Ultra-high Energy Resolution Research Articles

Design of an ultrahigh-energy-resolution and wide-energy-range soft X-ray beamline

A new ultrahigh-energy-resolution and wide-energy-range soft X-ray beamline has been designed and is under construction at the Shanghai Synchrotron Radiation Facility. The beamline has two branches: one dedicated to angle-resolved photoemission spectroscopy (ARPES) and the other to photoelectron emission microscopy (PEEM). The two branches share the same plane-grating monochromator, which is equipped with four variable-line-spacing gratings and covers the 20-2000 eV energy range. Two elliptically polarized undulators are employed to provide photons with variable polarization, linear in every inclination and circular. The expected energy resolution is approximately 10 meV at 1000 eV with a flux of more than 3 × 10(10) photons s(-1) at the ARPES sample positions. The refocusing of both branches is based on Kirkpatrick-Baez pairs. The expected spot sizes when using a 10 µm exit slit are 15 µm × 5 µm (horizontal × vertical FWHM) at the ARPES station and 10 µm × 5 µm (horizontal × vertical FWHM) at the PEEM station. The use of plane optical elements upstream of the exit slit, a variable-line-spacing grating and a pre-mirror in the monochromator that allows the influence of the thermal deformation to be eliminated are essential for achieving the ultrahigh-energy resolution.

Hybrid diamond-silicon angular-dispersive x-ray monochromator with 025-meV energy bandwidth and high spectral efficiency

We report on the design, implementation, and performance of an x-ray monochromator with ultra-high energy resolution (ΔE/E ≃ 2.7 × 10(-8)) and high spectral efficiency using x rays with photon energies E ≃ 9.13 keV. The operating principle of the monochromator is based on the phenomenon of angular dispersion in Bragg back-diffraction. The optical scheme of the monochromator is a modification of a scheme reported earlier [Shvyd'ko et al., Phys. Rev. A 84, 053823 (2011)], where a collimator/wavelength selector Si crystal was replaced with a 100-μm-thick type IIa diamond crystal. This modification provides a very-small-energy bandwidth ΔE ≃ 0.25 meV, a 3-fold increase in the aperture of the accepted beam, a reduction in the cumulative angular dispersion rate of x rays emanating from the monochromator for better focusing on a sample, a sufficient angular acceptance matching the angular divergence of an undulator source (≈ 10 μrad), and an improved throughput due to low x-ray absorption in the thin diamond crystal. The measured spectral efficiency of the monochromator was ≈ 65% with an aperture of 0.3 × 1 mm(2). The performance parameters of the monochromator are suitable for inelastic x-ray spectroscopy with an absolute energy resolution ΔE < 1 meV.

Ultra High Energy Resolution EELS Map Employing an Aberration-corrected STEM Equipped with a Monochromator

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

Direct observation of superconducting gaps in iron-based superconductors by laser ARPES

We have developed a new apparatus for carrying out ultra-high energy resolution laser-excited angle-resolved photoemission spectroscopy (laser ARPES). The achieved energy resolution is ~ 70 μeV at a sample temperature ~ 1.5 K. As a test case, we have measured the superconducting gap of elemental metal Sn (Tc = 3.7 K). We have used this laser-ARPES apparatus, to directly observe the superconducting gap of various iron-based superconductors. The spectra of FeTe0.6Se0.4 below Tc show a very clear temperature dependent superconducting coherence peak. Preliminary momentum-dependent results indicate an anisotropic superconducting gap in FeTe0.6Se0.4.

Direct observation of superconducting gaps in iron-based superconductors by laser ARPES

We have developed a new apparatus for carrying out ultra-high energy resolution laser-based angle-resolved photoemission spectroscopy (laser ARPES). From the spectra of the evaporated gold and superconducting (SC) gap of Sn (Tc=3.7K) sample, we conclude that the best energy resolution is ∼70μeV and the lowest sample temperature is ∼1.0K. We have used this laser-ARPES apparatus, to directly observe the SC gap of various iron-based superconductors. The spectra of FeTe0.6Se0.4 below Tc show a very clear temperature-dependent superconducting coherence peak, and the SC-gap nodes of KFe2As2 are found in the middle Fermi surface (FS) around the FS angle φ=±5.0°.

The Ultrahigh Resolution IXS Beamline of NSLS-II: Recent Advances and Scientific Opportunities

The ultrahigh resolution IXS beamline of NSLS-II is designed to probe a region of dynamic response that requires an ultrahigh energy and momentum resolution of up to 0.1 meV and < 0.1 nm−1 respectively, which is currently still beyond the reach of existing low and high frequency inelastic scattering probes. Recent advances at NSLS-II in developing the required x-ray optics and instrumentation based on the use of extremely asymmetric Bragg back reflections of Si have allowed us to achieve sub-meV energy resolution with sharp tails and high efficiency at a medium energy of around 9.1 keV, thereby validating the optical design of the beamline for the baseline scope and paving the way for further development towards the ultimate goal of 0.1 meV. The IXS beamline is expected to provide a broad range of scientific opportunities, particularly in areas of liquid, disordered and bio-molecular systems.

