Search

We present Kapitza conductance measurements of the bismuth/sapphire interface using depth- and time-resolved X-ray diffraction, for Bi film thicknesses ranging from 65 to 284 nm. Our measurements provide complementary information about heat transport in the films; we directly observe the thinnest film to be uniformly heated within 1 ns, whereas the thickest film sustains a large near-surface temperature gradient for several ns. The deduced Kapitza conductance is 1950 W/cm2/K. This value is close to the theoretical prediction using the radiation limit.

Solidification of single-crystal nickel-base superalloys introduces large-scale segregation of constituent elements and defects such as dislocations and mosaicity. By exploiting the energy tunability and interference capabilities of high-brilliance X-ray radiation, key structural features of the dendritic single crystals were mapped over large areas. Interference and diffraction of synchrotron X-rays revealed significant misorientations between individual dendrites in the as-solidified state. For the first time this mosaic structure was quantified for an array of dendrites and correlated with the density of “grown-in” dislocations whose density ranged from 107 to 108 cm−2. Absorption contrast permitted simultaneous mapping of the distribution of refractory metal additives (e.g. rhenium and tungsten), which segregated preferentially to the dendrite cores with a linear composition gradient toward the interdendritic regions. The results demonstrate that synchrotron X-ray imaging is promising for in situ studies of single-crystal structure and defects in nickel-base superalloys.

A time-resolved X-ray microprobe to study optical strong-field processes has been developed. Individual atoms or molecules located within the strong-field environment created by a focused ultrafast laser are probed by undulator-produced X-ray pulses to achieve spatial, temporal, spectral and polarization selectivity. Approximately 10 6 monochromatic X-rays per 100-ps pulse are focused into a ∼10 μm spot to selectively probe atoms in focal volumes where intensities up to 10 15 W/cm 2 can be present. In this paper, we describe the time-resolved X-ray microprobe and provide some illustrative examples from our work studying strong-field phenomena such as laser-modified absorption spectra, Coulomb explosion, transient laser-produced plasmas and molecular alignment.

Synchrotron X-ray-fluorescence (SXRF) analyses for Cu, Zn and Fe were carried out on magmatic minerals from intrusive and volcanic rocks in the Bingham – Park City Belt, Utah. The main goals of the study were to determine average Cu and Zn contents and zoning patterns in magmatic minerals in order to shed light on the chemical behavior of Cu and Zn during crystallization of the magmas. The combination of low limits of detection and small analytical footprint makes SXRF the preferred method for tests of this type. Results of the study show that the Zn is homogeneously distributed through most ferromagnesian minerals, commonly following Fe, for which it probably substitutes. The distribution of Cu is more complex, and includes Cu-rich patches of as yet unknown mineralogy that are widespread in all minerals. Median Cu and Zn contents for magmatic minerals, estimated excluding the Cu-rich patches, are lower than and higher than, respectively, those of volcanic rocks in the area. Copper thus must have behaved incompatibly, whereas Zn behaved compatibly during magmatic crystallization. Decreasing Cu concentrations from core to rim were observed in biotite from the volcanic rocks, but Cu and Zn zoning is absent in ferromagnesian minerals in the intrusive rocks. Altered rims on biotite in the intrusive rocks are enriched in Cu and locally in Zn thought to reflect postcrystallization migration of a late magmatic vapor phase.

Excellent x-ray optics for photons at around 10 keV can be expected with lithium metal. One of the best compound refractive lens designs [Lengeler et al., J. Appl. Phys. 84, 5855 (1998)] is now produced routinely in aluminum, and more recently has been demonstrated using beryllium [M. Kuhlmann et al. (unpublished)]. Here, we report a similar refractive lens made from lithium. At 10.87 keV, this lens has a ≃2 m focal length, more than 90% peak transmission, and an average transmission of 49%. The lens shows a very useful gain of up to 40. The full widths at half maximum (FWHM) of the focus are blurred by roughly 20 μm, resulting in a horizontal and vertical FWHM of 33 and 17 μm for an image distance of 2.13 m. The lens produces speckle on the x-ray beam, which is likely due to the inhomogeneities of the lens surface: Coherent x-ray scattering is useful in understanding imperfections in x-ray optics, such as mirrors and lenses. Better molding techniques should result in improved performance and enable microbeam techniques with this type of Li lens.

