Doll Andrin, Qi Mian, Pribitzer Stephan, Wili Nino, Yulikov Maxim, Godt Adelheid, Jeschke Gunnar
Laboratory of Physical Chemistry, ETH Zurich, Vladimir-Prelog-Weg 2, 8093 Zurich, Switzerland.
Phys Chem Chem Phys. 2015 Mar 21;17(11):7334-44. doi: 10.1039/c4cp05893c.
In order to enhance echo signals observed with selective pulses, equilibrium populations of the energy levels of S = 7/2 Gd(III) spin labels are rearranged with frequency-swept passage pulses. To transfer population from as many energy levels as possible, the 2 μs long passage pulses range over more than 1 GHz. Application of this technique at Q-band frequencies to three different Gd(III) complexes and spin dynamics simulations reveal large signal enhancements beyond 100% for Gd(III) complexes with zero-field splitting parameters below 1 GHz. For complexes with larger splittings, experimental enhancements are on the order of 90%. Moreover, population transfer is combined with distance measurements on a model system with a pair of Gd(III) ions. As a result, a signal enhancement of 85% is achieved without inducing changes in the obtained distance information. Besides this enhancement by population transfer, a dipolar modulation depth of 9% is demonstrated, which results in a total enhancement of 3.3 with respect to data obtained with monochromatic rectangular pulses. The limitations of the population transfer technique are discussed. In particular, the extraordinary broad pulse bandwidths caused heating effects and pulse distortions, which constrain the pulse length and thus the achievable signal enhancement.
为了增强用选择性脉冲观测到的回波信号，利用扫频通过脉冲重新排列了S = 7/2 Gd(III)自旋标记能级的平衡布居。为了使尽可能多的能级发生布居转移，2 μs长的通过脉冲扫频范围超过1 GHz。将该技术应用于Q波段频率下的三种不同Gd(III)配合物，并进行自旋动力学模拟，结果表明，对于零场分裂参数低于1 GHz的Gd(III)配合物，信号增强超过100%。对于分裂较大的配合物，实验增强约为90%。此外，在一个含有一对Gd(III)离子的模型系统上，将布居转移与距离测量相结合。结果，在不改变所获得距离信息的情况下，信号增强了85%。除了通过布居转移实现的这种增强外，还证明了9%的偶极调制深度，相对于用单色矩形脉冲获得的数据，这导致了3.3倍的总增强。讨论了布居转移技术的局限性。特别是，极宽的脉冲带宽会引起热效应和脉冲失真，这限制了脉冲长度，从而限制了可实现的信号增强。
