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Compelling Science and Synchrotron X-ray Sources By Gabrielle G. Long, Associate Director, Experimental Facilities Division, Advanced Photon Source, Argonne National Laboratory Dr. Long (Ph.D., Polytechnic Institute of Brooklyn, 1972) is an expert in x-ray scattering tech- niques and the microstructure of materials. She is currently associate director at the Advanced Photon Source, one of the nation's most powerful light sources, which is utilized by thousands of biological and physical scientists each year. She is a fellow of the American Physical Society and has been a member of the Materials Research Society's Public Affairs Committee and the Department of Energy's Basic Energy Sciences Advisory Committee. Before joining Argonne, Dr. Long was group leader at the National Institute of Standards and Technology's Center for Neutron Research. She has been named a Maria Goeppert-Mayer Distinguished Scholar. She is also an expert in designing and operating world-class instruments that serve many scientif- ic disciplines simultaneously. H istorically, x-ray research has been propelled by compelling scientific questions and by the push of powerful x-ray source technology. Hand in hand with x-ray source technology are the spectrometers, optics, detectors, and fast electronics that similarly enable the success of the scientific endeavor. In keeping with the sympo- sium's theme,"Instrumentation for a Better Tomorrow," I would like to illustrate the inter- play between important scientific results and the instrumentation that made them possible by selecting some achievements arising from a single technique, x-ray inelastic scattering, from the 1920s until today and speculating on what may become possible with future syn- chrotron sources now on the horizon. As the name implies, x-ray scattering involves x-rays impinging on matter and researchers observing the way they scatter. Elastic x-ray scat- tering occurs when the x-ray photons bounce off the object with no loss of energy; it can be used to characterize the static properties of physical systems. The atomic arrangement within the system can be derived from the angles at which the x-rays reflect, with x-ray crystallography being an important example of elastic x-ray scat- tering. Inelastic x-ray scattering, by contrast, involves observing how much of the x-rays' energy is absorbed and allows probing the dynamics of physical systems to learn about the X-ray crystallography image of the AcrB protein complex, a bacteria excitations that determine physical properties. that repels a wide range of antibiotics. Courtesy of Lawrence Livermore National Laboratory. 30 INSTRUMENTATION FOR A BETTER TOMORROW
Some of the earliest inelastic x-ray scattering experiments led to discoveries of great signifi- cance. In 1929, Compton (inelastic x-ray) scat- tering experiments on beryllium provided the first evidence for the validity of Fermi-Dirac, as opposed to Maxwell-Boltzmann, electron momentum distributions (DuMond, 1929). Since those early days, inelastic x-ray scattering has developed into a probe of collective electron excitations, structural chemistry and resonant excitations, and partial phonon densities of states. From the discovery of x-rays in 1895 by Röntgen until the early 1970s, x-ray sources and x-ray instrumentation changed little. Laboratory x-ray generators accelerated electrons from tungsten filament cathodes toward copper or molybdenum (or other X-ray scattering from ferroelectric stripe domains in a thin film of lead titanate three unit metal) anodes. These 1-kW machines remained cells thick. Courtesy of Argonne National Laboratory. the mainstay of x-ray laboratories until the invention of the rotating anode x-ray genera- tor. By rotating the hot spot of the anode away from the impinging electron beam, the power of laboratory x-ray generators increased from 1 kW to 12 kW and upward. Typically, progress in x-ray sources has been marked by increases of at least one and usu- ally many orders of magnitude in power and intensity. The next important development came in the form of x-ray optics, in which crystal ana- lyzers were bent to collect and focus the x-rays. Research included core electron excitations in low-Z materials. These x-ray Raman experiments (Black, 1990) offer the sensitivity of soft x-rays together with the penetration of hard x-rays. Thus, they deliver otherwise inac- cessible information on the electronic structure from deep within materials rather than from the surface. During the 1970s, first-generation synchrotron x-ray sources, such as the Stanford Synchrotron Radiation Laboratory (SSRL), came into operation. Circulating electrons in the storage ring produced intense x-ray beams that were used for physical measurements. While elementary particle physicists studied the newly discovered J/Y particle, which INSTRUMENTATION FOR A BETTER TOMORROW 31
