Theory of exchange splitting in a strong magnetic field. I. General formulation

A quantum statistical mechanical theory of ESR lineshapes is formulated for spin systems with exchange splitting. The theory is valid even for systems under the influence of strong magnetic fields and at low temperatures. A simple model Hamiltonian is deduced from a microscopic point of view. The magnetic susceptibility for these systems is also reduced to a tractable form which gives a firm basis for the analyses of related experiments.