Theory on transient effects in time resolved ESR spectroscopy

Abstract

A general theory is presented which in a unified way is able to explain the reported intensity behavior in the new field of time resolved ESR spectroscopy. Radicals with a short lifetime have been observed to have highly abnormal spin polarizations which have earlier been explained by a spin selective reaction. The production of polarization as well as other kinetic features are treated in a theoretical satisfactory way and the experimentally observed enhancement is directly related to fundamental quantities, such as the spin lattice relaxation time, the lifetime of the radical, and the polarization production per collision (the quantity calculated by the microscopic theories). It is shown that the time dependence of the intensity in many cases directly reflects the kinetic of the system and that the two commonly appearing cases of initial polarization and recombination polarization may be distinguished by the time behavior of the intensity. The effect of field inhomogeneities and different relaxation times T₁ and T₂ which becomes important when the radical decay is of the order of or faster than the spin lattice relaxation time is discussed. The method is based on a set of coupled Bloch equations modified to include a proper description of the radical kinetics, the polarization production, and the quenching of polarization due to Heisenberg spin exchange. Solutions of these equations are presented for a number of commonly appearing systems.