We introduce a new technique for probing the microscopic relaxation of magneto-viscoelastic materials consisting of magnetic particles embedded in a natural rubber matrix. Transversely coherent x-rays from a high brilliance synchrotron source are scattered by the magnetic particles, forming a speckle pattern at low scattering angles. The time dependence of this pattern is recorded with a CCD area detector while the sample is cyclically perturbed by a reversal of the magnetic field direction. The corresponding time-resolved scattering pattern probes both the dynamics of the particles and the relaxation of the matrix in which they are embedded. X-ray photon correlation spectroscopy (XPCS) reveals characteristic time scales for this relaxation by applying the intensity auto-correlation function to the time dependent speckle pattern. For low angle scattering, the wave vector dependence of the relaxation rate exhibits power law length scaling.

The Sector 7 undulator beamline (7 ID) of the Advanced Photon Source (APS) is dedicated to time-resolved X-ray research [1] . Silicon avalanche photodiodes (APDs) are used as the primary point detector for time-resolved Bragg diffraction experiments for their fast recovery time ( [2] . Common practice has been to attenuate the monochromatic beam entering the experimental hutch to an appropriately low flux [3] . For these high-flux experiments, an APD operated in proportional mode is a better detector choice due to a large dynamic range and linearity. With the ZT4212 ZTEC, EPICS based oscilloscope, the operating procedure to use an APD in proportional mode has been improved. This article shows the setup and operating procedure for this oscilloscope and demonstrates its application to measuring time-resolved rocking curves of laser excited semiconductors.

Charge production, charge redistribution, and ion fragmentation are explored in the decay of a Xe $K$-shell vacancy in XeF${}_{2}$. Coincidence measurements of all ionic fragments in XeF${}_{2}$ provide evidence that an interatomic-Coulombic-decay-like (ICD-like) process plays a role in the cascade decay. The signature of the ICD-like process is an enhancement of the total number of electrons ejected as compared to the case of atomic Xe. The results indicate that the F atoms participate in the decay cascade within the first few femtoseconds after core-hole formation and that fragmentation begins during the decay process.

We present a new design for mounting a cryogenically cooled Si crystal which gives greatly improved beam stability. The design has been successfully implemented at the University of Michigan, Howard University, Bell Laboratories-Lucent Technologies Collaborative Access Team (MHATT-CAT) 7ID Beamline of the Advanced Photon Source. Before the installation of the new crystal mount, our Si (lll) cryogenically cooled monochromator was sensitive to the pressure fluctuations of the liquid nitrogen coolant, such that the angle of incidence on the first crystal varied linearly with the applied pressure in the cooling lines, causing beam motion of about 250 μm, 60 m from the source. The key element of the design is a symmetrically positioned cooling manifold which balances the forces caused by pressure fluctuations. With this new mount, the typical beam stability is now about 10 μm, comparable to the source stability.

Thanks to its low x-ray absorption, lithium should be the material of choice for x-ray refractive lenses. This article discusses some of the measurements done to verify lithium’s relevant properties. Both x-ray transmission and refraction are consistent with expectations. The lens gain suffers from broadening that is related to small-angle scattering.

Lithium metal is the best material for refractive lenses that must focus x-rays with energies below 15 keV, but to date no lens from Li has been reported. This letter demonstrates focusing of 10 keV x-rays with a one-dimensional sawtooth lens made from Li. The lens’ theoretical gain is 4.5, with manufacturing imperfections likely responsible for the threefold gain that is observed. Despite the Li reactivity the lens is stable over months of operation if kept under vacuum.

The Sector 7 undulator beamline (7‐ID) of the Advanced Photon Source (APS) is dedicated to time‐resolved x‐ray research and is capable of ultrafast measurements on the order of 100 ps. Beamline 7‐ID has a laser laboratory featuring a Ti:Sapphire system (average power of 2.5 W, pulse duration

The recent developments in X-ray detectors have opened new possibilities in the area of time-resolved pump/probe X-ray experiments; this article presents the novel use of a PILATUS detector to achieve X-ray pulse duration limited time-resolution at the Advanced Photon Source (APS), USA. The capability of the gated PILATUS detector to selectively detect the signal from a given X-ray pulse in 24 bunch mode at the APS storage ring is demonstrated. A test experiment performed on polycrystalline organic thin films of alpha-perylene illustrates the possibility of reaching an X-ray pulse duration limited time-resolution of 60 ps using the gated PILATUS detector. This is the first demonstration of X-ray pulse duration limited data recorded using an area detector without the use of a mechanical chopper array at the beamline.

Ultrafast laser excitation of an InGaAs/InAlAs superlattice (SL) creates coherent folded acoustic phonons that subsequently leak into the bulk (InP) substrate. Upon transmission, the phonons become "unfolded" into bulk modes and acquire a wave vector much larger than that of the light. We show that time-resolved x-ray diffraction is sensitive to this large-wave vector excitation in the substrate. Comparison with dynamical diffraction simulations of propagating strain supports our interpretation.

