THE COMBINATION HARMONICS IN 4-PULSE ELECTRON-SPIN-ECHO ENVELOPE MODULATION OF A S=1/2, I=5/2 SPIN SYSTEM UNDER ORIENTATIONAL DISORDER

The general expression for the echo intensity generated by the recently designed four-pulse electron-spin-echo envelope modulation experiment was derived, and a detailed analysis of this expression in terms of the relative contributions of the various terms to the electron-nuclear double-resonance frequencies and their combinations was performed. The terms contributing to the echo intensity were grouped according to the total number of generated EPR coherences in order to select those responsible for the combination harmonics. The latter are important since in orientationally disordered systems they exhibit better resolution than in the basic frequencies, and they can be used to determine the anisotropic hyperfine interaction and the nuclear quadrupole interaction. When the nuclear Zeeman interaction dominates the nuclear spin Hamiltonian, a significant number of terms in the final expression for the echo intensity have negligible contributions to the echo intensity, and they can be omitted to reduce computing time. A specific analysis was performed for the spin system S = 1/2, I = 5/2 in orientationally disordered samples with a large g anisotropy, which enables orientation-selective experiments. The lineshape and resolution of the combination peaks were explored through variations of the relative orientations of the g, hyperfine, and nuclear quadrupole tensors and the orientation of the external magnetic field. It is demonstrated that although the combination peaks are often dominated by the sum-combination harmonic corresponding to the nuclear \1/2]-\-1/2] transition, under several conditions of orientation selectivity it is possible to obtain combination lines split to five lines from which the quadrupolar interaction can be determined. (C) 1994 Academic Press, Inc.