The GRANIT spectrometer

Abstract The existence of quantum states of matter in a gravitational field was demonstrated recently in the Institut Laue–Langevin (ILL), Grenoble, in a series of experiments with ultra cold neutrons (UCN). UCN in low quantum states is an excellent probe for fundamental physics, in particular for constraining extra short-range forces; as well as a tool in quantum optics and surface physics. The GRANIT is a follow-up project based on a second-generation spectrometer with ultra-high energy resolution, permanently installed in ILL. It has been constructed in framework of an ANR grant; and will become operational in 2011.

Synchrotron Mössbauer spectroscopy using high-speed shutters.

A new method of performing Mössbauer spectroscopy with synchrotron radiation is demonstrated that involves using a high-speed periodic shutter near the focal spot of a microfocused X-ray beam. This fast microshuttering technique operates without a high-resolution monochromator and has the potential to produce much higher signal rates. It also offers orders of magnitude more suppression of unwanted electronic charge scattering. Measurement results are shown that prove the principle of the method and improvements are discussed to deliver a very pure beam of Mössbauer photons (E/ΔE ≃ 10(12)) with previously unavailable spectral brightness. Such a source will allow both Mössbauer spectroscopy in the energy domain with the many advantageous characteristics of synchrotron radiation and new opportunities for measurements using X-rays with ultra-high energy resolution.

Improved detection limits in PIXE analysis employing wavelength dispersive X-ray spectroscopy

A wavelength dispersive X-ray spectroscopy was employed to measure the proton induced Lα X-ray emission spectra of Ag, Pd and Cd targets in order to lower detection limits for trace amounts of Pd and Cd in silver matrix. The measurements were performed with a Johansson-type crystal spectrometer having an energy resolution below the natural linewidths of the measured L X-ray lines. As a direct consequence of such ultrahigh experimental energy resolution, detection limits of only few tens of ppm were reached. The method presented in this work can be used in general to improve substantially PIXE detection limits compared to standard energy dispersive spectroscopy for the measurement of trace elements with atomic number in the close vicinity of the atomic number of the target matrix element.

Focusing X-Rays with Curved Multiplate Crystal Cavity

An overview is given of the study on X-ray focusing using the Fabry-Perot type multi-plate silicon crystal cavities consisting of compound refractive lenses. Silicon (12 4 0) is used as the back reflection for cavity resonance at the photon energy of 14.4388 keV. Measurements of focal length of the transmitted beam through the crystal cavities show enhanced focusing effect due to the presence of back diffraction. Also, an incident beam with ultrahigh energy resolution can improve the focusing owing to the wider acceptance angle of the back diffraction. Considerations based on the excitation of dispersion surface within the framework of X-ray dynamical diffraction theory are also presented to reveal the origin of this enhanced focusing.

Development of ultrahigh energy resolution gamma spectrometers for nuclear safeguards

We are developing superconducting ultrahigh resolution gamma-detectors for non-destructive analysis (NDA) of nuclear materials, and specifically for spent fuel characterization in nuclear safeguards. The detectors offer an energy resolution below 100 eV FWHM at 100 keV, and can therefore significantly increase the precision of NDA at low energies where line overlap affects the errors of the measurement when using germanium detectors. They also increase the peak-to-background ratio and thus improve the detection limits for weak gamma emissions from the fissile Pu and U isotopes at low energy in the presence of an intense Compton background from the fission products in spent fuel. Here we demonstrate high energy resolution and high peak-to-background ratio of our superconducting Gamma detectors, and discuss their relevance for measuring actinides in spent nuclear fuel.

Studying the microscopic nature of diffusion with helium-3 spin-echo

Helium-3 spin-echo (3HeSE) is a powerful, new experimental technique for studying dynamical phenomena at surfaces with ultra-high energy resolution. Resolution is achieved by using the 3He nuclear spin as an internal timer, to enable measurement of the energy changes of individual atoms as they scatter. The technique yields a measurement of surface correlation in reciprocal space and real time, and probes the nanometre length scales and picosecond to nanosecond timescales that are characteristic of many important atomistic processes. In this article we provide an introductory description of the 3HeSE technique for quasi-elastic scattering measurements and explain how it can be used to obtain unique insights into the motion of adsorbates. We illustrate the technique by reviewing recent measurements, starting with simple hopping and then showing how correlations, arising from adsorbate interactions, can be observed. The final measurements demonstrate how the absence of such correlations, when expected, are used to question the conventional description that attributes the coverage dependence of surface processes entirely to pairwise forces between adsorbates. The emphasis throughout is on the characteristic signatures of adsorbate motion that can be seen in the data, without recourse to a detailed theoretical analysis. Numerical simulations using the Langevin equation are used to illustrate generic behaviour and to provide a quantitative analysis of the experiment.

