OTHER DESIGNATIONS: Small-angle x-ray scattering (SAXS), wide-angle x-ray scattering (WAXS), grazing-incidence small-angle x-ray scattering (GISAXS), x-ray Raman scattering, Compton scattering, inelastic x-ray scattering (IXS), resonant inelastic x-ray scattering (RIXS), nuclear resonant scattering (NRS), x-ray photon correlation spectroscopy (XPCS).

PURPOSE: Hard x-ray scattering techniques represent a broad extension of x-ray diffraction methods to the enormous range of systems that are not perfectly ordered or static. Problems addressed include:

• Liquid–vapor, liquid–liquid, and molecular film interfaces
• Colloids, solution-phase proteins, polymers
• Collective dynamics in soft materials
• Short-range order in amorphous materials
• Phonons and elementary excitations in solids
• Electron momentum distribution in solids

HOW THE TECHNIQUE WORKS: Monochromatic x-rays (single wavelength, single energy) impinge upon the sample of interest. The scattered x-rays are detected and their intensities are measured as a function of the angle of scattering. The angular range can be small as in SAXS (low momentum transfer) or wide as in WAXS (high momentum transfer). There are two principal variants of the technique:

1. Elastic. The energy (wavelength) of the detected x-ray is the same as that of the incident x-ray.
2. Inelastic. The energy of the detected x-ray is lower than that of the incident x-ray. The lost energy is carried away by a vibrational, electronic, or magnetic excitation. The detection system in this case requires a spectrometer to measure the energy loss.

UNIQUENESS: The high intensity of synchrotron radiation permits the study of dilute samples where the atoms of interest constitute only one-millionth or less of the total population. The accessible range of momentum transfer is very favorable. Time-resolved SAXS/WAXS studies on systems such as polymer phase formation and mechanical processing are enabled. Tunability is essential for RIXS, a unique probe of atomic-orbital ordering. Highly coherent x-rays are essential for XPCS studies of the dynamics of small particles, information that cannot be obtained in any other way.

EXAMPLES:

Transport Properties of Molten Aluminum Oxide
Direct Evidence of Holons in Strontium Copper Oxide
Model Lipid Membranes Help Demystify Membrane Fusion
Grasping the Structure of Insect Muscle
A Switch in Time — Coherent Control of Pulsed X-Ray Beams

Levitated liquid aluminum oxide sample in a super-cooled state at ~1800° C.

The transport properties (such as viscosity or thermal conductivity) of high-temperature oxide melts are of considerable interest for a variety of applications, including modeling the Earth’s mantle, optimizing aluminum production, confining nuclear waste, and investigating the use of aluminum in aerospace propulsion. However, it is difficult to obtain data on the microscopic transport properties of high-temperature liquid oxides because the chemical reactivity of the material precludes the use of traditional containers. In addition, kinematic restrictions on neutron scattering make it impossible to study acoustic modes of energy absorption, and black-body radiation restricts the utility of (visible) light scattering. Researchers have sought to circumvent these limitations by studying aluminum oxide using high-resolution inelastic x-ray scattering. Molten spheres 3–4 mm in diameter were suspended in an oxygen gas jet, allowing a clear path for the incident and scattered x-ray beams. The spectra obtained when the x-rays were widely scattered (high-Q spectra) were well described by theory, but the spectra in the cases where the x-rays were more narrowly scattered (low-Q spectra) diverged significantly from hydrodynamic theory. The low-Q spectra require a frequency-dependent viscosity and impose previously unknown experimental constraints on the behavior of liquids.

H. Sinn, B. Glorieux, L. Hennet, A. Alatas, M. Hu, E.E. Alp, F.J. Bermejo, D.L. Price, and M.-L. Saboungi, “Microscopic dynamics of liquid aluminum oxide,” Science 299, 2047 (2003).

Energy vs momentum plot (red indicates high spectral intensity). The necklace feature cor- responds to a holon-antiholon spectrum.

High-temperature superconductivity in cuprate materials, colossal magnetoresistance in manganates, and unusually intense nonlinear optical responses in nickelates are all phenomena thought to result from very strong interactions among electrons. According to theory, some of these compounds should also exhibit other unusual physical properties, such as charge fluctuations in the form of exotic quanta called “holons.” To examine this phenomenon, a method sensitive to valence-band excitations must be employed. Researchers used resonant inelastic x-ray scattering (RIXS) to obtain direct evidence of holons in a strontium copper oxide antiferromagnet. In earlier work, the researchers demonstrated that tuning the incident x-ray energy near an absorption edge results in a large enhancement of the valence-band scattering, making possible detailed momentum-resolved studies of high-Z materials. In addition, because of the high brightness of third-generation synchrotrons, scattering studies over the entire Brillouin zone were feasible. Features in the inelastic x-ray scattering spectra indicate a particle-hole excitation and comparison with numerical studies shows good agreement with holon formation. Considered together, these observations are consistent with the presence of spin-charge separation and the formation of holons and antiholons in this cuprate. Future efforts, perhaps feasible at high-brightness facilities, may allow much higher resolution experiments that can capture the full range of spin-charge behavior in these interesting materials.

