Theory of chemically induced magnetic polarization. Effects of S–T±1 mixing in strong magnetic fields

Abstract

Chemically induced magnetic polarization due to mixing of the singlet and M_S=±1 triplet sublevels of a diffusing radical pair in a strong magnetic field is investigated by an approximate stochastic Liouville model which considers separately the two‐level S–T₋₁ and S–T₁ mixings. As previously found for S–T₀ mixing, the stochastic Liouville equations for S–T₋₁ level mixing can be written as a Bloch‐type equation with diffusion, and a similar equation for the S–T₁ mixing can be obtained simply by changing the sign of the magnetic field. These equations give a simple vector model of chemically induced polarization by S–T_±1 mixing, including the all‐important diffusion through the S–T₋₁ level crossing. For simple Brownian diffusion and an exchange interaction that decays exponentially with radical separation, the stochastic Liouville equations for S–T₋₁ mixing can be converted to a single integral equation for the resulting polarization, and a similar equation can be derived for the S–T₁ case. The polarizations are small and can be well approximated by a single term in the Neumann series solution of the integral equation. Usually the entire S–T_±1 polarization comes from S–T₋₁ mixing in the region where these levels cross. Significant polarizations occur only in those radical pairs where either a very large hyperfine interaction provides rapid singlet–triplet mixing or slow diffusion enables the pair to remain a relatively long time in the level crossing region.