Ultrahigh-Resolution Vacuum Ultraviolet Light Source System for Extremely Low Energy Photoelectron Spectroscopy

We propose an efficient monochromatization method for the new ultrahigh-resolution Ar, Kr, and Xe light sources to be used for extremely low energy photoelectron spectroscopy (ELEPES). By fully utilizing the short-wavelength transmittance limits known as the Urbach tail of LiF, CaF2, and sapphire, we can extract only the longest wavelength lines of the resonance lamp with Ar, Kr, and Xe gases by using LiF heated up to ∼50 °C as well as CaF2 and sapphire at room temperature. The full width at half maximum of the longest wavelength Xe I resonance line is found to be narrower than 500 µeV. Using these light sources, we can perform ELEPES in laboratories at three photon energies with ultrahigh energy resolution and high efficiency.

Sixth Harmonic of A Nd:YVO4 Laser Generation In KBBF for ARPES

We report that a deep ultraviolet (DUV) laser from the sixth harmonic of a 1064 nm laser has been firstly used as light source in an ultrahigh energy-resolution angle-resolved photoemission spectroscopy (ARPES). The wavelength is 177.3nm obtained by using the second harmonic KBe2BO3F2 crystal with a frequency tripled 1064nm Nd:YVO4 laser. The large flux (1014 – 1015 photons/s) and narrow line width (0.26meV) are suitable for the ultrahigh-energy resolution ARPES. The laser-ARPES can be a powerful tool to study the electronic structure at and near the Fermi level of the superconductor and correlated materials. The laser-ARPES has worked more than 500 h already.

A versatile system for ultrahigh resolution, low temperature, and polarization dependent Laser-angle-resolved photoemission spectroscopy

We have developed a low temperature ultrahigh resolution system for polarization dependent angle-resolved photoemission spectroscopy (ARPES) using a vacuum ultraviolet (vuv) laser (hnu=6.994 eV) as a photon source. With the aim of addressing low energy physics, we show the system performance with angle-integrated PES at the highest energy resolution of 360 mueV and the lowest temperature of 2.9 K. We describe the importance of a multiple-thermal-shield design for achieving the low temperature, which allows a clear measurement of the superconducting gap of tantalum metal with a T(c)=4.5 K. The unique specifications and quality of the laser source (narrow linewidth of 260 mueV, high photon flux), combined with a half-wave plate, facilitates ultrahigh energy and momentum resolution polarization dependent ARPES. We demonstrate the use of s- and p-polarized laser-ARPESs in studying the superconducting gap on bilayer-split bands of a high T(c) cuprate. The unique features of the quasi-continuous-wave vuv laser and low temperature enables ultrahigh-energy and -momentum resolution studies of the spectral function of a solid with large escape depth. We hope the present work helps in defining polarization dependent laser excited angle-resolved photoemission spectroscopy as a frontier tool for the study of electronic structure and properties of materials at the sub-meV energy scale.

Absolute photoionization cross sections with ultra-high energy resolution for Ar, Kr, Xe and N 2 in inner-shell ionization regions

The high-resolution absolute photoionization cross sections for Ar, Kr, Xe and N 2 in the inner-shell ionization region have been measured using a multi-electrode ion chamber and monochromatized synchrotron radiation. The energy ranges of the incident photons for the target gases were as follows: Ar: 242–252 eV (2p Rydberg excitation), Kr: 1650–1770 eV (near the 2p ionization thresholds), Xe: 665–720 eV (near the 3d ionization thresholds) and 880–1010 eV (near the 3p ionization thresholds), N 2: 400–425 eV (N 1s excitation and ionization). It is the first time to measure the absolute ionization cross sections of Ar, Kr, Xe and N 2 over the present energy ranges with the energy resolution of over 10,000. The natural lifetime widths of 2 p 3 / 2 − 1 4 s , 2 p 3 / 2 − 1 3 d , 2 p 3 / 2 − 1 4 d and 2 p 1 / 2 − 1 4 s resonances for Ar, 3 d 5 / 2 − 1 6 p resonance for Xe, and 1 s − 1 π g * ( v ′ = 0 ) resonance for N 2 have been obtained based on the cross sections determined. The ionization energies into the Ar + ( 2 p 3 / 2 − 1 ), Ar + ( 2 p 1 / 2 − 1 ) and Xe + ( 3 d 5 / 2 − 1 ) ionic states are also determined using the Rydberg formula.