An experimental comparison of models for performing dead-time corrections of photon-counting detectors at synchrotron sources is presented. The performance of several detectors in the three operating modes of the Advanced Photon Source is systematically compared, with particular emphasis on asymmetric fill patterns. Several simple and well known correction formulas are evaluated. The results demonstrate the critical importance of detector speed and synchrotron fill pattern in selecting the proper dead-time correction.

Tunable, polarized, microfocused x-ray pulses were used to record x-ray absorption spectra across the K edges of Kr{sup +} and Kr{sup 2+} produced by laser ionization of Kr. Prominent 1s {yields} 4p and 5p excitations are observed below the 1s ionization thresholds in accord with calculated transition energies and probabilities. Due to alignment of 4p hole states in the laser-ionization process, the Kr{sup +} 1s {yields} 4p cross section varies with respect to the angle between the laser and x-ray polarization vectors. This effect is used to determine the Kr{sup +} 4p{sub 3/2} and 4p{sub 1/2} quantum state populations, and these are compared with results of an adiabatic strong-field ionization theory that includes spin-orbit coupling.

Time-resolved x-ray absorption spectroscopy monitors the transient species generated by one-photon detachment of an electron from aqueous bromide. Hydrated bromine atoms with a lifetime of ca. 17 ns were observed, nearly half of which react with excess Br- to form Br2-. The K-edge spectra of the Br atom and Br2- anion exhibit distinctive resonant transitions that are absent for the Br- precursor. The absorption spectra indicate that the solvent shell around a Br0 atom is defined primarily by hydrophobic interactions, in agreement with a Monte Carlo simulation of the solvent structure., 6 pages, 4 figures + supplement, will be submitted to PRL

In the Ge on Si model heteroepitaxial system, metal patterns on the silicon surface provide unprecedented control over the morphology of highly ordered Ge islands. Island shape including nanorods and truncated pyramids is set by the metal species and substrate orientation. Analysis of island faceting elucidates the prominent role of the metal in promoting growth of preferred facet orientations while investigations of island composition and structure reveal the importance of Si-Ge intermixing in island evolution. These effects reflect a remarkable combination of metal-mediated growth phenomena that may be exploited to tailor the functionality of island arrays in heteroepitaxial systems.

We observe the time evolution of ground-state ion alignment in a laser-produced plasma. Krypton ions produced in a strong, linearly polarized optical laser field $({10}^{14}--{10}^{15}\phantom{\rule{0.3em}{0ex}}\mathrm{W}∕{\mathrm{cm}}^{2})$ are aligned along the field polarization axis. Using microfocused, tunable x rays from Argonne's Advanced Photon Source, we measure orbital alignment as a function of time. For plasma densities of the order of ${10}^{14}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$, the alignment decays within a few nanoseconds. A quantitative model explains the decay in terms of electron-ion collisions in the plasma. By applying an external magnetic field, we are able to suppress the disalignment and induce coherent spin precession of the Kr ions, thus providing an in situ monitor of magnetic fields in a plasma.

The first crystal mount of the double‐crystal Si (111) cryogenically cooled monochromator of the 7ID beamline at the Advanced Photon Source (APS) is slightly sensitive to pressure variations in the cryogenic lines. Pressure variations during a liquid nitrogen cryocooler fill every 4 hours move the beam by tens of microns. Pressure variations due to the cryocooler closed‐loop pressure control with a heater element (around 0.3 psi) move the beam by 5 microns every 15 seconds. We have recently stabilized the coolant pressure with a simple pressure regulator that is in use at many beamlines of the APS. This paper shows the improvements in beam position stability made using this simple yet effective pressure‐regulation circuit. We also recently added beam‐position feedback to the second‐crystal Bragg angle of the monochromator. The Experimental Physics and Industrial Control System (EPICS) Enhanced Proportional‐Integral‐Differential (EPID) feedback record implementation resulted in an additional improvement of the standard deviation of the beam position to 0.5 μm.

Lithium’s low x‐ray absorption should give refractive x‐ray optics their highest possible performance. We made low‐cost lithium lenses that are easy to align and match the ∼ 1 mm beam size. This paper summarizes lithium lens performance to date, and what might be done in the future.

We have developed a synchrotron-based, time-resolved x-ray microprobe to investigate optical strong-field processes at intermediate intensities (10(14) - 10(15) W/cm2). This quantum-state specific probe has enabled the direct observation of orbital alignment in the residual ion produced by strong-field ionization of krypton atoms via resonant, polarized x-ray absorption. We found strong alignment to persist for a period long compared to the spin-orbit coupling time scale (6.2 fs). The observed degree of alignment can be explained by models that incorporate spin-orbit coupling. The methodology is applicable to a wide range of problems.

