Demonstration of simultaneous experiments using thin crystal multiplexing at the Linac Coherent Light Source.

Multiplexing of the Linac Coherent Light Source beam was demonstrated for hard X-rays by spectral division using a near-perfect diamond thin-crystal monochromator operating in the Bragg geometry. The wavefront and coherence properties of both the reflected and transmitted beams were well preserved, thus allowing simultaneous measurements at two separate instruments. In this report, the structure determination of a prototypical protein was performed using serial femtosecond crystallography simultaneously with a femtosecond time-resolved XANES studies of photoexcited spin transition dynamics in an iron spin-crossover system. The results of both experiments using the multiplexed beams are similar to those obtained separately, using a dedicated beam, with no significant differences in quality.

All-diamond optical assemblies for a beam-multiplexing X-ray monochromator at the Linac Coherent Light Source.

A double-crystal diamond (111) monochromator recently implemented at the Linac Coherent Light Source (LCLS) enables splitting of the primary X-ray beam into a pink (transmitted) and a monochromatic (reflected) branch. The first monochromator crystal, with a thickness of ∼100 µm, provides sufficient X-ray transmittance to enable simultaneous operation of two beamlines. This article reports the design, fabrication and X-ray characterization of the first and second (300 µm-thick) crystals utilized in the monochromator and the optical assemblies holding these crystals. Each crystal plate has a region of about 5 × 2 mm with low defect concentration, sufficient for use in X-ray optics at the LCLS. The optical assemblies holding the crystals were designed to provide mounting on a rigid substrate and to minimize mounting-induced crystal strain. The induced strain was evaluated using double-crystal X-ray topography and was found to be small over the 5 × 2 mm working regions of the crystals.

Milli-electronvolt monochromatization of hard X-rays with a sapphire backscattering monochromator.

A sapphire backscattering monochromator with 1.1 (1) meV bandwidth for hard X-rays (20-40 keV) is reported. The optical quality of several sapphire crystals has been studied and the best crystal was chosen to work as the monochromator. The small energy bandwidth has been obtained by decreasing the crystal volume impinged upon by the beam and by choosing the crystal part with the best quality. The monochromator was tested at the energies of the nuclear resonances of (121)Sb at 37.13 keV, (125)Te at 35.49 keV, (119)Sn at 23.88 keV, (149)Sm at 22.50 keV and (151)Eu at 21.54 keV. For each energy, specific reflections with sapphire temperatures in the 150-300 K region were chosen. Applications to nuclear inelastic scattering with these isotopes are demonstrated.

X-ray Bragg diffraction in asymmetric backscattering geometry.

We observe three effects in the Bragg diffraction of x rays in backscattering geometry from asymmetrically cut crystals. First, exact Bragg backscattering takes place not at normal incidence to the reflecting atomic planes. Second, a well-collimated (approximately 1 microrad) beam is transformed after the Bragg reflection into a strongly divergent beam (230 microrad) with reflection angle dependent on x-ray wavelength--an effect of angular dispersion. The asymmetrically cut crystal thus behaves like an optical prism, dispersing an incident collimated polychromatic beam. The dispersion rate is approximately 8.5 mrad/eV. Third, parasitic Bragg reflections accompanying Bragg backreflection are suppressed. These effects offer a radically new means for monochromatization of x rays not limited by the intrinsic width of the Bragg reflection.

X-ray interferometry with microelectronvolt resolution.

We demonstrate an interferometer for hard x rays with two back-reflecting sapphire crystal mirrors--a prototype x-ray Fabry-Pérot interferometer. A finesse of 15 and 0.76 mu eV broad Fabry-Pérot transmission resonances are measured by the time response of the interferometer. Interference patterns are observed directly in spectral dependences of reflectivity.

Anomalous isotopic effect on the lattice parameter of silicon.

The difference Delta(a)=a(30)-a(28) of the lattice parameter of 30Si and 28Si crystals is measured over a temperature range from 4.7 to 700 K. In disagreement with existing knowledge, the strongest isotopic effect is not detected at the lowest achieved temperature T=4.7 K. An anomalous behavior is observed: The relative difference |Delta(a)/a| attains its maximum value of 56.8(5) ppm at T=75(10) K. The anomalous behavior is attributed to the influence of phonon modes with negative Grüneisen parameters. At T=700 K the effect still amounts to 30% of the maximal value. The experimental data are consistent with an approach based on the density-functional perturbation theory.