Solution of the spin Hamiltonian with orthorhombic hf and g tensors (I = S = 1/2). I. Theory

A theory was developed appropriate for analyzing ESR spectra characterized by H = βH ⋅ g ⋅ S+S ⋅ A ⋅ I, where S = I = 1/2 and g and A tensors are of orthorhombic symmetry using the perturbational method. The theory reproduced the reported solutions of special cases; the Breit–Rabi solution, when g and A are isotropic, the zero field solution, when H = 0, and the solution for axial symmetry solved under the limited conditions of H along or perpendicular to z (the principal coordinate of g and A). The perturbation was found to become zero when H is along one of x, y, and z. Therefore, the theory gives the exact solution for the orientations, from which resonance field ESR parameters can be derived.