M.Z. Hasan, Y-D. Chuang, Y. Li, P. Montano, M. Beno, Z. Hussain, H. Eisaki, S. Uchida, T. Gog, and D.M. Casa, “Direct spectroscopic evidence of holons in a quantum antiferromagnetic spin-1/2 chain,” Intl. J. Mod. Phys.B 17, 3479 (2003).

The diffraction pattern (a) from the distorted hexagonal phase suggests that the lipid might have formed distorted tubes (b).

Cellular membrane fusion is one of the most common ways for molecules to enter or exit cells. Understanding the details of membrane fusion may help scientists prevent viruses such as HIV from fusing to and thereby infecting human cells; it may also lead to the design of systems facilitating drug delivery or gene therapy.The primary structural component of a cellular membrane is a double layer of lipid molecules. To understand how structural transitions such as fusion occur, researchers used x-ray diffraction to study the transition from planar lipid layers (lamellar phase) to hexagonally stacked lipid tubes (inverted hexagonal phase). The experiments were performed on model membranes that were made of hundreds of lipid bilayers on a silicon substrate. The composition of the samples was varied along with temperature and water content. The results showed that, when a binary lipid mixture (DOPC/DOPE) was dehydrated, a new lipid structure was found. Though resembling the well-known hexagonal phase structure in which the lipids form circular tubes, the lipids in this new structure appear to form noncircular tubes that stack into a distorted hexagonal lattice. This implies that under the mechanical stress induced by dehydration, the two lipid species in the mixture might demix so as to lower the overall free energy of the system, a mechanism that might be utilized in membrane fusion to lower the free energy barriers in the fusion pathway.

References: L. Yang and H.W. Huang, “Observation of a membrane fusion intermediate structure,” Science 297, 1877 (2002);L. Yang, L. Ding, and H.W. Huang, “New phases of phospholipids and implications to the membrane fusion problem,” Biochemistry 42, 6631 (2003).

Myosin heads in their pre- and post-powerstroke state.

Researchers have achieved the first detailed view of resting muscle filaments poised to contract, a long-sought window into the biochemical cycle that causes muscle contraction. Muscle fibers contain two sets of protein filaments, made of myosin and actin, that “telescope” past each other to stretch or shorten the muscle. The shortening motion is driven by myosin “heads” that bind to, bend against, and then release adjacent actin filaments (a “powerstroke”). Researchers had previously viewed the end state of the powerstroke, but no one had reported the orientation of the unbound heads in myosin’s relaxed initial state until recently. Researchers recorded low-angle x-ray diffraction patterns from flight muscle fibers of giant waterbugs. The results confirmed that resting myosin filaments have stacked rings or “crowns” of eight heads each — two heads per myosin molecule. One head of each myosin projects about 90° from the filament axis; the other tucks inward. Each of the four projecting heads rotates slightly about the “neck,” which connects to a pivot on the myosin filament. The rotation positions the head to bind tightly to the actin filament and tilt forcefully by 45° relative to the axis of the filament. Such a powerstroke would move the head and bound actin filament 10 nanometers down the axis. The four inward-pointing heads each touch an adjacent projecting head. These contacts may restrain both myosin heads from cleaving adenosine triphosphate (ATP) molecules — the powerstroke fuel — until the right moment. As if optimized to do so, the relaxed outward myosin heads seem poised to bind actin and begin powerstroking.

H.A. Al-Khayat, L. Hudson, M.K. Reedy, T.C. Irving, and J.M. Squire, “Myosin head configuration in relaxed insect flight muscle: X-ray modeled resting cross-bridges in a pre-powerstroke state are poised for actin binding,” Biophys. J. 85, 1063 (2003).

Geometry for a subpicosecond x-ray switch.

Tracking changes in molecular structure during chemical and biochemical reactions requires the ability to switch hard x-ray beams on and off rapidly enough to capture the motion of the atoms (a subpicosecond time scale). An elegantly simple and highly adaptable x-ray "switch" uses vibrations in a crystal to modulate the transmission of coherent x-ray pulses. The switch consists of a specially cut and aligned germanium crystal placed in the path of the synchrotron x-ray beam to produce two diffracted beams. The crystal is also coherently excited by a femtosecond laser pulse, resulting in lattice vibrations that transiently rearrange the atoms in the crystal. Inside the crystal, the x-ray beam consists of two standing waves: the α-wave has its nodes on the atomic planes and thus experiences low absorption, while the β-wave has its antinodes on the atomic planes and thus experiences enhanced absorption. This affects the transmission of the incident x-ray beam, redistributing energy between the two exiting beams. By simply varying the time delay between the x-rays and the laser, researchers can switch the exiting beams on and off or change their relative strengths. An even faster switch might result from perturbation of the electron distribution around atoms in the crystal lattice or through the use of optical rather than acoustic modes of lattice vibration (one period is typically about 30 femtoseconds).

M.F. Decamp, D.A. Reis, P.H. Bucksbaum, B. Adams, J.M. Caraher, R. Clarke, C.W.S. Conover, E.M. Dufresne, R. Merlin, V. Stoica, and J.K. Wahlstrand, "Coherent control of pulsed x-ray beams,"Nature 413, 825 (2001).