Beamline Technique 01: Low-Energy Spectroscopy

OTHER DESIGNATIONS: Vacuum-ultraviolet (VUV) spectroscopy, photoelectron spectroscopy (PES), angle-resolved photoelectron spectroscopy (ARPES), photon-ion spectroscopy, infrared (IR) spectroscopy, terahertz (THz) spectroscopy, ultraviolet photoemission spectroscopy (UPS), cold-target recoil-ion momentum spectroscopy (COLTRIMS), photoelectron-photoion coincidence (PEPICO) spectroscopy, Fourier transform infrared (FTIR) spectroscopy.

PURPOSE: Probes that use the VUV region of the spectrum (10–100 eV) are very well matched to the elucidation of bonding in solids, surfaces, and molecules; to the investigation of electron–electron correlations in solids, atoms, and ions; and to the study of reaction pathways in chemical dynamics. At the lowest end of this energy range (below 1 eV) we have infrared, far-infrared, and terahertz spectroscopies, which are well matched to vibrational modes and other modes of excitation. Problems addressed include:

Complex materials
Surfaces, clusters
Atomic and molecular physics, astrophysics
Combustion, chemical dynamics
Biological systems

HOW THE TECHNIQUE WORKS: VUV spectroscopy is not a single technique but a myriad of techniques. The sample of interest, either solid or gaseous, is illuminated with light and the various product particles (electrons, ions, or fluorescent photons) are detected and analyzed. In ARPES experiments, for example, the sample is a single crystal having a clean, well-characterized surface. The directions and energies of the emitted photoelectrons are measured. Analysis of this data yields incisive information on the way electrons move within the sample. In gas-phase experiments, the detected particles can be electrons, ions, molecules, or clusters. Auxiliary techniques include mass spectrometry, time-of-flight and coincidence detection.

UNIQUENESS: The universal demand in this area is for a high signal rate with very high resolving power. The high brightness and small spot size achievable with synchrotron radiation, particularly at third-generation sources, permits the design and operation of very advanced monochromators, spectrometers, and electron-energy analyzers.

EXAMPLES:

Electron Waves in the Fermi Sea
Understanding the Fundamental Processes in Combustion
Dimensional Crossover in Layered Strongly Correlated Metals
Spin Interactions in Magnetic Oxides
Electron Excitations At-A-Glance

Electron Waves in the Fermi Sea

The momentum distribution of electrons at the Fermi energy in a two-dimensional metal.

The electrons that conduct electricity in metals and semiconductors are important because they determine all of the major properties of conductors: not only those that are well understood but also those of the more exotic materials such as the high-temperature superconductors. Recent angle-resolved photoelectron spectroscopy (ARPES) experiments on a system comprising a monolayer of indium on a silicon substrate display the properties of a “nearly-free-electron” two-dimensional metal. If the electrons were perfectly free, those with energies near the Fermi energy would reside around a single perfect circle in momentum space. It is seen from the figure that the basic circle is repeated and folded back on itself in a highly convoluted way. This is because the electron waves are lapping against the background atomic lattice. From this kind of information one can deduce the way electrons move in solids.

E. Rotenberg, H. Koh, K. Rossnagel, H.W. Yeom, J. Schäfer, B. Krenzer, M.P. Rocha, and S.D. Kevan, “Indium √ 7 × √3 on Si(111): A nearly free electron metal in two dimensions,” Phys. Rev. Lett. 91, 246404 (2003).

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Understanding the Fundamental Processes in Combustion

Laminar-flow burner assembly.

Combustion seems to be well understood in terms of average energy output, high-concentration intermediates, and major products. However, an understanding of flame chemistry is required for global models of combustion and also for controls relevant to emissions. Because of the complexity of the fluid dynamics of a “real” flame and the highly reactive nature of chemical states, many important rate constants have never been measured directly, nor have all the species included in theoretical models been directly observed. Scientists have developed a laminar-flow burner assembly that allows the real-time monitoring of the processes in an actual flame. It also permits the introduction of dopants so that the changes in chemistry that they produce can be studied. In these flames, the temperature and concentration profiles can be mapped to very high precision, a consequence of the laminar flow and the low-pressure conditions that make the flame reaction zone much larger than under atmospheric conditions. These benefits result in a machine with both increased sensitivity and near-universal selectivity. Combined with high-flux, continuous vacuum-ultraviolet (VUV) radiation, this machine offers the sensitivity and flexibility needed to study the complexities of combustion.

