Study of Spin-Correlated Radical Ion Pairs in Irradiated Solutions by Optically Detected EPR and Related Techniques

Abstract

The strong Coulomb attraction and recombination dramatically shorten the lifetime of radical ion pairs generated by ionizing irradiation in organic solutions, which complicates the use of conventional EPR spectroscopy to study these short-lived radical ions. However, the recombination of the oppositely charged ions gives birth to a fluorescence response of the irradiated media. This response appears to depend on the same properties of the radical ions that are studied by EPR spectroscopy. The dependence can be revealed with an external magnetic field, thus allowing a quantitative study of hyperfine couplings, spin-orbit interaction, paramagnetic relaxation times of radical ions, whose lifetime can amount to only a few nanoseconds. In this chapter we consider experimental approaches, both steady-state and time-resolved, which are based on the registration of the fluorescence response influenced by an external magnetic field. These are Optically Detected EPR, MARY (Magnetically Affected Reaction Yield) spectroscopy, and the technique of Time-Resolved Magnetic Field Effect (TR MFE) in recombination fluorescence. A brief history, a theoretical background, methodological details, as well as some unique experimental results obtained with these techniques are discussed.