Reported here are measurements of the penetration depth and spatial distribution of photoelectron (PE) damage excited by 18.6 keV X-ray photons in a lysozyme crystal with a vertical submicrometre line-focus beam of 0.7 µm full-width half-maximum (FWHM). The experimental results determined that the penetration depth of PEs is 5 ± 0.5 µm with a monotonically decreasing spatial distribution shape, resulting in mitigation of diffraction signal damage. This does not agree with previous theoretical predication that the mitigation of damage requires a peak of damage outside the focus. A new improved calculation provides some qualitative agreement with the experimental results, but significant errors still remain. The mitigation of radiation damage by line focusing was measured experimentally by comparing the damage in the X-ray-irradiated regions of the submicrometre focus with the large-beam case under conditions of equal exposure and equal volumes of the protein crystal, and a mitigation factor of 4.4 ± 0.4 was determined. The mitigation of radiation damage is caused by spatial separation of the dominant PE radiation-damage component from the crystal region of the line-focus beam that contributes the diffraction signal. The diffraction signal is generated by coherent scattering of incident X-rays (which introduces no damage), while the overwhelming proportion of damage is caused by PE emission as X-ray photons are absorbed.

X-ray diffraction and vanadium x-ray absorption near-edge structure (XANES) data have been obtained for (V1-xCrx)(2)O-3 samples containing several concentrations of Cr, crossing the metal-insulator transition boundary. For single-phase single-crystal samples our theoretical results are generally in good qualitative agreement with our experimental single-crystal XANES, for both crystal orientations relative to the incident-beam electric vector. However, an anomalous peak occurs for both orientations in the K pre-edge of the single-crystal sample containing 1.2% Cr, a paramagnetic insulator sample that is in the concentration regime corresponding to the room-temperature two-phase (coexistence) region of the phase diagram. Upon increasing the temperature of the 0.4% Cr powdered material to 400 K so that one enters the two-phase region of the phase diagram, a similar peak appears and then diminishes at 600 K. These results, as well as experiments done by others involving room-temperature and low-temperature XANES of a 1.1% Cr sample, suggest that this feature in the V pre-edge structure is associated with the appearance under some circumstances of a small amount of highly distorted VO6 octahedra in the interface region between coexisting metal and insulating phases. Finally, we find that, for the two-phase regime, the concentration ratio of the metal-to-insulating phase varies between different regions from a sample batch of uniform composition made by the skull melting method.

Recently, strategies to reduce primary radiation damage have been proposed which depend on focusing X-rays to dimensions smaller than the penetration depth of excited photoelectrons. For a line focus as used here the penetration depth is the maximum distance from the irradiated region along the X-ray polarization direction that the photoelectrons penetrate. Reported here are measurements of the penetration depth and distribution of photoelectron damage excited by 18.6 keV photons in a lysozyme crystal. The experimental results showed that the penetration depth of ~17.35 keV photoelectrons is 1.5 ± 0.2 µm, which is well below previous theoretical estimates of 2.8 µm. Such a small penetration depth raises challenging technical issues in mitigating damage by line-focus mini-beams. The optimum requirements to reduce damage in large crystals by a factor of 2.0-2.5 are Gaussian line-focus mini-beams with a root-mean-square width of 0.2 µm and a distance between lines of 2.0 µm. The use of higher energy X-rays (> 26 keV) would help to alleviate some of these requirements by more than doubling the penetration depth. It was found that the X-ray dose has a significant contribution from the crystal's solvent, which initially contained 9.0%(w/v) NaCl. The 15.8 keV photoelectrons of the Cl atoms and their accompanying 2.8 keV local dose from the decay of the resulting excited atoms more than doubles the dose deposited in the X-ray-irradiated region because of the much greater cross-section and higher energy of the excited atom, degrading the mitigation of radiation damage from 2.5 to 2.0. Eliminating heavier atoms from the solvent and data collection far from heavy-atom absorption edges will significantly improve the mitigation of damage by line-focus mini-beams.

A new strategy is presented to reduce primary X-ray damage in macromolecular crystallography. The strategy is based on separating the diffracting and damaged regions as much as feasible. The source of the radiation damage to macromolecular crystals is from two primary mechanisms: the direct excitations of electrons by absorption, and inelastic scattering of the X-rays. The first produces photoelectrons with their accompanying Auger electrons from relaxation of the core hole and the second creates Compton electrons. The properties of these two mechanisms and calculations of primary X-ray damage quantify how to modify the spatial distribution of X-rays to reduce the deleterious effects of radiation damage. By focusing the incident X-rays into vertical stripes, it is estimated that the survival (the time during which quality diffraction data can be obtained with a given X-ray flux) of large crystals can be increased by at least a factor of 1.6, while for very small platelet crystals the survival can be increased by up to a factor of 14.

