Two Spin Probes Method for the Investigation of Intermediate Radical Species and their Molecular Dynamics

Introduction

Investigations of intermediate paramagnetic products of
photochemical reactions helps one to understand mechanism of
such reactions. These intermediate species, are as a rule
sufficiently stable at low temperatures and it is easy to detect
them by EPR spectroscopy. Information concerning the structure of
the radicals can be obtained using this technique. Typical spectra
of radical pairs (RP) arise if the distance between two radical is
not more than 7 A - 10 A. In this case the dipole-dipole magnetic
interaction of the electron spins with each is large enough to see
a  corresponding splitting in the EPR spectrum. In the case of
crystal matrix anisotropy interations do not take part within the
width of lines and it is possible to see the hyperfine splitting
(HFS) more distinctly, so giving more information about the
nature of the radicals in RP.

The spin Hamiltonian of two electrons in a triplet state is (1).