The effect of Hartmann-Hahn mismatching on polarization inversion spin exchange at the magic angle
The effect of Hartmann-Hahn mismatching on polarization inversion spin exchange at the magic angle
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DOI：
10.1016/s1090-7807(02)00036-8
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发表时间：
2002-12-01
影响因子：
2.2
通讯作者：
Cross, TA
中科院分区：
文献类型：
--
作者：
Fu, RQ;Tian, CL;Cross, TA
研究了在幻角极化反转自旋交换(PISEMA)过程中Hartmann-Hahn失配Delta=omega(Jeff)-omega(1s)的影响，其中omega(Jeff)和omega(1s)分别表示在幻角和N-15 RF自旋锁定场下H-1有效自旋锁定场的幅度。在PISEMA演化过程中，精确的哈特曼-哈恩匹配条件(即A=0.816)对PISEMA实验产生了最大偶极标度因子0.816，而任何失配都会在每个频率切换Lee-Goldburg周期的前半部分和后半部分产生两个不同的有效场。失配对标度因子的影响强烈地依赖于在每个FSLG周期内从一个有效场到另一个有效场的跃迁角度以及周期时间。在FSLG周期时间较长的低RF自旋锁定幅度下，随着omega(1s)大于omega(Jeff)，标度因子迅速变小。另一方面，当omega(1s)<；ω(Ef).Δ的变化对比例因子的影响相对较小。因此，由于失配效应，RF不均匀的存在可能显著地加宽偶极维度中的线宽。较高的射频自旋锁定幅度使得标度因子的变化相对较小，此外，与FSLG序列的触发器同步的N-15 RF自旋锁定场的倾斜幅度使FSLG周期内两个有效场之间的跃迁角度最小化。实验表明，这种倾斜幅度不仅产生相同的标度因子，而且与矩形振幅相比，导致更窄的偶极线宽。(C)2002年埃尔塞维尔科学公司(美国)。版权所有。
The effect of the Hartmann-Hahn mismatch Delta = omega(eff) - omega(1S) during polarization inversion spin exchange at the magic angle (PISEMA) has been investigated, where omega(eff) and omega(1S) represent the amplitudes of the H-1 effective spin-locking field at the magic angle and the N-15 RF spin-locking field, respectively. During the PISEMA evolution period, the exact Hartmann-Hahn match condition (i.e., A = 0) yields a maximum dipolar scaling factor of 0.816 for PISEMA experiments, while any mismatch results in two different effective fields for the first and second half of each frequency switched Lee-Goldburg (FSLG) cycle. The mismatch effect on the scaling factor depends strongly on the transition angle from one effective field to the other within each FSLG cycle as well as on the cycle time. At low RF spin-lock amplitudes in which the FSLG cycle time is relatively long, the scaling factor rapidly becomes smaller as omega(1S) becomes greater than omega(eff). On the other hand, when omega(1S) < ω(eff). there is relatively little effect on the scaling factor with variation in Δ. As a result, the presence of RF inhomogeneities may significantly broaden the line-width in the dipolar dimension because of the mismatch effect. Higher RF spin-lock amplitudes result in a relatively small variation for the scaling factor, Furthermore, ramped amplitude of the N-15 RF spin-lock field in synchronization with the flip-flop of the FSLG sequence minimizes the transition angle between the two effective fields within the FSLG cycle, It is shown experimentally that such a ramped amplitude not only gives rise to the same scaling factor but also results in a narrower dipolar line-width in comparison with the rectangular amplitude. (C) 2002 Elsevier Science (USA). All rights reserved.