Toroidal Grating Monochromator Research Articles

Multispectral time-resolved energy-momentum microscopy using high-harmonic extreme ultraviolet radiation.

A 790-nm-driven high-harmonic generation source with a repetition rate of 6kHz is combined with a toroidal-grating monochromator and a high-detection-efficiency photoelectron time-of-flight momentum microscope to enable time- and momentum-resolved photoemission spectroscopy over a spectral range of 23.6-45.5eV with sub-100fs time resolution. Three-dimensional (3D) Fermi surface mapping is demonstrated on graphene-covered Ir(111) with energy and momentum resolutions of ≲100 meV and ≲0.1 Å-1, respectively. The tabletop experiment sets the stage for measuring the kz-dependent ultrafast dynamics of 3D electronic structure, including band structure, Fermi surface, and carrier dynamics in 3D materials as well as 3D orbital dynamics in molecular layers.

Analysis of higher harmonic contamination with a modified approach using a grating analyser.

Soft x-ray spectra of the toroidal grating monochromator (TGM) at the reflectivity beamline of Indus-1 synchrotron source are analyzed for higher harmonic contribution. A diffraction grating of central line spacing 1200 l/mm is used to disperse the monochromatic beam received from TGM to quantify the harmonic contents in the 50-360 Å wavelength range. In order to calculate the harmonic contamination, conventionally the intensity of higher order peak is divided by first order peak intensity of the desired wavelength. This approach is found to give wrong estimate as first order peak itself is overlapped by higher order peaks. In the present study, a modified approach has been proposed to calculate harmonic contamination where the intensity contributions of overlapping orders have been removed from the first order diffraction peak of the desired wavelength. It is found that the order contamination in the TGM spectra is less than 15% in the wavelength range of 90-180 Å. The total harmonic contribution increases from 6%-60% in the wavelength range of 150-260 Å. The critical wavelength of Indus-1 is 61 Å hence the harmonic contamination below 90 Å is significantly low. The results obtained with modified approach match well with those obtained by quantitative analysis of multilayer reflectivity data. The obtained higher harmonics data are used to fit the transmission of aluminum edge filter in the 120-360 Å wavelength range.

Improvement on a new concept of beamline delivering high purity VUV photons starting at 7.3 eV

We report recent improvements and performance on the Toroidal Grating Monochromator (TGM) beamline at the Laboratório Nacional de Luz Síncrotron – LNLS. Compared to normal incidence monochromators, (NIMs), TGMs provide substantially wider energy range, less resolving power and very poor higher harmonic rejection (HHR). A new concept, MIRHACLLE [1], developed at LNLS, allows reaching more than five orders of harmonic rejection (NIM gives 10%). Previously, we were not able to reach below 12 eV using MIRHACLLE. Here we report improvements extending the lower limit to 7.3 eV. Furthermore, HHR is maintained down to the lower limit using a special gas mixture, the upper limit given by 330 eV is kept.

Optimization of the 3 m TGM beamline, at CAMD, for constant initial state spectroscopy

The 3 m toroidal grating monochromator (TGM) VUV beamline at CAMD/LSU was realigned to achieve better illumination of the monochromator gratings with the goal of substantially increasing the flux at the higher photon energies. This is partly accomplished through a tilting of the monochromator (by about of 13.5°), with respect to the plane defined by the synchrotron, providing a smaller grazing angle at the initial mirrors. The improved performance of the beamline permits resonant photoemission studies at Gd 4d core threshold without resorting to second order light, which we demonstrate for Gd doped HfO 2.

Ionic yield and dissociation pathways from soft X-ray double-ionization of alcohols

We present data from inner shell excitation and double-ionization of alcohols of interstellar interest, methanol and ethanol at energies around C 1s edge. The measurements were taken at the Brazilian Synchrotron Light Laboratory (LNLS), employing soft X-ray photons (100–310 eV) from a toroidal grating monochromator (TGM) beamline. The experimental set up consists of a high vacuum chamber with a time-of-flight mass spectrometer TOF-MS. Mass spectra were obtained using coincidence techniques. Mean kinetic energies, abundances and dissociation pathways for each ionic pair in coincidence have been obtained from the analysis of the corresponding peak shapes and slopes observed in photoelectron photoion photoion (PE2PICO) coincidence mass spectra. The main dissociation channels of double ionized methanol were associated to the ions pairs H + + COH +, H + + CO +, H + + CH 2 + and H + + CH +. In the case of double ionized ethanol, the main dissociation channels were associated to the ions pairs CH 3 + + COH +, H + + COH +, H + + CH 2 + and H + + C 2H 2 +. About 40% and 25% of the dissociation channels of doubly ionized methanol and ethanol, respectively, lead to the production of reactant COH + ion, important for the formation of other organic molecules.

