The effect of spin relaxation on ENDOR spectra recorded at high magnetic fields and low temperatures
The effect of spin relaxation on ENDOR spectra recorded at high magnetic fields and low temperatures
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DOI：
10.1006/jmre.2000.2261
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发表时间：
2001-02-01
影响因子：
2.2
通讯作者：
Goldfarb, D
中科院分区：
文献类型：
--
作者：
Epel, B;Pöppl, A;Goldfarb, D
一个简单的理论模型描述了脉冲戴维斯电子-核双共振（ENDOR）实验，其中电子自旋S = 1/2耦合到核自旋I = 1/2，以解释在低温下观察到的不寻常的w波段（95 GHz） ENDOR效应。该模型考虑了四能级系统的热极化和所有内部弛豫过程，分别由电子和核自旋弛豫时间T-1e和T-1n以及交叉弛豫时间T-1x表示。结果表明,足够高的条件下热自旋极化,原子核可以表现出不对称的恩多光谱两种情况:第一个当t(混合)远远大于T-1e T-1n,比T-1e T-1x大得多,在恩多信号从一个歧管-和β多方面的积极,第二当十字架——比重复和/或核弛豫时间长时间(t(混合)远低于T-1e远低于t (R)和T-1n T-1x > t (R))。在这种情况下，ENDOR信号的极化与前一种情况相反，a流形中的线为正，而β流形中的线为负。这种情况更有可能在实验中遇到，因为它不需要很长的混合时间，而且是核跃迁饱和的结果。利用该模型再现了[Cu（咪唑）(4)]Cl-2 w波段H-1 ENDOR振幅的实验t（mix）和t(R)依赖关系，并确定了t -1e和t -1x的值远大于t -1e。在ENDOR光谱中不对称的存在是有用的，因为它直接提供了超精细耦合的标志。所提出的模型允许实验人员调整实验参数，如t（mix）和t(R)，以优化所需的光谱外观。(C) 2001学术出版社。
A simple theoretical model that describes the pulsed Davies electron-nuclear double resonance (ENDOR) experiment for an electron spin S = 1/2 coupled to a nuclear spin I = 1/2 was developed to account for unusual W-band (95 GHz) ENDOR effects observed at low temperatures. This model takes into account the thermal polarization along with all internal relaxation processes in a four-level system represented by the electron- and nuclear-spin relaxation times T-1e and T-1n, respectively, and the cross-relaxation time, T-1x. It is shown that under conditions of sufficiently high thermal spin polarization, nuclei can exhibit asymmetric ENDOR spectra in two cases: the first when t(mix) much greater than T-1e and T-1n, T-1x much greater than T-1e, where ENDOR signals from the a manifold are negative and those of the beta manifold positive, and the second when the cross- and/or nuclear-relaxation times are longer than the repetition time (t(mix) much less than T-1e much less than t(R) and T-1n, T-1x > t(R)). In that case the polarization of the ENDOR signals becomes opposite to the previous case, the lines in the a manifolds are positive, and those of the beta manifold are negative. This case is more likely to be encountered experimentally because it does not require a very long mixing time and is a consequence of the saturation of the nuclear transitions. Using this model the experimental t(mix) and t(R) dependencies of the W-band H-1 ENDOR amplitudes of [Cu(imidazole)(4)]Cl-2 were reproduced and the values of T-1e and T-1x much greater than T-1e were determined. The presence of asymmetry in the ENDOR spectrum is useful as it directly provides the sign of the hyperfine coupling. The presented model allows the experimentalist to adjust experimental parameters, such as t(mix) and t(R), in order to optimize the desired appearance of the spectrum. (C) 2001 Academic Press.