Electron spin echo envelope modulation spectroscopy on orientationally disordered systems: Line shape singularities in S=1/2, I =1/2 spin systems

The electron spin echo envelope modulation of S = 1/2, I = 1/2 spin systems is analyzed for orientationally disordered systems. Analytical expressions are derived for the position of the combination frequencies which can be observed in primary (two-pulse) ESEEM experiments. The conditions for which line shape singularities of the basic and combination frequencies emerge are conveniently mapped in a standard graph. Using this graph it is demonstrated that for virtually all combinations of hyperfine parameters the ESEEM spectra will contain interpretable features. Especially the application of multifrequency techniques (i.e., variation of the nuclear Larmor frequency) will enable full use of the line shape singularities of the combination frequencies.