Photodissociation of organic molecules in star-forming regions

The presence of methyl alcohol or methanol (CH 3 OH) in several astrophysical environments has been characterized by its high abundance that depends on both the production late and the destruction rate. In the present work, the photoionization and photodissociation processes of methanol have been experimentally studied, employing soft X-ray photons (100-310 eV) from a toroidal grating monochromator (TGM) beamline of the Brazilian Synchrotron Light Laboratory (LNLS). Mass spectra were obtained using the photoelectron photoion coincidence (PEPICO) method. Kinetic energy distribution and abundances for each ionic fragment have been obtained from the analysis of the corresponding peak shapes in the mass spectra. Absolute photoionization and photodissociation cross sections were also determined. We have found, among the channels leading to ionization, about 11-16% of CH 3 OH survive the soft X-rays photons. This behavior, together with an efficient formation pathways, may be associated with the high column density observed in star-forming regions. The three main photodissociation pathways are represented by COH + (or HCO + ) ion release (with ejection of H 2 + H), the dissociation via C-O bond rupture (with strong charge retention preferentially on the methyl fragment) and the ejection of a single energetic (2-4 eV) proton. Since methanol is very abundant in star forming regions, the produced protons could be an alternative route to molecular hydrogenation or a trigger for secondary dissociation processes or even to promote extra heating of the environment.

Ionic yield and dissociation pathways from soft X-ray multi-ionization of acetic acid

We present data from inner shell excitation and multi-ionization of acetic acid with synchrotron radiation at at energies around C 1s edge. The measurements were taken at the Brazilian Synchrotron Light Laboratory (LNLS) and at center for advanced microstructures & devices (CAMD), employing soft X-ray photons (200–310 eV) from a toroidal grating monochromator (TGM) beamline. The main dissociation route were the secondary decay after deferred charge separation (SDDCS) process and deferred charge separation (DCS) processes. The main dissociation channels were associated to the ions pairs H + + C + , H + + CH + , H + + CH 2 + , H + + O + , H + + CO + and H + + COH + . About 70–80% of the dissociation channels leads to the production of energetic (3–7 eV) protons. The ejection of reactant radical COOH + was seen in about 7–10% of the channels.

Delivering high-purity vacuum ultraviolet photons at the Brazilian toroidal grating monochromator (TGM) beamline

Vacuum ultraviolet beamlines based on grating monochromators share the problem of high-order harmonics contamination. In the low-photon energy range up to 25 eV, this problem may be overcome by letting the light beam pass through a high-pressure gas region. Here the photons with energies higher than the gas ionization threshold are absorbed and dumped. We have recently proposed and demonstrated a simple and inexpensive type of mixed gas metallic film filter. We present here the design and results of a complete gas-phase harmonics filter based on a very efficient and affordable differential pumping system which currently works at a bending magnet beamline at LNLS. Besides the efficient depletion of high-order harmonics, a noble gas harmonic filter has additional advantages. It can also provide a precise photon energy calibration, as well as a means for determining the resolving power. The use of such a gas filter is very important for photochemistry and molecular dynamics where even a very small portion of high-energy photons can mask the fragments generated by the first order photons.