In semiconductor epitaxy, a central challenge for the formation of ordered arrays of nanostructures, such as quantum dot islands, is the realization of processing routes to control surface-mediated growth mechanisms with high spatial precision and reproducibility across macroscopic lengths. To this end we have recently demonstrated a simple route for the directed assembly of heteroepitaxial islands based on rudimentary metal patterning. Here we show that the same metal patterns on the silicon surface that lead to island ordering radically modify island morphology resulting in shapes such as nanorods and truncated pyramids that are set by the choices of metal species and substrate orientation. These effects reflect a remarkable combination of metal-mediated growth phenomena that may be exploited to tailor the functionality of island arrays.

We present the first measurements of excitons in bulk liquid He, performed at several densities. Inelastic x-ray scattering was used to take these measurements of $^{4}\mathrm{He}$. The exciton measurements are very similar to those for solid $^{4}\mathrm{He}$, with the exciton excitations occurring at nearly the same energy transfers for the same pressure. This similarity suggests that excitons in solid helium rely mostly on short range order. The measured peaks for the liquid are asymmetric in nature and fit well to two Gaussians, suggesting that at least two exciton transitions are present in the signal peak.

Lithium's low x-ray absorption should give refractive x-ray optics their highest possible performance. We made low-cost lithium lenses that are easy to align and match the ~ 1 mm beam size. This paper summarizes lithium lens performance to date, and what might be done in the future.

We have recently redesigned the first crystal mount of our cryogenically cooled monochromator to reduce its sensitivity to pressure fluctuations in the cryogenic lines feeding the Si (111) crystal. With the use of a fluorescence‐based X‐ray beam position monitor (BPM) placed 19 m away from the monochromator, much operational experience has been gained on the sensitivity of the beam position and intensity to small changes in the cooling system. In this presentation, we will describe our X‐ray BPM design and performance and will provide examples of changes that have made the beam position more stable on our beamline. One such change for example has been the top‐up operation of the Advanced Photon Source (APS), which has reduced the thermal drifts associated with the ring current decay.

For rapid and efficient tests of novel X‐rays optics, such as lithium‐based compound refractive lenses, we have built a fast X‐ray sensitive CCD imaging system. We report on the linearity, response and resolution of the microscope‐based imaging system. For the low magnifications used here (X2‐X10), we find that a thinly doped YAG screen has a poorer resolution than a thick YAG screen. We provide an example of its use in testing a new 2D focusing multiprism X‐ray lens.

We measure transient strain in ultrafast laser-excited Ge by time-resolved x-ray anomalous transmission. The development of the coherent strain pulse is dominated by rapid ambipolar diffusion. This pulse extends considerably longer than the laser penetration depth because the plasma initially propagates faster than the acoustic modes. X-ray diffraction simulations are in agreement with the observed dynamics.

We demonstrate a hard x-ray probe of laser-aligned small molecules. To align small molecules with optical lasers, high intensities at nonresonant wavelengths are necessary. We use 95 ps pulses focused to 40 mum from an 800 nm Ti:sapphire laser at a peak intensity of 10^12 W/cm^2 to create an ensemble of aligned bromotrifluoromethane (CF3Br) molecules. Linearly polarized, 120 ps x-ray pulses, focused to 10 mum, tuned to the Br 1s --> sigma* pre-edge resonance at 13.476 keV, probe the ensemble of laser-aligned molecules. The demonstrated methodology has a variety of applications and can enable ultrafast imaging of laser-controlled molecular motions with Angstrom-level resolution., 4 pages, 4 figures, RevTeX4, corrected typos

We characterize the time evolution of ion spatial distributions in a laser-produced plasma. Krypton ions are produced in strong, linearly and circularly polarized optical laser fields (1014–1015 W/cm2). The Kr+ ions are preferentially detected by resonant x-ray absorption. Using microfocused, tunable x rays from Argonne’s Advanced Photon Source, we measure ion densities as a function of time with 10 μm spatial resolution for times ≤50 ns. For plasma densities of the order of 1014 cm−3, we observe a systematic expansion of the ions outward from the laser focus. We find the expansion timescale to be independent of the plasma density though strongly dependent on the plasma shape and electron temperature. The former is defined by the laser focus, while the latter is controlled by the laser polarization state. We have developed a fluid description assuming a collisionless quasineutral plasma, which is modeled using a particle-in-cell approach. This simulation provides a quantitative description of the observed...