ESEEM of Orientationally Disordered Systems (II). Lineshape Singularities in S = 1 2, I = 1 Systems with Small Quadrupole Interaction

The electron spin-echo envelope-modulation (ESEEM) spectra of S = 1 2, I = 1 systems with small quadrupole interaction (for instance, deuterium nuclei ²D) are analyzed for orientationally disordered systems. It is shown that the commonly accepted description of deuterium ESEEM spectra using theory developed for the S = 1 2, I = 1 2 case is valid only in the limit of small hyperfine (HFI) and nuclear quadrupole interactions (NQI) relative to the nuclear Zeeman interaction. For larger HFI, apart from the "simple" basic (ν_α and ν_β) and combination (ν_α ± ν_β) features, the "higher" basic (2ν_α and 2ν_β) and combination [2ν_α(β) ± ν_β(α), 2ν_α ± 2ν_β] harmonics appear in the spectra for I = 1. Analytical expressions are derived that define the positions of the higher combination harmonics. The presence of small NQI will complicate the basic and combination lineshapes, giving rise to line broadenings and/or to observable line splittings. These, in turn, can be interpreted in terms of the relative angle between the principal axes of the HFI and NQI tenser. The approach developed is demonstrated on the example of silver atoms stabilized in γ-irradiated AgNO₃/D₂O matrices.