Photodissociation of organic molecules in star-forming regions

Fragments from organic molecule dissociation (such as reactive ions and radicals) can form interstellar complex molecules like amino acids. The goal of this work is to experimentally study photoionization and photodissociation processes of acetic acid (CH$_3$COOH), a glycine (NH$_2$CH$_2$COOH) precursor molecule, by soft X-ray photons. The measurements were taken at the Brazilian Synchrotron Light Laboratory (LNLS), employing soft X-ray photons from a toroidal grating monochromator (TGM) beamline (100 - 310 eV). Mass spectra were obtained using the photoelectron photoion coincidence (PEPICO) method. Kinetic energy distribution and abundances for each ionic fragment have been obtained from the analysis of the corresponding peak shapes in the mass spectra. Absolute photoionization and photodissociation cross sections were also determined. We have found, among the channels leading to ionization, that only 4-6% of CH$_3$COOH survive the strong ionization field. CH$_3$CO$^+$, COOH$^+$ and CH$_3^+$ ions are the main fragments, and the presence of the former may indicate that the production-destruction process of acetic acid in hot molecular cores (HMCs) could decrease the H$_2$O abundance since the net result of this process converts H$_2$O into OH + H$^+$. The COOH$^+$ ion plays an important role in ion-molecule reactions to form large biomolecules like glycine.

Espectroscopia de fotoelétrons de limiares de átomos e moléculas

A threshold photoelectron spectrometer applied to the study of atomic and molecular threshold photoionization processes is described. The spectrometer has been used in conjunction with a toroidal grating monochromator at the National Synchrotron Radiation Laboratory (LNLS), Brazil. It can be tuned to accept threshold electrons (< 20 meV) and work with a power resolution of 716 (~18 meV at 12 eV) with a high signal/noise ratio. The performance of this apparatus and some characteristics of the TGM (Toroidal Grating Monochromator) beam line of LNLS are described and discussed by means of argon, O2 and N2 threshold photoelectron spectra.

Photoelectron spectroscopy study on Li substituted NiO using PES beamline installed on Indus-1

Photoelectron spectroscopy beamline based on a toroidal grating monochromator (TGM) is recently commissioned on Indus-1 storage ring. It has been used to carry out valence band photoemission study of Li substituted NiO. In this paper initially a brief description of the beamline components and the experimental station for angle integrated photoemission experiment is presented. The later part of this paper is devoted to studies carried out on Li x Ni 1− x O with x=0.0, 0.35 and 0.5 samples. Thin pellets of polycrystalline samples were used for the measurements reported here. Valence band spectra recorded on polycrystalline Li x Ni 1− x O samples show drastic changes in various features as compared to that of pure NiO. The prominent changes are: (i) change in the relative contributions of Ni-3d and O-2p emissions, (ii) change in the peak position of Ni-3d from the top of the valance band of NiO and (iii) no noticeable change in the Ni satellite peak. These results are evaluated in terms of earlier findings in pure and low Li doped NiO.

CAMD 6-m toroidal grating monochromator beamline

The design and performance of a 6-m 20° deflection-angle toroidal grating monochromator (TGM) installed on the 1.5 GeV CAMD electron storage ring is described. A novel front-end optical configuration based on float-glass mirror technology enables effective use of 28 mrad of bending magnet radiation. Ray tracing is used to evaluate and optimize the theoretical resolution and transmission of the TGM. Experiments that characterize the CAMD bending magnet source parameters and evaluate the performance of the float-glass mirror are described. Alignment procedures and measurements that establish beamline performance (resolution and flux) are presented. The CAMD TGM covers 10–200 eV, delivers good flux at moderate resolution (∼2×10 12 photons/s in a 0.1% bandwidth) and achieves an ultimate resolving power of 2000.

A low energy monochromator beamline at ELETTRA: conceptual design and scientific aims

A beamline using a Low Energy Monochromator Optimized at Normal incidence (LEMON) has been designed to match the bending magnet radiation characteristics of ELETTRA, the third generation source in Trieste. The beamline will use 4×6 mrad 2 of the emitted radiation cone and will be equipped with two interchangeable monochromators: one Normal Incidence Monchromator (NIM) and one Toroidal Grating Monochromator (TGM), both sharing the same focusing optics, entrance and exit slits. The expected performance of the beamline will be a photon energy range from 5 to 200 eV, a resolving power ranging from 100 000 to 500 and a photon flux up to 1×10 11 ph/s/0.1BW on the sample.

