Gaffney B J, Silverstone H J
National High Magnetic Field Laboratory and Institute for Molecular Biophysics, Florida State University, 1800 East Paul Dirac Drive, Tallahassee, Florida, 32310, USA.
J Magn Reson. 1998 Sep;134(1):57-66. doi: 10.1006/jmre.1998.1526.
Looping transitions occur in field-swept electron magnetic resonance spectra near avoided crossings and involve a single pair of energy levels that are in resonance at two magnetic field strengths, before and after the avoided crossing. When the distance between the two resonances approaches a linewidth, the usual simulation of the spectra, which results from a linear approximation of the dependence of the transition frequency on magnetic field, breaks down. A cubic approximation to the transition frequency, which can be obtained from the two resonance fields and the field-derivatives of the transition frequencies, along with linear (or better) interpolation of the transition-probability factor, restores accurate simulation. The difference is crucial for accurate 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 transferrin oxalate are treated as examples.
循环跃迁发生在扫场电子磁共振谱中靠近避免交叉的位置，涉及一对在避免交叉前后的两个磁场强度下发生共振的能级。当两个共振之间的距离接近线宽时，由跃迁频率对磁场的依赖性的线性近似得出的通常的谱模拟就会失效。跃迁频率的三次近似可以从两个共振场和跃迁频率的场导数获得，再加上跃迁概率因子的线性（或更好的）插值，就能恢复精确的模拟。这种差异对于固定角度下的精确线形至关重要，比如在取向单晶中，但对于粉末谱来说，这种差异在相对强度上的变化较小。以红宝石中的自旋-3/2 Cr³⁺和草酸转铁蛋白中的自旋-5/2 Fe³⁺为例进行了讨论。
