ESR of the quartet states of randomly oriented molecules: Calculation of the line shape and detection of the zero-field splitting

The ESR spectrum of the quartet states of randomly oriented molecules with axial symmetry is discussed. The absorption curves of the Δm = 1, Δm = 2, and Δm = 3 transitions are calculated in first order perturbation approximation. The line broadening is explicitly taken into account. The experimental spectrum of an alternant hydrocarbon agrees sufficiently well with the theoretical predictions. The shape of the absorption curve permits the ready determination of the zero-field parameters of quartet molecules.