VUV photoemission studies of candidate Large Hadron Collider vacuum chamber materials

In the context of future accelerators and, in particular, the beam vacuum of the Large Hadron Collider (LHC), a 27 km circumference proton collider to be built at CERN, VUV synchrotron radiation (SR) has been used to study both qualitatively and quantitatively candidate vacuum chamber materials. Emphasis is given to show that angle and energy resolved photoemission is an extremely powerful tool to address important issues relevant to the LHC, such as the emission of electrons that contributes to the creation of an electron cloud which may cause serious beam instabilities and unmanageable heat loads on the cryogenic system. Here we present not only the measured photoelectron yields from the proposed materials, prepared on an industrial scale, but also the energy and in some cases the angular dependence of the emitted electrons when excited with either a white light (WL) spectrum, simulating that in the arcs of the LHC, or monochromatic light in the photon energy range of interest. The effects on the materials examined of WL irradiation and /or ion sputtering, simulating the SR and ion bombardment expected in the LHC, were investigated. The studied samples exhibited significant modifications, in terms of electron emission, when exposed to the WL spectrum from the BESSY Toroidal Grating Monochromator beam line. Moreover, annealing and ion bombardment also induce substantial changes to the surface thereby indicating that such surfaces would not have a constant electron emission during machine operation. Such characteristics may be an important issue to define the surface properties of the LHC vacuum chamber material and are presented in detail for the various samples analyzed. It should be noted that all the measurements presented here were recorded at room temperature, whereas the majority of the LHC vacuum system will be maintained at temperatures below 20 K. The results cannot therefore be directly applied to these sections of the machine until measurements at cryogenic temperatures, i.e., in the presence of cryosorbed gas layers, are obtained. However, these results are directly relevant to all the warm regions of the LHC vacuum system, such as the experimental vacuum chambers and warm element vacuum chambers in the insertion regions.

Use of spherical gratings in a toroidal grating mount for vuv-soft x-ray synchrotron radiation

A toroidal grating monochromator (TGM) based beamline was designed earlier and it is being setup on the 450 MeV Indus-1 synchrotron radiation source. This beamline operates in 40–1000 A wavelength range and gives a moderate resolution (∼500) at high throughput. Now-a-days, spherical gratings are giving very high resolution in some complex monochromators. We have evaluated the use of spherical gratings in the simple rotation mount of a TGM. The motivation has been to evaluate if the existing TGM beamline, without any hardware changes, can be used with the much cheaper spherical gratings. The two monochromators, employing toroidal and spherical gratings respectively, have been compared by performing detailed ray tracing calculations. Our study shows that, with the same mechanical hardware, a real spherical grating monochromator gives better performance compared to the conventional TGM with respect to the spectral resolution and the flux. Furthermore, for moderate resolution this improvement is realized even when the exit slit is not moved to keep the spherical grating in-focus.

TGM-to-SGM conversion at NSLS beamline U7A: Recyclying of beamline components (abstract)

The U7A bending magnet toroidal grating monochromator (TGM) beamline has been converted into a spherical grating monochromator (SGM) type, without constructing any new vacuum components. That is, the mirror, slit, and grating chambers have been recycled. The inherited optical arrangement and the recycling concept prevented the use of ‘‘standard’’ SGM Kirkpatrick-Baez collecting and focusing mirrors at the front end. We show that this modification is not a serious detriment to the optical properties of the resulting beamline. Specifically, the resolution is completely unaffected and the throughput is approximately 2/3 of the ‘‘standard’’ SGM value. The flux and photon energy resolution of the U7A beamline throughout its 200–1000 eV operating photon energy range will be presented. Three end stations are being constructed for this beamline, featuring soft x-ray absorption and photoemission chambers optimized for operation in the carbon K-, oxygen K-, and transition metal L-edge ranges. Two of these end stations will be preceded by refocusing mirrors that will focus the soft x-rays to &amp;lt;1 mm spot size onto their samples.

