ESEEM OF ORIENTATIONALLY DISORDERED-SYSTEMS .2. 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, deuter ium nuclei D-2) are analyzed for orientationally disordered systems. I t is shown that the commonly accepted description of deuterium ESEEM s pectra using theory developed for the S = 1/2, I = 1/2 case is valid o nly in the limit of small hyperfine (HFI) and nuclear quadrupole inter actions (NQI) relative to the nuclear Zeeman interaction. For larger H FI, apart from the ''simple'' basic (nu(alpha) and nu(beta)) and combi nation (nu(alpha) +/- nu(beta)) features, the ''higher'' basic (2 nu(a lpha) and 2 nu(beta)) and combination [2 nu(alpha(beta)) +/- nu(beta(a lpha)), 2 nu(alpha) +/- 2 nu(beta)] harmonics appear in the spectra fo r I = 1. Analytical expressions are derived that define the positions of the higher combination harmonics. The presence of small NQI will co mplicate the basic and combination lineshapes, giving rise to line bro adenings 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 t he example of silver atoms stabilized in gamma-irradiated AgNO3/D2O ma trices. (C) 1995 Academic Press, Inc.