Synchrotron Radiation


Overview

Synchrotron radiation is emitted by electrons orbiting in a storage ring. It provides ultra-bright ultraviolet light and soft X-rays which are used for a wide range of analytical techniques. As surface scientists we are primarily interested in finding out how atoms are bonded at a surface and how their electrons determine the bonding. Analyzing photoelectrons ejected from a surface by soft X-rays provides chemical analysis with optimum surface sensitivity because the photoelectrons are emitted at their minimum escape depth (only a few atomic layers deep). The energy spectrum of photoelectrons reflects the energy levels of surface atoms, which identify an atom and its neighbors via chemical shifts.

Overview of photoelectron spectroscopy: F.J. Himpsel and I. Lindau, Ch. 9, Photoemission and Photoelectron Spectra, Encyclopedia of Applied Spectroscopy, Edited by David L. Andrews, Wiley-VCH, (Weinheim 2009). ISBN: 978-3-527-40773-6.


Undulators

Undulators are periodic magnet structures that represent the state of the art in producing synchrotron radiation. They exhibit a spectral brilliance up to 10,000 times higher than conventional bending magnets. Undulators at the Advanced Light Source (ALS) cover the spectral range from 100-1000 eV photon energy, which contains the sharpest core level of each element. Undulators at the Synchrotron Radiation Center (SRC) are optimized for the 10-100 eV energy range, which represents the best energies for exciting the valence electrons.


Display Spectrometer

A special photoelectron analyzer displays an image of the photoelectrons emitted in various directions. With the high brightness at an undulator at the ALS such images take only a few seconds to acquire. That makes it possible to quickly explore electrons emitted in all directions and thereby obtain a complete picture of electrons in solids and at surfaces.

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Emission pattern of photoelectrons from the (111) surface of diamond, measured in two seconds at the ALS.

The most common method for describing electrons in solids is local density theory. The 1998 Nobel Prize in Chemistry was awarded for the development of this method. It works so well that it has been difficult to test its limits and to find ways to take its accuracy to the next level. As a fundamental characteristic of the electrons one can choose their band width, which reflects the strength of their interactions. This number can be obtained from a series of images of photoelectrons emitted by synchrotron radiation at different energies, such as those shown below. These data from diamond achieve 1% accuracy, ten times better than previous experiments. It is found that the band width is 7% too small in local density theory, but well-described by quasiparticle calculation, the latest theory of electrons in solids. This type of measurement has been performed for protopes of an insulator (lithium fluoride), a semiconductor (diamond), and a semi-metal (graphite).

Examples:
LiF:  Himpsel et al., Phys. Rev. Lett. 68, 3611 (1992);
Diamond:  Jimenez et al., Phys. Rev. B 56, 7215 (1997);
Graphite:  Heske et al., Phys. Rev. B 59, 4680 (1999).

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Images of photoelectrons from diamond at various energies below the top of the valence band (in eV).


Mapping of Electrons at the Fermi Level

Many properties of a solid are determined by electrons close to the Fermi level, which represents the highest occupied energy level. Examples are the electrical conductivity, magnetoresistance, superconductivity, and magnetism. Only a narrow energy slice around the Fermi level is relevant for these properties. The width of this slice is about 3.5 kT, corresponding to the states that are thermally accessible (kT=25 meV at room temperature). In recent years the resolution of photoemission experiments has improved such that electrons within a few meV around Fermi level can be distinguished. Angle-resolved photoemission provides the information about the momentum of these electrons, which is not accessible by measuring conductivity and magnetism directly.

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Energy bands of nickel close to the Fermi level, measured with high-resolution at the SRC. The magnetic splitting between spin up and spin down electrons is clearly resolved near the Fermi level (Energy = 0). The slope of the dark lines corresponds to the electron velocity. From Altmann et al., Phys. Rev. Lett. 87, 137201 (2001).


Photon in, Photon out

Synchrotron light sources provide tunable X-rays and thereby enable X-ray absorption spectroscopy (XAS). This technique measures excitations from the core level of a specific atom into its empty valence orbitals. As the resulting core hole gets re-filled, the atom emits either electrons or X-rays. Electrons dominate for shallow core levels, which are the sharpest. The characterisctic fluorescence photons emitted by deeper (but broader) core levels are needed for detecting dilute atoms, such as in catalysts or for environmental contaminants. XAS of the sharpest core levels is difficult with fluorescence detection because of the small cross section, but it has become possible with the leap in spectral brilliance created by undulators. The spectral range of the ALS is suited particularly well for semiconductors, organic materials, biomolecules, and magnetic materials (see the table of the sharpest core levels of the elements and their binding energies).

XAS has been used for a wide variety of applications. A few samples from Beamline 8.0 at the ALS are given below. A bendable upstream mirror allows a wide range of focal points located at various endstations. For radiation-sensitive organic and bio-materials the light can be defocused as well. Various photon detectors allow for an optimized trade-off between spectral selectivity and detection efficiency. The nearby Molecular Foundry provides many capabilities for synthesizing nanostructures, as well as for identifying orbitals by state-of-the-art calculations of XAS spectra.

Examples:
Buried layer of B dopants in silicon:   Carlisle et al., Appl. Phys. Lett. 67, 34 (1995)
Magnetic nanocrystals protected by organic molecules:   Perez-Dieste et al., Appl. Phys. Lett. 83, 5053 (2003)
Orientation of DNA molecules attached to a surface:   Petrovykh et al., J. Am. Chem. Soc. 128, 2 (2006)
Protein immobilized at a surface:   Liu et al., Langmuir 22, 7719 (2006)
Dyes for biomimetic solar cells:   Cook et al., J. Chem. Phys. 131, 194701 (2009).
Atom-specific orbitals in a donor-absorber-acceptor complex:   Zegkinoglou et al., J. Phys. Chem. C 117, 13357 (2013).

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Two endstations in a row for photon in / photon out spectroscopy of nanostructures at the ALS.


Supported by NSF-DMR, NSF-CHE, and DOE-BES.
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