Resonant Photoemission from Complex Cuprates and Nickelates

Synchrotron-excited resonant-photoemission measurements at rare-earth 4d --> 4f and transition-metal 3p --> 3d thresholds have been carried out using a variety of complex cuprates and nickelates on stations 6.1 (grazing-incidence monochromator) and 6.2 (toroidal-grating monochromator) at the SRS CLRC Daresbury Laboratory. The systems studied are Nd(2)Ni(1 - x)Cu(x)O(4), La(2 - x)Sr(x)Ni(1- y)Fe(y)O(4 + delta) and Bi(2)Sr(2)Ca(1 - x)Y(x)Cu(2)O(8 + delta). A combination of EDC and constant-initial-state data is used to examine the 4f and 3d contributions to the valence-band density of states and their binding-energy positions relative to the Fermi energy. This allows the study of the valence states of the transition-metal ions and their modulation on doping. For La(2 - x)Sr(x)Ni(1 - y)Fe(y)O(4 + delta), this approach is used to infer a valence state of >/= 3.0 for Fe. In the case of Bi(2)Sr(2)Ca(1 - x)Y(x)Cu(2)O(8 + delta), the effect of Cu valence modulation on the 3p resonance is observed as x is varied. This is discussed in the light of controversy surrounding shifts in core-level photoemission with doping for this system.

A system for magnetic circular dichroism measurements with a 10 m toroidal grating monochromator

A system of remotely-controlled apertures have been installed in the Synchrotron Radiation Center 10-m toroidal grating monochromator (TGM) for production of circularly-polarized light. In addition to allowing normal operation of the beamline, the apertures can be adjusted to block any fraction of the incoming beam without excessive heating and outgassing. This selective blocking of the light produces an appreciable elliptical polarization of the incoming light. Over the energy range of the 10 m TGM high energy grating, from 200 to 800 eV, the beamline optics do not significantly affect the polarization. The 10-m instrumental energy range overlaps the binding energies of the L-shell core levels of the important transition-metal materials. Calculations have been performed to estimate the performance of the aperture system as a function of energy. The results indicate that for an aperture position transmitting 20% of the synchrotron flux, the degree of polarization should be about 85% and should be reasonably constant over the photon energy range. With the light masked, magnetic circular dichroism was observed by measuring total yield with different orientations of sample magnetization and photon spin. Thin films of magnetic materials on magnetic and non-magnetic substrates have been studied.

High resolution in the soft x-ray range from a toroidal grating monochromator

A resolving power, E/ΔE, of ≥13 000 has been achieved with the modified 6 m/160° toroidal grating monochromator (TGM) installed on Beam Line 8-1 at the Stanford Synchrotron Radiation Laboratory. The resolving power of the TGM was increased by replacing the entrance and exit slits with high-precision slits, masking the horizontal part (short radius) of the grating, and improving the TGM scanning mechanisms. To determine the performance of the monochromator, we measured the dependences of resolution and photon flux on the entrance- and exit-slit widths, the exit-slit position, and the masking of the grating. The monochromator resolution in the energy range of 25–65 eV was derived from photoionization measurements of extremely narrow core-excitation resonances in He and Ne. With 10-μm vertical entrance- and exit-slit widths and 32% mask opening of the grating, the monochromator has a resolution (full-width at half maximum) of 5.0±0.7 meV at a photon energy of 64.5 eV and a flux of 2×107 photons/s/100 mA. The results suggest a simple procedure for converting a TGM with moderate resolution into a high-resolution monochromator with a moderate reduction in photon flux due to masking the grating, beyond the reduction attributable to the slit widths.

Experimentally determined resolution of a six meter toroidal grating monochromator

The resolution of a 6-m toroidal grating monochromator (TGM), installed at the University of Wisconsin Synchrotron Radiation Center (SRC), is experimentally determined. Narrow resonance lines of He, Ne, Ar, Kr and Xe in the 8–73 eV energy range were used to measure the resolution. The experimental resolution is compared with that obtained using a ray-tracing program SHADOW, in which theoretical mirror and grating specifications were used. Agreement between the experimental data and ray-tracing calculations is excellent. The effects of masking the grating and limiting the horizontal acceptance of the entrance and exit slits are also examined. Finally, it was found during the resolution measurements that the higher-order components contribute significantly to the amount of light obtained at a given grating position. For the low-energy grating, assuming only first- and second-order light, 70% of the light is first-order and 30% is second-order at a given grating position. For the medium energy grating, 80% of the light is first-order and 20% is second-order.