Electron spin relaxation at low field

Abstract

The low field ESR lineshape and the electron spin–lattice relaxation correlation function are calculated using the stochastic Liouville theory for an effective electron spin quantum number S = 1. When an axially symmetric permanent zero field splitting provides the dominant relaxation mechanism, and when it is much larger than the rotational diffusion constant, it is shown that both electron spin correlation functions 〈S¹_n(0)S¹_n(t)〉 (n = 0,1) are characterized by the same relaxation time τ_S = (4D_R)⁻¹. This confirms the conjectures made by Schaefle and Sharp, J. Chem. Phys., 2004, 121, 5287 and by Fries and Belorizky, J. Chem. Phys., 2005, 123, 124510, based on numerical results using a different formalism. The stochastic Liouville approach also gives the paramagnetically enhanced nuclear spin relaxation time constants, T₁ and T₂, and the ESR lineshape function I(ω). In particular, the L-band (B₀ = 0.035 T) ESR spectrum of a low symmetry Ni(II)-complex with a cylindrical ZFS tensor is shown to be detectable at sufficiently slowly reorientation of the complex. The analysis shows that the L-band spectrum becomes similar to the zero-field spectrum with a electron spin relaxation time τ_S = (4D_R)⁻¹.