Laser pump/XAFS probe study of Ge using the high efficiency facility at PNC/XOR CAT at sector 20 of the Advanced Photon Source (APS) has discovered some surprising results of how the excited electrons/hole (e/h) decay. The 200 femtosecond (fs) pulse laser is triggered at the frequency of the APS ring so as to use the x-rays of one pulse of the 24 singlet mode at 100% efficiency. The higher efficiency of the use of the ring's x-rays (about two orders more efficient than usual) allowed the measurement of XAFS spectra at a large number of delay times between laser pump and x-ray probe and at different laser powers. The XAFS data determined the time dependence of the relative distances and their vibration amplitudes of the first and second Ge neighbors. The laser pulse excites a classical long wavelength optical mode that within 0.1 picoseconds (ps) or so excites only the vibrations of the 1st neighbor while to excite the second neighbor requires decay of the excited e/h through coupling with incoherent phonons. The surprising result is that this decay is delayed 11 ps, independent of the laser power as it is increased by a factor of two.

Ti K and Ti L-2,L-3 x-ray absorption fine-structure near-edge spectra of SrTiO3 thin films grown coherently on Si(001) reveal the presence of a ferroelectric (FE) distortion at room temperature. This unique phase is a direct consequence of the compressive biaxial strain achieved by coherent epitaxial growth.

We report x‐ray absorption near edge structure (XANES) measurements of four closely related perovskite materials: SrTiO3, CaTiO3, CaZrO3, and SrZrO3. This data is used to address the conceptually important, early EXAFS experiment of Perel and Deslattes. That experiment attempted to distinguish between the then‐competing short‐range and long‐range theories of EXAFS by cross‐material comparison of the EXAFS for the metal ions in the four perovskites reported here. Their inconclusive result is surprising, given the modern understanding of EXAFS. Our new measurements show strong disagreements with the prior results at multiple edges. When analyzed in qualitative, conceptual framework of the original study, our new results are in strong agreement with the short‐range order theory. This solves a historical puzzle in the early scientific development of x‐ray absorption spectroscopy.

Laser-pump/x-ray-probe measurements were made on Ge films with nominal time-delay steps of 18 ps. The response of the Ge lattice to the excitation of a high efficiency 200 fs pulse laser operating at 800 rim wavelength and at the Advanced Photon Source (APS) ring frequency of 272 kHz is probed by x-ray absorption fine structure (XAFS) measurements which determine the time dependences of the local structure. Initially the lattice response is dominated by a large increase of the RMS nearest neighbor bond disorder that decays within a single delay step. The next nearest neighbor RMS disorder shows an increase delayed by similar to 30 ps, consistent with only optical modes dominating the initial lattice response and other phonons being excited more slowly. The different rate of excitation of optical than the rest of phonons require a different mechanism for exciting the optical phonons. Because the laser excites electrons from binding to anti-binding states, a Franck-Condon like effect is suggested as the cause of this initial response, instead of the standard hot electron-hole coupling to phonons.

Edward A. Stern, RN,is an Informatics Nurse and Lead Consultant for NothingBetter Healthcare Solutions, Kingstowne, VA.

Experiments have shown that the transition temperature of 4-75-unit-cell-thick PbTiO3 films grown on an insulating SrTiO3 substrate range between 550 and 980 K. At high temperatures (below T-c), the films are in a 180 degrees stripe domain state, polarized perpendicular to the surface, and transform at about room temperature to a single macroscopic domain. This transition is curious because the strong depolarizing field is expected to quench the spontaneous polarization. Indeed, a 10-unit-cell-thick BaTiO3 film grown on the same substrate was found to remain in the paraelectric state even at room temperature. To understand these phenomena, we present a free energy model based on the bulk perovskite ferroelectricity model that we have previously developed. The model takes into account two interacting order parameters, the average spontaneous local off-center displacements (pseudospins) and the condensed soft mode. The model shows that at high temperatures the PbTiO3 films should be in the stripe domain state and predicts in reasonable quantitative agreement with experiment the stripe period as a function of temperature and film thickness. It further predicts that at about room temperature the films would transform into a single domain state with vanishing electrical polarization but with large ionic displacements. Finally, the model explains why the properties of PbTiO3 and BaTiO3 are so different from each other.

Local atomic and crystal structures around Cu and Mn atoms in Mn1.68Cu0.6Ni0.48CO0.24O4 spinel samples fabricated by metal-organic decomposition synthesis at different annealing temperatures were investigated by X-ray absorption fine structure analysis. There are two distinct copper cations, Cu1+ and Cu2+, both of which maintain tetrahedral coordination. The bond-length distances are Cu1+-O = 2.00 A and Cu2+-O = 1.80 A. The manganese cations are for the most part octahedral. The spinels prepared at low temperature (600 degrees C) contain smaller (Mn4+-O = 1.88 A) undistorted MnO6 octahedrons corresponding to Mn4+ valence, whereas the manganese octahedrons in high-temperature materials (800 degrees C and higher) were larger and had a pronounced tetragonal distortion pertaining to Mn3+ oxidation state (Mn3+-O = 1.93 A and 2.11 A). By rising the fabrication temperatures, relative concentration of the species of Mn4+ and Mn3+ varies as a result of the reaction represented by Cu1+ + Mn4+ => Cu2+ + Mn3+, implying irreversible temperature-induced structural transformation. Atomic coordinates in the low-temperature phase are similar to those found in the ideal cubic spine] with oxygen parameter u = 0.27, whereas local environments of the Cu and Mn atom correspond to the tetragonal CuMn2O4 phase (space group J4(1)/atnd). Unlike in CuMn2O4, orientation of the lattice distortions is random, however, the long-range cubic spinel structure is retained at all time. (c) 2005 Elsevier B.V. All rights reserved.