A negative-ion TPC with ultra-high energy resolution for 0-ν double beta decay search in 136Xe

Future searches for the neutrino-less double beta decay mode in candidate nuclei must confront the need for sensitivities at the level of 10 – 50 meV effective neutrino mass. Current techniques may not be able to scale simultaneously to the needed mass of active isotope with both improved energy resolution and much higher levels of background rejection. To address these severe challenges, a novel approach based on a negative-ion time projection chamber filled primarily with high-pressure 136Xe gas is developed. In addition to the provision of a nearly ideal active fiducial boundary surface, an energy resolution of ∼5 × 103 FWHM appears possible at the Q-value of 2.48 MeV. The background rejection advantages of detailed track information and a single monolithic detector are addressed. A barium daughter-tagging scheme based on mobility differences and a new approach addressing the goal of efficient electron capture and release in high-density gas exploiting chemical reactions are introduced. Many R&D issues are suggested by this new concept.

Realization of Fabry-Perot resonators for hard X rays using micro- and nanotechnology

An X-ray Fabry-Perot resonator has been proposed and investigated for more than three decades. The difficulty in realizing such a resonator is mainly due to the strict requirement on coherence for cavity resonance, which is governed by the energy resolution of the incident X-ray photons and the size of the resonator. With the aid of modern micro-and nanotechnology for silicon, such as X-ray lithography, miniature resonators, approximately a few micrometers or smaller, can be prepared. Together with X-ray photons of ultrahigh-energy resolution from synchrotron sources, the required conditions on coherence for cavity resonance are fulfilled. Several X-ray cavities prepared by this means are used in X-ray back-diffraction experiments, where back diffraction acts as mirror reflection for X rays. Resonance interference fringes are observed unambiguously. Attempts to use multilayered materials for cavity fabrication are discussed in relation to X-ray synchrotron experiment. Possible future applications of this X-ray resonator are also mentioned.

Photodissociation dynamics in “hyper-rovibronic” detail: Exploring the potential of millimeter/submillimeter-wave spectroscopy in molecular reaction dynamics experiments

Over the years, many techniques for studying molecular reaction dynamics have been developed and fine-tuned to probe chemical dynamics at an ever-increasing level of detail. Unfortunately, this progress has frequently come at the price of high experimental cost and great complexity. In this regard experiments employing direct absorption have a distinct advantage in that they are comparatively simple in setup and they probe nascent product distributions directly. Even though the low product number densities in molecular-beam experiments put severe constraints on the noise and sensitivity requirements of detectors, Nesbitt and co-workers [J. Chem. Phys. 86, 3151 (1987); Rev. Sci. Instrum. 58, 807 (1987); J. Chem. Phys. 85, 4890 (1986); J. Chem. Phys. 107, 5661 (1997); Chem. Phys. Lett. 258, 207 (1996)] have demonstrated the use of direct infrared absorption in a variety of molecular reaction dynamics studies. In analogous experiments, this article explores the use of millimeter- and submillimeter-wavelength radiation in direct absorption experiments in a molecular beam. The comparatively simple and inexpensive setup demonstrates the utility of combining new commercial solid-state millimeter/submillimeter-wavelength sources with hot-electron bolometer detectors to directly probe parent and product hyperfine rovibronic levels and their Doppler-resolved velocity distributions in a molecular beam. For example, in open-shell products with nuclear spin, the ultrahigh energy resolution of the rotational spectroscopy easily resolves nuclear quadrupole hyperfine structure and lambda doublets in both ground and excited spin-orbit states as well as in ground and excited vibrational levels. Two molecular beam examples are given: (1) detection of “hyper-rovibronic” structure in ClO (ΠΩ=3∕2,1∕22, ν=0–8, J=112–712, Λ,F) following the mode-specific photodissociation of OClO (AA22←XB12, ν1=14–15), and (2) coherent transient absorption of HCN following the 266 nm photodissociation of sym-triazine/argon clusters.

Quantifying the benefits of ultrahigh energy resolution for gamma‐ray spectrometry

Cryogenic Gamma-ray spectrometers operating at temperatures of ∼0.1 K provide an order of magnitude better energy resolution than conventional germanium detectors. Ultra-high energy resolution improves the accuracy of non-destructive analysis of nuclear materials, since a better separation of lines reduces statistical errors as well as systematic errors from background subtraction and efficiency correction. We are developing cryogenic Gamma-spectrometers based on bulk tin absorbers and superconducting molybdenum-copper sensors for nuclear forensics and non-proliferation applications. Here we quantify the improvements in accuracy for isotope analysis with cryogenic detectors in terms of detector performance for different cases of line separation, line intensity ratios and background levels. Precise measurements of isotope ratios are crucial in the context of nuclear attribution, since they provide signatures of composition, age, origin, intended purpose and processing history of illicit nuclear materials. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)