determined the energy at which the ring was operated, the production of x-rays was ancil- lary. Later, SSRL became a dedicated x-ray source, but we turn our attention now to the second-generation source at Brookhaven, the National Synchrotron Light Source (NSLS). The NSLS was built for the purpose of producing x-ray light. It made use of x-ray beams primarily from bending magnets. The NSLS delivered x-rays six orders of magnitude more intense than x-rays from rotating anodes. Along with the creation of intense x-ray sources, the development of optimized optics and detection methods played an important role in enabling x-ray inelastic scattering experi- ments. The back-scattering geometry and the development of spherically bent crystal analyz- ers (Dorner et al., 1986) contributed greatly to making experiments more efficient, robust, and ultimately practical. Also, as the photon flux increased, so did the demands of the science, from 1 eV to 0.1 eV and, finally, to approximately 1 meV resolution (Burkel, 2000), stimulat- ing the development of a new generation of novel monochromators (Sinn et al., 2001). Three premier synchrotron sources of hard x-rays are now in operation: the European Synchrotron Radiation Facility (ESRF), in France; the Super Photon Ring 8-GeV (or SPring-8), in Japan; and the Advanced Photon Source (APS), in the United States. Third- generation synchrotron x-ray sources primarily make use of radiation from insertion devices placed in straight sections between the bending magnets. These insertion devices--wigglers and undulators--are long devices that deflect the electrons rapidly back and forth and consequently deliver additional photon intensity. The new science coming from the APS depends on its unique beam characteristics. A very high degree of collima- tion makes it possible to efficiently monochromate hard x-ray beams to the approximate- ly 1 meV mentioned above. These beams are used for inelastic x-ray scat- tering studies of lattice dynamics, which until then could be studied only by neutron scattering. Today, inelastic x-ray scattering is used to probe dynamics such as the collective vibra- tions (phonons) in a crystal, valence FIGURE 17 Novel sample environments enable research into matter under extreme conditions within Earth. Inelastic x-ray scattering from levitated (containerless) molten alumina at high temperature is used to gain information on the liquid dynamics within Earth's mantle. The levitated liquid aluminum oxide sample in a supercooled state at approximately 1800°C is shown. 32 INSTRUMENTATION FOR A BETTER TOMORROW
FIGURE 18 (Above left) Nuclear resonant inelastic x-ray scattering from iron and iron compounds at high temperature and high pressure in a diamond anvil cell (Above right) is used to study the geophysics of Earth's core. electrons near the Fermi level, excitations monitored through a nuclear resonance, core and valence electrons, and spin-polarized electrons related to magnetism. Inelastic x-ray scattering now probes materials with unusual non-Fermi-like behavior, such as the high critical temperature (high-Tc) superconductors. New calculation tech- niques are used to describe these materials, which are compared with measurements of the electronic excitation spectrum. Techniques such as angle-resolved photoemission are sur- face- rather than bulk-sensitive and suffer from final state effects and/or sample charging. Inelastic x-ray scattering has none of these limitations and is used successfully to measure charge excitations as a function of incident photon energy. Inelastic x-ray scattering results (Hill et al., 1998) on Nd2CuO4, for example, indicate the presence of a charge-transfer-type excitation involving the oxygen 2p and Cu 3d orbitals; they compare well with cluster cal- culations for the CuO planes in high-Tc materials. In another area, both the spectroscopy and the sample levitation technique were novel. Molten aluminum oxide is of interest for modeling Earth's mantle, for optimizing alu- minum production, and for confining nuclear waste. Kinematic restrictions on neutron scattering make it impossible to reach acoustic modes in liquid oxides, and the high- temperature regime is inaccessible by light scattering because of black-body radiation. Another factor making it difficult to obtain data is the chemical reactivity of aluminum oxide, which melts at about 2327 K and is an extremely aggressive material in the liquid state, precluding the use of traditional containers and forcing the scattering measurements to be performed in a containerless environment. Aluminum spheres 3 mm in diameter INSTRUMENTATION FOR A BETTER TOMORROW 33
