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.
