Powder magnetic resonance spectra in the presence of planar rotational jumps

The effect on magnetic resonance line shapes of powder due to inplane jumps and planar rotational diffusion is discussed. Two approaches are used to calculate the line shapes: (i) A group theoretical method developed previously is applied to describe the effect of discrete jumps about a principal axis of a nonaxial Hamiltonian. (ii) A formalism is developed which considers a continuous range of jump angles, distributed about a preferred angle. This formalism reduces to the discrete jump case or the planar diffusion case in the proper limits. Numerical results for some specific cases in which the jump process involves rotations by π/2 and π/3 are presented. It is shown that the discrete jumps lead in the intermediate rate region to the appearance of conspicuous features in the spectra typical of the jump process. When the jump angles are diffused these peaks broaden out and disappear completely in the rotational diffusion limit. Experimental results on the radical AsO4−4 in powder samples of KH2AsO4 which exhibit these predicted features are presented.