted region, which in turn accounts for the observed changes in diffusion behavior.