Structural transformations around both V and Cr atoms in (V1-xCrx)2O3 across its metal-insulator transition (MIT) at x approximately 0.01 are studied by extended x-ray absorption fine-structure technique. Our new results for Cr made possible by the use of a novel x-ray analyzer that we developed reveal the substitutional mechanism of Cr doping. We find that this system has a buckled structure with short Cr-V and long V-V bonds. This system of bonds is disordered around the average trigonal lattice ascertained by x-ray diffraction. Such local distortions can result in a long range strain field that sets in around dilute Cr atoms in microscopic regions. We suggest that such locally strained regions should be insulating even at small x. The possibility of local insulating regions within a metallic phase, first suggested by Rice and Brinkman in 1972, remains unaccounted for in modern MIT theories.

Diffuse x-ray scattering measurements on the cubic paraelectric phase of single-crystal PbTiO3 are presented. No diffuse scattering sheets are found, in contrast to sheets observed in the cubic paraelectric phases of BaTiO3 and KNbO3. A quantitative analysis of the diffuse scattering indicates that the soft-mode branch contribution to sheets is negligible in all three ferroelectric perovskites. Differences in the diffuse scattering are explained by differences in the disordering of the local displacements, thus resolving a long-standing controversy.

In order to better understand ferroelectricity in thin films, it is important to explore the atomic-scale structure and the spatial distribution of polarization near the interfaces. We present sub-Angstrom-resolution electron density maps of three ultrathin PbTiO3 films grown epitaxially on SrTiO3 (001) substrates. The maps were obtained by analysis of synchrotron x-ray scattering measurements of Bragg rod intensities using the recently developed coherent Bragg rod analysis method. A four- and a nine-unit-cell-thick film were studied at room temperature, and a nine-unit-cell-thick film was studied at 181 degrees C. The results show that at room temperature, the PbTiO3 films are polar, monodomain, and have their polarization oriented away from the substrate. The four-unit-cell film may be the thinnest monodomain perovskite film found to be in the polar phase. At 181 degrees C, the electron density map of the nine-unit-cell film is consistent with the presence of 180 degrees stripe domains. In the monodomain samples, details of the atomic-scale structure of the PbTiO3/SrTiO3 interface are observed, which may provide evidence for the nature of the positive charge layer required to stabilize polarization in monodomain films.

A facility at PNC-CAT in the Advanced Photon Source measures with sub-nanosecond time resolution both XAFS and diffraction on femtosecond laser-excited samples. XAFS measures with relatively high efficiency the time for the laser excitation to couple to the lattice, the sample temperature after reaching thermal equilibrium, any ablation of the sample with time, and, in many cases can distinguish between the amorphous and crystalline states. Preliminary measurements on 200 nm thick polycrystal Ge films indicate that the time for transferring the laser excitation to thermal heating of the lattice is less than 2 nanoseconds when the initial temperature is 560 K.

Synchrotron-based laser pump/x-ray probe experiments on a ns or faster time scale have typically used ultrafast lasers suffering from the limitation of low repetition rates on the order of 1-10 kHz. This severely limits the data collection efficiency, since the x-ray bunches repeat at a much higher rate. This is particularly critical for XAFS experiments, which require very large S/N ratios (similar to 10(4) or better) to extract useful EXAFS far above the absorption edge. PNC-CAT has developed an apparatus based on a laser repeating at the rate of the APS bunch repetition frequency of 272 kHz, allowing an increase in data collection efficiency by two orders of magnitude. We have used this apparatus to obtain preliminary results for the time-resolved EXAFS from a 200nm thick polycrystalline Ge film which has been heated by the laser.

We present Generalized Diffractometer Control (gdc), a diffractometer‐control software package developed specifically for high‐precision measurements of Bragg rods; we discuss its features and analyze its performance in data collection. gdc, implemented at several APS beamlines, controls a six‐circle diffractometer in either Eulerian or kappa geometry, yet does not assume a mechanically ideal diffractometer; instead, the measured directions of the diffractometer axes (and the direction of the incident beam) are input parameters. The Labview‐based program features a graphical interface, making it straightforward to find all the commands and operations. Other features include optimized scans along Bragg rods, straightforward background subtraction, and extensive sets of pseudomotors.