A Fast-Cycling Magnet, 3He Cryostat Instrument for Spin- Polarized EXAFS and X-Ray MCD

我们提出了一种同时用于x射线磁圆二色性（XMCD）和硬x射线自旋极化EXAFS （SPEXAFS）实验的新仪器。1992年，我们的研究小组首次在稀释的顺磁性生物无机样品上展示了软x射线MCD，利用了发热焦球菌红氧还蛋白中的铁中心。使用低于4K的温度和接近6特斯拉的磁场来充分磁化样品。虽然这个实验是一个成功的原理证明，但需要更好的仪器来充分利用这项技术的巨大潜力。在XMCD实验中，比较了磁化样品对左圆极化和右圆极化x射线的吸收。所得的差谱可以揭示x射线吸收元件磁矩的取向。与磁场方向的比较揭示了该物质是铁磁耦合还是反铁磁耦合。在相关的SPEXAFS实验中，利用EXAFS随光子螺旋度的变化来区分相邻原子的自旋取向。因此，两种技术的结合揭示了大量关于局部磁结构的信息。新仪器的目标是使稀释生物无机样品的XMCD和SPEXAFS的灵敏度提高一到两个数量级。我们希望通过快速切换磁场消除单色器和光束的不稳定性来实现这一目标。我们还希望通过更快的探测器捕获更大的立体角来提高荧光光子检测的效率。在与商业供应商协商后，设计了一个1特斯拉超导磁体和~0.4 K 3He低温恒温器，可以实现这一目标。为了进行这些实验，我们将首先在国家同步加速器光源（NSLS）、斯坦福同步加速器辐射实验室（SSRL）和伯克利先进光源（ALS）使用弯曲磁铁束线的圆偏振x射线。在这些初步研究之后，我们将使用ALS目前正在开发的更强大的椭圆摆动源。更好的XMCD和SPEXAFS仪器将允许在许多生物无机系统中表征顺磁性金属团簇。适合研究的混合金属和/或混合价簇包括氮酶P-簇和Fe- Mo辅助因子，氢化酶和CO脱氢酶中的Ni-Fe中心以及光系统II中的Mn簇。为这些问题开发的方法将普遍适用于几乎所有顺磁性过渡金属配合物。

We propose to develop a new instrument for both x-ray magnetic circular dichroism (XMCD) and hard x-ray spin- polarized EXAFS (SPEXAFS) experiments. In 1992, our group was the first to demonstrate soft x-ray MCD on a dilute, paramagnetic bioinorganic sample, using the Fe center in the proteinPyrococcus furiosus rubredoxin. Temperatures below 4K and fields near 6 Tesla were used to fully magnetize the sample. Although this experiment was a successful proof of principal, better instrumentation is needed to fully exploit the tremendous potential of this technique. In the XMCD experiment, the absorption of left- and right- circularly polarized x-rays by a magnetized sample is compared. The resulting difference spectrum can reveal the orientation of the magnetic moment of the x-rav absorbing element. Comparison with the field direction then reveals whether this species is ferromagnetically or antiferromagnetically coupled with respect to the bulk magnetization. In the related SPEXAFS experiment, changes in the EXAFS with photon helicity are used to distinguish the spin-orientation of neighboring atoms. The combination of the two techniques thus reveals a great deal about the local magnetic structure. The goal for the new instrument is to achieve one to two orders of magnitude greater sensitivity for the XMCD and SPEXAFS of dilute bioinorganic samples. We hope to accomplish this primarily by canceling monochromator and beam instabilities with a rapidly switching magnetic field. We also hope to increase the efficiency of fluorescence photon detection by capturing a larger solid angle with a much faster detector. In consultation with commercial vendors, a design with a 1 Tesla superconducting magnet and a ~0.4 K 3He cryostat has been developed that can accomplish this goal. To conduct these experiments, we will initially employ circularity-polarized x-rays from bend magnet beamlines at the National Synchrotron Light Source (NSLS), the Stanford Synchrotron Radia tion Lab (SSRL), and the Advanced Light Source (ALS) in Berkeley. Following these initial studies we will use a more powerful elliptical wiggler source currently under development at the ALS. Better XMCD and SPEXAFS instrumentation will allow characterization of paramagnetic metal clusters in many bioinorganic systems. Mixed-metal and/or mixed-valence clusters appropriate for study include nitrogenase P- clusters and Fe- Mo cofactor, Ni-Fe centers in hydrogenase and CO dehydrogenase, and the photosystem II Mn cluster. The methods developed for these problems will be generally applicable to nearly all paramagnetic transition metal complexes.