March 2004 NSLS-II Workshop: Inelastic X-ray Scattering Breakout Session

Peter Abbamonte welcomed the participants of the IXS-session and briefly described the purpose of the session to determine plans for beamlines and science at NSLS-II.

The first speaker was Kevin Smith (Boston University), who described Soft X-ray emission, Resonant Soft X-ray Scattering (RIXS), and Resonant Soft X-ray Emission (RSXE). The energy range of the incident and scattered photon is between 50 and 1000eV, but he is slowly moving also to lower photon energies. The bulk of his work is still done at beamline X1B at the NSLS X-ray ring, but some experiments have been tested at beamline U5U at the NSLS VUV ring.

The purpose of his experiments is to study the electronic structure of complex materials. So far, photoemission spectroscopy has been the main probe, but it has significant limitations in the sample quality. RIXS and SXE also determine the valence band structure, and are excellent tools to measure excitations across a gap (semiconductor, superconductor). The main disadvantage is the huge loss in intensity of about 6 orders of magnitude, which warrens the use of high-intensity beamlines at insertion devices (undulators).

The spectrometer consists of a grazing-incidence spherical grating with two collimating slits. The energy resolution is about 100meV at 500eV, which is just sufficient. Higher energy resolution, however, would result in a significant higher loss of intensity. The time to collect one spectrum of a typical sample takes about 40 minutes. The next generation of instrument, which is developed by Joseph Nordgren from Uppsala University, Sweden, uses first a parabolic mirror to collimate the radiation emitted by the sample. This trick should allow improving the energy resolution to less than 10meV at 100eV, which should be sufficient to resolve many-body effect. There might be a need for multiple analyzers to measure the q-dependence in just a few, selected systems.

Kevin then described the results of three different experiments based on SXE, RIXS, and RSXE. He used SXE to study the bandgap in AlxGa1-xN as a function of x. He measured the N or Ga pDOS separately, and even observed Ga-N hybridization, which was evident from Ga-emission at the N-edge. Using RIXS, he studied dd-excitations and charge-transfer excitations in Cr-doped V₂O₃, but he observed also some additional transitions, which cannot be explained with existing models. He then explained experiments using RSXE, which he used to determine the pDOS for specific lattice positions. He could clearly resolve two small peaks at the C K-edge in copper phthalocyanine, which were never observed with photoemission spectroscopy. Theory, however, predicts that both exist.

Kevin finally discussed radiation damage in his samples, which requires continuous translation of the sample. In some samples, it is almost negligible, while in others, mostly carbon-compounds, it is a serious problem. He concluded his talk with a suggestion for the insertion device installed for soft x-ray scattering, which should be an EPU to switch the polarization of the incident beam. The frequency of switching does not have to be high, since he wants to align the polarization of the incident beam with different crystallographic directions in the sample.

The next presentation was an introduction for the hard x-ray beamlines. Boris Podobedov (NSLS) described the lattice and insertion devices, which are planned for the IXS beamlines. The plan, so far, is a symmetric lattice with identical length of all insertion devices. IXS is definitely a candidate for a longer straight section. With the present technology, the ideal insertion device is an MGU10 (mini-gap undulator with a period of 10 mm), but if the technology improves, it will be an SCU14 (superconducting undulator with a period of 14 mm). Another interesting device, especially for resonant scattering, might be a quasi-periodic undulator, which shifts the peak of higher harmonic away from the monochromator harmonics. The disadvantage is a loss in intensity.

The first presentation on hard x-ray inelastic scattering was given by Harald Sinn from the Advance Photon Source. He presented the present high-resolution IXS beamline at the APS, and gave some ideas for a complimentary instrument at the NSLS. The present instrument utilizes an artificial channel-cut monochromator, which is tunable from 21.5-21.7 keV with an energy resolution of 1.3 meV. Another device reaches 0.9 meV at 9 keV. The flux is about 10¹⁰ photons/s. The analyzer consists of diced wafers glued into spherical glass mirrors. The depth of the dicing cuts is several millimeters. At the moment, they use 4 analyzers at different momentum transfers.

The science at the beamline concentrates on the dispersion of phonons in systems that cannot be investigated with inelastic neutron scattering, which has been the standard probe to measure S(q,?). Due to the relation between the neutron momentum and its energy, it is not possible to use neutrons to measure v_liquid.