were suspended in an oxygen gas jet and heated with a laser to between 2300 and 3100 K (Figure 17). The high momentum-transfer data (Sinn et al., 2003) were well described by kinetic theory, but it was found unexpectedly that the viscosity increased in the hydrody- namic limit, where it had been expected to decrease. The results provide a description of liq- Over the years, uid dynamics in relatively uncharted regions between hydrodynamics and kinetic theory. innovative Nuclear resonance inelastic x-ray scattering (NRIXS) (Sturhahn, 2004) became possible and even practical with the advent of the third-generation sources. One of the ways in developments which a nucleus can reach an excited state is by the absorption of a photon. Such an exci- tation is important when the energy of the photon is very close to the difference between the nuclear ground state and a nuclear excited state. By using samples enriched with in x-ray nuclear isotopes that have large resonant cross sections and special timing techniques, one can perform nuclear resonant inelastic scattering to study lattice dynamics. NRIXS gives sources and access to the partial phonon density of states of the resonant isotope only, thereby deliver- ing unique isotope selectivity (Sturhahn et al., 1995). This has proven important in two instrumentation very different areas: geophysics and biophysics. have greatly In geophysics, nuclear resonant inelastic scattering is providing the phonon density of states in iron and iron compounds at high pressures and high temperatures. The data enlarged the deliver essential information needed to characterize iron deep within Earth (Figure 18). Iron, which is abundant in Earth, transforms from a bcc phase (a-Fe) to an fcc phase (g- Fe) at moderate pressures and temperatures, to an hcp phase (e-Fe) at higher pressures. parameter space With NRIXS, it was possible to measure the phonon density of states directly as a function of pressure and temperature and to compare the thermodynamic and elastic parameters in our physical directly with Debye's model. It was found that g-Fe has lower Debye temperatures and average sound velocities than e-Fe (Shen et al., 2004). world that can Inelastic x-ray scattering measurements are making profound contributions in another be explored. unanticipated area--biophysics. Vibrational spectroscopy probes the structure, dynamics, and reactivity of biological molecules, which has so far been done using resonance Raman, infrared, and femtosecond coherence spectroscopies. Limitations on those methods include selection rules that prevent the observation of many important active-site vibra- tions and difficulties in the assignment of the observed vibrational frequencies with expected normal modes. To meet these challenges, nuclear resonant vibrational spectroscopy reveals the complete vibrational spectrum of a probe nucleus. It can select a single atom from a complex molecule because only the vibrational dynamics of the probe nucleus contribute to the detected signal. It is now being used to identify and characterize 34 INSTRUMENTATION FOR A BETTER TOMORROW
iron-ligand modes at protein active sites (Leu et al., 2004). For heme proteins, these include in-plane iron vibrations that had not been reported in resonance Raman experi- ments and the iron-imidazole stretch that had not been identified in six-coordinate pro- teins. Such identifications are a crucial step toward quantifying the reactive energetics of iron porphyrins and heme proteins. Over the years, innovative developments in x-ray sources and instrumentation have great- ly enlarged the parameter space in our physical world that can be explored. These devel- opments led to a rich interaction between the experimental opportunities and the grand challenges that face many fields of science. Today, we can measure an exceedingly broad range of parameters, from those relevant to the functioning of proteins to parameters rel- evant to geophysics deep within Earth. These developments have also enabled crystal structure determinations from micrometer-sized crystals and have revolutionized protein crystallography, opening the door to the high-throughput determination of protein struc- ture. With the realization of fourth-generation x-ray sources, the peak brightness will be 10 orders of magnitude greater than current synchrotrons, the x-ray light will be coherent, and the pulses will be three orders of magnitude shorter. The possibility of going beyond meV toward meV would open up another regime for inelastic x-ray scattering. REFERENCES Black, D.R. 1990. Ph.D. dissertation, Colorado State University. Burkel, E. 2000. Rep. Prog. Phys. 63: 171-232. Dorner, B., E. Burkel, and J. Peisl. 1986. Nucl. Instrum. Methods A 246: 450-451. DuMond, J.W.M. 1929. Phys. Rev. 33: 643-658. Hill, J.P., C.-C. Kao, W.A.L. Caliebe, M. Matsubara, A. Kotani, J.L. Peng, and R.L. Greene. 1998. Phys. Rev. Lett. 80: 4967-4970. Leu, B.M., M.Z. Zgierski, G.R.A. Wyllie, W.R. Scheidt, W. Sturhahn, E.E. Alp, S.M. Durbin, and J.T. Sage. 2004. J. Am. Chem. Soc. 126: 4211-4227. Shen, G., W. Sturhahn, E.E. Alp, J. Zhao, T.S. Tollner, V.B. Prakapenka, Y. Meng, and H.-R. Mao. 2004. Phys. Chem. Minerals 31: 353-359. Sinn, H., E.E. Alp, J. Barraza, G. Bortel, E. Burkel, D. Shu, W. Sturhahn, J.P. Sutter, T.S. Toellner, and J. Zhao. 2001. Nucl. Inst. Methods A467: 1545-1548. Sinn, H., B. Glorieux, L. Hennet, A. Atalas, M. Hu, E.E. Alp, F.J. Bermejo, D.L. Price, and M.-L. Saboungi. 2003. Science 299: 2047-2049. Sturhahn, W. 2004. J. Phys. Condens. Mat. 16: S497-S530. Sturhahn, W., T.S. Toellner, E.E. Alp, X. Zhang, M. Ando, Y. Yoda, S. Kikuta, M. Seto, C.W. Kimball, and B. Dabrowski. 1995. Phys. Rev. Lett. 74: 3832. INSTRUMENTATION FOR A BETTER TOMORROW 35