T.A. Cool, K. Nakajima, T.A. Mostefaoui, F. Qi, A. McIlroy, P.R. Westmoreland, M.E. Law, L. Poisson, D.S. Peterka, and M. Ahmed, “Selective detection of isomers with photoionization mass spectrometry for studies of hydrocarbon flames,” J. Chem. Phys. 119, 8356 (2003).

[Back to top] Dimensional Crossover in Layered Strongly Correlated Metals
Comparison ARPES spectra at two different temperatures.		Many new materials consist of one- or two-dimensional building blocks, loosely connected into a three-dimensional whole, resulting in highly anisotropic physical properties. Small interactions between the building blocks may induce a whole variety of unusual transitions. One of the most intriguing is a “dimensional crossover” in layered metals where, in the direction perpendicular to the layers, transport properties change from insulator-like at high temperatures to metallic-like at low temperatures, while remaining metallic over the whole temperature range in the direction parallel to the layers. Scientists examined two such layered metallic systems with angle-resolved photoemission spectroscopy (ARPES) and electronic transport measurements. They found a crossover in the number of effective dimensions — from two to three — with decreasing temperature. Within the conventional theories, this phenomenon is hard to understand. Recent photoemission experiments have shown that this change in effective dimensionality correlates with changes in the electronic spectra: sharp features associated with “quasiparticle-like excitations” only exist in the low-temperature three-dimensional-like phase, while they are absent in the effectively two-dimensional, high-temperature phase. T. Valla, P.D. Johnson, Z. Yusof, B. Wells, Q. Li, S.M. Loureiro, R.J. Cava, M. Mikami Y. Mori, M. Yoshimura, and T. Sasaki, “Coherence-incoherence and dimensional crossover in layered strongly correlated metals,” Nature (London) 417, 627 (2002).

[Back to top] Spin Interactions in Magnetic Oxides
Schematic representation of the spins (red and cyan cones) at the manganese sites of LaMnO₃, showing their antiferromagnetic orientation.		The parent compound for many of the so-called “colossal magnetoresistance” (CMR) materials is lanthanum manganese oxide (LaMnO₃). These CMR materials have been instrumental in enabling recent advances in magnetic storage devices, yet their physical properties are yet to be fully understood. LaMnO₃ is an antiferromagnet, meaning that adjacent spins are aligned in opposite directions. When placed in the field of a large laboratory magnet, the spin of an electron can resonantly flip between two spin states. This resonance occurs at a characteristic frequency, usually in the microwave portion of the spectrum. If a stronger field is available, the resonance can be shifted into the terahertz or far-infrared spectral range. With this in mind, a group of researchers has developed an electron spin resonance (ESR) system using a high-field magnet and Fourier-transform infrared (FTIR) spectrometer system. With this system, spin resonance can be studied as a function of both magnetic field and frequency. The sharpness of the resonance is an indicator of how strongly the spin interacts with other excitations in the system. This new ESR system provides a method for measuring the resonance lifetime without sweeping the field (which can cause the resonance to change). Terahertz light from a synchrotron source provides the high power and brightness required to pass through the magnet’s narrow windows. These studies of LaMnO₃ explain how the spins are “canted” relative to the crystal structure, and provide values for the interaction strength between different spins in the material. D. Talbayev, L. Mihaly, and J. Zhou, “Antiferromagnetic resonance in LaMnO₃ at low temperature,” Phys. Rev. Lett. 93, 017202 (2004).

[Back to top] Electron Excitations At-A-Glance
Two-dimensional spectrum of hydrogen chloride.		Important questions in many areas of science and technology often hinge upon how atoms, molecules, and ions behave in a specific environment. For example, detailed knowledge of the electronic structure of solids must be built upon a thorough understanding of isolated atomic and molecular systems. With high-brightness synchrotron light and advanced data-acquisition technology, researchers have been able to record a complete, angle-resolved, two-dimensional picture of electron emission from a diatomic molecule, hydrogen chloride, over a comprehensive energy range. By plotting the energy of the incoming photons on the vertical axis and the kinetic energy of the ejected electrons on the horizontal axis, scientists can study the interplay between various excitations and photoemission processes as the photon energy is scanned across an ionization threshold. The locations and intensities of emission lines identify processes such as excitation to unoccupied molecular orbitals, excitation to Rydberg orbitals, and ionization into the continuum. Diagonal lines indicate the absorption of x-rays by electrons in outer (valence) molecular orbitals rather than deep inside the chlorine atom. E. Kukk, A. Wells, N. Berrah, B. Langer, J.D. Bozek, O. Nayandin, M. Alshehri, A. Farhat, and D. Cubaynes, “Angle-resolved two-dimensional mapping of electron emission following Cl 2p excitations in the HCl molecule,” Phys. Rev. A 57, R1485 (1998).

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