Two independent techniques are used to obtain the background function for the x-ray-absorption fine structure (XAFS) of pure Pb at energies that are normally inaccessible because they are in the edge. The results of the two techniques are shown to give the same \ensuremath{\chi}(k) above a threshold energy in the absorption edge, where the background is 70% of the edge step. The reliability of the XAFS starting at unprecedentedly low k values (1.5 A${\mathrm{\r{}}}^{\mathrm{\ensuremath{-}}1}$ above the Fermi energy) allows sensitive tests of several details of theoretical XAFS calculations. Electron-hole-excitation losses in addition to the plasmon-pole loss term are shown to be important, and the Hedin-Lundqvist many-body self-energy is found to be superior to that of Dirac-Hara for pure Pb. One of the presented methods of background removal can be employed for general XAFS analysis, and is easily automated.

Near-edge x-ray-absorption fine structure of Pb : a comparison of theory and experiment

Amino acids are the essential molecular components of living organisms on Earth, but the proposed mechanisms for their spontaneous generation have been unable to account for their presence in Earth's early history1. The delivery of extraterrestrial organic compounds has been proposed as an alternative to generation on Earth2,3,4,5, and some amino acids have been found in several meteorites6,7,8,9. Here we report the detection of amino acids in the room-temperature residue of an interstellar ice analogue that was ultraviolet-irradiated in a high vacuum at 12 K. We identified 16 amino acids; the chiral ones showed enantiomeric separation. Some of the identified amino acids are also found in meteorites. Our results demonstrate that the spontaneous generation of amino acids in the interstellar medium is possible, supporting the suggestion that prebiotic molecules could have been delivered to the early Earth by cometary dust, meteorites or interplanetary dust particles.

Amino acids from ultraviolet irradiation of interstellar ice analogues.

The delivery of extraterrestrial organic molecules to Earth by meteorites may have been important for the origin and early evolution of life. Indigenous amino acids have been found in meteorites-over 70 in the Murchison meteorite alone. Although it has been generally accepted that the meteoritic amino acids formed in liquid water on a parent body, the water in the Murchison meteorite is depleted in deuterium relative to the indigenous organic acids. Moreover, the meteoritical evidence for an excess of laevo-rotatory amino acids is hard to understand in the context of liquid-water reactions on meteorite parent bodies. Here we report a laboratory demonstration that glycine, alanine and serine naturally form from ultraviolet photolysis of the analogues of icy interstellar grains. Such amino acids would naturally have a deuterium excess similar to that seen in interstellar molecular clouds, and the formation process could also result in enantiomeric excesses if the incident radiation is circularly polarized. These results suggest that at least some meteoritic amino acids are the result of interstellar photochemistry, rather than formation in liquid water on an early Solar System body.

Racemic amino acids from the ultraviolet photolysis of interstellar ice analogues

The Nuclear Resonance Beamline at ESRF is dedicated to the excitation of nuclear levels by synchrotron radiation. The sources of radiation and optical elements are optimized to provide an intense, highly monochromatic, collimated and stable X-ray beam of small cross-section at the Mossbauer transition energies between 6 and 30 keV. The set-up of the beamline allows to perform studies in diffraction, small angle scattering, forward scattering and incoherent scattering. Equipment is available to maintain the sample at variable temperature and magnetic field. Fast detectors and timing electronics serve to separate the delayed nuclear scattering from the “prompt” electronic scattering and to measure the time spectra of nuclear radiation with sub-nanosecond resolution. The general lay-out and the parameters of the beamline are reported. Typical domains of applications are discussed and illustrated by first experimental results.

