X-ray Detection on Fe-H Vibrations
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1262-W07-10
X-ray Detection on Fe-H Vibrations Hongxin Wang,1,2 Yisong Guo,1 Saeed Kamali,1 and Stephen P. Cramer1,2 1 2
Department of Applied Science, University of California, Davis, CA 95616, USA Lawrence Berkeley National Lab, Berkeley, CA 94720, USA
ABSTRACT X-ray detection on hydrogen related events is difficult due to its extremely small scattering factor. In this report, we have used nuclear resonance vibrational spectroscopy (NRVS) to examine the nature of the Fe–H vibrational modes in several FeH model complexes, which shines light on the possible measurements on the Fe-H vibrations inside real biological systems in the future. INTRODUCTION Hydrogenase catalyzes the reversible dihydrogen (H2) production and oxidation [1,2], and is one of the most important enzymes in nature. The enzyme and/or its functional models are possible candidates for bio-hydrogen production as clean fuel in the future [3]. For NiFe hydrogenases, Ni-A (as-isolated state) has a oxo or hydroxo ligand bridging in between the Ni and the Fe inside the NiFe center, while Ni-R is proposed to have a hydride in the same position instead [1]. In the fields of energy and materials science, iron, iron oxide and other iron complexes are important materials used for solar hydrogen production [4] and hydrogen storage [5]. For these materials and their applications, the interaction between Fe and H is highly interested as well. While X-ray spectroscopy is good for its element specificity, its detection on light elements, especially on hydrogen, is usually very difficult. Therefore no direct X-ray observation has been reported so far for the Fe-H interactions mentioned above. Nuclear resonance vibrational spectroscopy (NRVS) measures the vibrational modes coupled with the 57Fe nuclear transition at 14.4 KeV. It is an element and isotope specific measurement. In this report, we will illustrate how to use the method to probe the Fe-H vibrational modes in various FeH model complexes, which will serve as the first step towards the Fe-H measurements for real biological systems and/or other complicated FeH systems in the future. EXPERIMENTS The NRVS spectra were recorded with a published procedure [6,7] at APS (ID03) in the USA and at SPring-8 (BL09XU) in Japan. At APS, as shown in figure 1, the X-ray beam from the synchrotron radiation storage ring went through a high heat load monochromator (pre-mono, C(1,1,1)xC(1,1,1)) to produce 14.4 KeV radiation with 1 eV in energy resolution and then a high-resolution monochromator (HRM, 2Si(10,6,4)x2Si(4,0,0)) to produce 14.4 KeV radiation with 1 meV (8 cm-1) resolution. At SPring-8, the pre-mono (Si(1,1,1)xSi(1,1,1)) produced 14.4KeV radiation with 1.5 eV resolution, while and the HRM (Ge(4,2,2)x2Si(9,7,5)) produced 14.4KeV radiation with 1.2 meV resolution.
During all NRVS measurements, the samples were maintained at a low temperature using a liquid helium cryostat. The real sample temperatures were calculated [6] using the ratio of antistokes to stokes intensities according to S(-E)/S(E)=exp(-E/kT) [6]. Th
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