SIMULATION METHODS FOR LOOPING TRANSITIONS

Looping transitions occur in field-swept electron magnetic resonance s pectra near avoided crossings and involve a single pair of energy leve ls that are in resonance at two magnetic field strengths, before and a fter the avoided crossing. When the distance between the two resonance s approaches a linewidth, the usual simulation of the spectra, which r esults from a linear approximation of the dependence of the transition frequency on magnetic field, breaks down. A cubic approximation to th e transition frequency, which can be obtained from the two resonance f ields and the field-derivatives of the transition frequencies, along w ith linear (or better) interpolation of the transition-probability fac tor, restores accurate simulation. The difference is crucial for accur ate line shapes at fixed angles, as in an oriented single crystal, but the difference turns out to be a smaller change in relative intensity for a powder spectrum. Spin-3/2 Cr3+ in ruby and spin-5/2 Fe3+ in tra nsferrin oxalate are treated as examples, (C) 1998 Academic Press.