Nuclear Resonance Beamline at ESRF

Nuclear resonant scattering techniques with synchrotron radiation (SR) are introduced on a basic level. We focus on the theoretical background and on experimental aspects of two popular methods with a widening range of applications, nuclear resonant inelastic x-ray scattering and synchrotron Mossbauer spectroscopy. The inelastic method provides specific vibrational information, e.g., the phonon density of states. The Mossbauer method permits determination of hyperfine interactions. All nuclear resonance techniques take full advantage of the unique properties of SR: intensity, collimation, time structure, and polarization. As a result both methods discussed here have led to novel applications for materials under extreme conditions, proteins with biological functionality, and magnetic nanostructures.

https://iopscience.iop.org/article/10.1088/0953-8984/16/5/009/pdf

Nuclear resonant spectroscopy

We report the observation of nuclear resonant diffraction of synchrotron radiation by a synthetic multilayer. The nuclear period of the ${[}^{57}$Fe(22 \AA{})/Sc(11 \AA{})/Fe(22 \AA{})/Sc(11 \AA{})\ifmmode\times\else\texttimes\fi{}25 multilayer was chosen to be twice the electronic period to obtain a pure nuclear Bragg reflection. Strong enhancement of the radiative scattering channel provided a fast time response for the $^{57}\mathrm{Fe}$ resonant scattering at the Bragg peak, with a decay time of 4 ns. The nuclear multilayer will be useful as a narrow bandpass monochromator for synchrotron radiation.

Resonant diffraction of synchrotron radiation by a nuclear multilayer.

Multielectron excitations involving deep and shallow core levels have been identified for the first time in the x-ray-absorption fine-structure (XAFS) energy range for crystalline samples, RbBr and \ensuremath{\beta}-${\mathrm{PbO}}_{2}$. The XAFS oscillations have been fitted and subtracted from the experimental data using theoretical standards. The positions of steps in the extracted background are in excellent agreement with the Z+1 approximation. The ability to determine this structure has important implications for XAFS and multiexcitation studies.

Multielectron x-ray photoexcitation observations in x-ray-absorption fine-structure background

Specular scattering of x rays by electrons in a material becomes large at grazing angles below the critical angle for external reflection. In contrast, we observe specular scattering by resonant nuclei to peak at the critical angle and decrease at smaller angles. This is the result of the influence of the electronic scattering on the nuclear response of the system and is explained using an optical model. A distorted-wave Born approximation shows the specular nuclear signal should be large at angles where the electronic reflectivity varies rapidly with angle.

/pdf/angular-dependence-of-specular-resonant-nuclear-scattering-149smc7pj2.pdf

Angular dependence of specular resonant nuclear scattering of x rays

The time evolution of the coherent decay of a collective nuclear excitation (nuclear exciton), induced by 14.4 keV synchrotron radiation, was studied in a system of two stainless steel foils, where one was vibrated by ultrasound (US). The US vibrations disrupted the coherent nuclear emission from the two targets. However, at times corresponding to multiples of the US period, spike increases in the decay were observed. The spikes are interpreted as echoes of the initial nuclear exciton. {copyright} {ital 1996 The American Physical Society.}

Nuclear Exciton Echo Produced by Ultrasound in Forward Scattering of Synchrotron Radiation.

We report the first observation of excitation of the 6.2 keV nuclear level of ${}^{181}\mathrm{Ta}$ by synchrotron radiation. The absolute energy of the level was determined to be $6214\ifmmode\pm\else\textpm\fi{}2$ eV. The time distribution of nuclear forward scattering of synchrotron radiation by a 7.5 $\ensuremath{\mu}$m Ta metal foil was measured, and a decay time of $530\ifmmode\pm\else\textpm\fi{}80$ ns was observed. Analysis of the time distribution reveals an inhomogeneous broadening of the nuclear resonance in this sample $6\ifmmode\pm\else\textpm\fi{}2$ natural linewidths.

G. S. Brown

Papers

Resonant diffraction of synchrotron radiation by a nuclear multilayer.

Multielectron x-ray photoexcitation observations in x-ray-absorption fine-structure background

Angular dependence of specular resonant nuclear scattering of x rays

Nuclear Exciton Echo Produced by Ultrasound in Forward Scattering of Synchrotron Radiation.

Nuclear scattering of synchrotron radiation by 181TA.