Harald's first example demonstrated the capability of IXS to measure the phonon-dispersion in liquid sapphire between 2000°C and 3000°C. These measurements allowed determining the viscosity and the speed of sound. The low resolution (1.8 meV), though, limited the maximum value of the viscosity. The next sample, MgB₂, was extremely small, which is also not an advantage for inelastic neutron scattering. For that experiment, he lowered the energy resolution to 6 meV to get as many phonons as possible. The last example, PuGa, was also a small sample; furthermore, Pu is not really well suited for neutron scattering.

Harald then showed some possibilities for hard IXS at NSLS-II and instrumentation. Since NSLS-II cannot compete with the APS with flux between 15 and 20keV with present technology, research at NSLS-II should concentrate on the energy range below 15keV. One possibility is to install a 5 m spectrometer in vertical scattering geometry, which would be an excellent complementary instrument to the planned APS HERIX instrument in horizontal geometry. This will allow utilizing different polarization dependence of the incident and scattered photons. Another advantage of NSLS-II is the smaller source, which will allow focusing the beam to a smaller spot, which is important to study phonons in micro crystals. He concluded his talk with the emphasis to start the development of new analyzers and their reproducible production NOW.

Yuri Shvyd'ko recently came up with an idea to get high energy resolution at lower energies (2 meV at 10 keV), but this idea has not been tested yet. The development of new techniques requires the collaboration of several groups over several years.

Uwe Bergmann from Stanford Synchrotron Radiation Center gave an overview of IXS with medium energy resolution (0.1-1 eV). His first examples showed (non-resonant) X-ray Raman Scattering (XRS) data measured at the O K-edge of water. Uwe used XRS to measure the EXAFS of water as a function of temperature (ice, water at room temperature, and water just below the boiling point). The data analysis shows that the water molecules form ring-like configurations with about 2.1 hydrogen bonds per molecule at room temperature. This value drops to 2.0 hydrogen bonds at 90°C. Monte-Carlo calculations reflect this trend with values of 3.5 and 3.3, respectively. These findings were quite a surprise for many researchers in the water-community.

His next example showed the O XANES in supercritical water, which surprisingly is very similar to the gas-phase spectrum of water, which has already been measured with conventional soft x-ray absorption spectroscopy. The present interpretation is that several Rydberg-levels are observed, although there is still some discussion weather these might be vibrational excitations.

Uwe then showed the utilization of resonant inelastic x-ray scattering (RIXS) to study the structure of photosystem II at different stages in its process to combine water and CO₂ to create oxygen and sugar. This process involves the absorption of four photons in the optical spectrum. The molecule (photosystem II) has four Mn ions, which are the active site, and which play a crucial role in the whole process. Conventional XANES shows subtle changes in the Mn K-edge in the different stages, but RIXS can clearly separate these transitions, which are attributed to de-localized electrons.

The time to collect one spectrum with decent statistical error is about 5 h, but the sample has to be changed after about 2 h due to radiation damage, despite a continuous translation of the sample in the beam. This situation will worsen for NSLS-II, but a higher flux will still improve the overall situation, since IXS needs as many phonons as possible.

John Hill then collected contributions from the audience for IXS at NSLS-II.

Everybody agreed that three different beamlines are required, one for soft IXS, one for medium energy resolution IXS (100 meV), and one for high-energy resolution IXS (1 meV). The hard x-ray beamlines will significantly benefit from a longer straight section than the planned 4m-long sections. The hard x-ray community also agreed that research and development for analyzers with an energy resolution of 1 meV between 8 and 10 keV should start right now, since Yuri Shvyd'ko cannot do everything by himself. R&D has to be devoted to Si-drift array detectors for large analyzer arrays as well. Furthermore, analyzer crystals in general have to be improved to improve the efficiency and to reduce the counting time.

The low-resolution beamline should be separated into two different stations, one for q-dependent studies, and one with several analyzers to investigate low-concentration samples. The hutches for these programs have to be large in order to get a decent energy resolution.

The main advantage of IXS at the NSLS-II is the small source-size, which allows focusing the beam into a smaller spot than at the APS. For micro-focused experiments, NSLS-II can compete with the APS in the energy range above 15keV.

Radiation damage issues require continuous translation of the sample in several cases. When the incident energy has to be scanned, continuous monochromator scans like in QEXAFS are useful, which require simultaneous change of the undulator gap (or of the current in a superconducting undulator). This mode of operation is already available at a few APS beamlines.

The sample environment will cover all areas of extreme conditions like high and low temperatures, high magnetic fields, and high pressure.

From a theoretical point of view, IXS is the ideal technique to study S(q,).

Last Modified: January 31, 2008
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