ETH Zürich, Laboratory of Physical Chemistry, 8093 Zürich, Switzerland.
J Magn Reson. 2012 Oct;223:187-97. doi: 10.1016/j.jmr.2012.08.005. Epub 2012 Aug 15.
Two-pulse electron spin echo envelope modulation (ESEEM) line widths are influenced by transverse electron spin relaxation, which is in turn induced by local field fluctuations. Simultaneous analysis of the decays of the unmodulated and modulated parts of the ESEEM signal provides deeper insight into the relaxation of a spin system consisting of an electron spin 1/2 coupled to N(I) nuclei with spin 1/2. Standard two-pulse ESEEM formulas either do not account for relaxation or assume uniform relaxation for all lines. In general, the relaxation rates on allowed and forbidden transitions may not be the same. Experimental results obtained on a single crystal of Cu(II)-doped L-histidine suggest that such a difference exists. Theoretical considerations show that in such a case the product rule for two-pulse ESEEM does not extend to expressions including relaxation. Product rules in general do not properly account for relaxation in three-pulse ESEEM and HYSCORE experiments. Decay of the apparently non-oscillatory part of the two-pulse echo may be strongly affected by modulation interference. Such interference of difference frequencies of matrix nuclei may cause a rather flat initial feature, which was previously attributed solely to non-exponential phase relaxation of electron spin transitions due to spin diffusion of the matrix nuclei. In addition, the sometimes observed drastic initial decay of the time domain signal is related to modulation interference of multiple-quantum coherences that arise from a strong cross-suppression effect.
双脉冲电子自旋回波包络调制（ESEEM）线宽受横向电子自旋弛豫的影响，而横向电子自旋弛豫又是由局部场波动引起的。对 ESEEM 信号未调制和调制部分的衰减进行同时分析，可以更深入地了解由电子自旋 1/2 与自旋 1/2 的 N(I)核耦合组成的自旋系统的弛豫情况。标准的双脉冲 ESEEM 公式要么不考虑弛豫，要么假设所有线的弛豫都是均匀的。一般来说，允许跃迁和禁止跃迁的弛豫速率可能不一样。在 Cu(II)掺杂 L-组氨酸单晶上获得的实验结果表明存在这种差异。理论考虑表明，在这种情况下，双脉冲 ESEEM 的乘积定则不适用于包括弛豫的表达式。一般来说，三脉冲 ESEEM 和 HYSCORE 实验中的乘积定则不能正确考虑弛豫。双脉冲回波中显然非振荡部分的衰减可能会受到调制干扰的强烈影响。这种基质核差频的干扰可能会导致初始特征相当平坦，这以前归因于由于基质核的自旋扩散而导致的电子自旋跃迁的非指数相位弛豫。此外，有时观察到的时域信号的剧烈初始衰减与多量子相干的调制干扰有关，这是由于强烈的交叉抑制效应引起的。
