CONTROL OF RADICAL PAIR LIFETIMES BY MICROWAVE IRRADIATION - APPLICATION TO PHOTOSYNTHETIC REACTION CENTERS

Radicals produced by illumination or ionizing radiation are often prod uced in pairs, which quickly decay by recombination or by diffusion an d subsequent reactions. For maximizing the yield of products, and for facilitating the study of reaction pathways, it is desirable to minimi ze the probability of radical pair recombination. We present a way of controlling the radical pair lifetime through the application of a pul se of resonant microwaves in the presence of a magnetic field. Herewit h, two radical pair triplet states are coherently populated, from whic h the pair cannot recombine directly to the singlet ground state becau se of spin conservation. We illustrate the method with a photosyntheti c photochemical reaction, where we have achieved an increase in the ra dical pair lifetime of up to two orders of magnitude.