Embedded electron spin decoherence as indicator of the matrix material structure
In this work the problem of characterizing matrix material structure from embedded electron spin decoherence is studied both theoretically and experimentally. Theoretical calculation using nuclear spin bath model and cluster correlation expansion method shows that the positions of decoherence time scale extremums among single crystal orientations of the matrix material coincide with those of the nearest neighbour proton dipolar couplings. This finding is confirmed by single crystal pulsed EPR experiment performed on $γ$-irradiated malonic acid (MA). Electron spin decoherence decay profile in polycrystalline matrix material is obtained from the orientation dependence as an average over sampled orientations on a Fibonacci grid. In addition, it is pointed out theoretically that a further removal of crystal ordering in the nuclear spin bath can reduce decoherence time scale from the polycrystalline value. This prediction is verified experimentally by the Hahn echo time decay scale in a new amorphous polymorph of MA, obtained for the first time by mechanical milling. Thus the embedded electron spin decoherence can be viewed as a quantitative indicator for studying structures and/or structure changes of the matrix material.