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Synchrotron radiation (SR) contains a broad spectrum of high-energy X- rays, and the brilliance and focus that dedicated synchrotron light sources can achieve is much greater than what is attainable with laboratory X- ray sources. The high flux and brightness allow good quality XAS data to be obtained on a much shorter timescale than with bench-top X-ray spectrometers. Beamlines of third-generation synchrotrons that host XAS experiments, such as B18 at the Diamond Light Source, use high-precision optical equipment to provide a tuneable monochromatic X-ray beam of excellent focus and energy resolution. Hence, X-ray absorption spectroscopy benefits greatly from the use of synchrotron light.

As an introduction to the synchrotron XAS experiments that are presen- ted in Chapter 3 of this thesis, the following sections will briefly describe the basic theory of XAS and the production of synchrotron radiation.

1.4.1

Synchrotron Radiation

Synchrotron radiation (SR) was discovered as a by-product that occurred in particle accelerators. Purpose-built 3rd-generation synchrotrons such as Diamond Light Source generate synchrotron light of a broad energy spec- trum that ranges from infrared (IR), visible (vis), and ultraviolet (UV) light up to the energy of X-rays.

When charged particles (e.g. electrons) travel at almost the speed of light, their energy increases as a function of their velocity. Upon entering a magnetic field which forces the travelling high-energy electrons to alter their flight path, this change in direction slows the electrons down, they

loose kinetic energy. This energy is emitted in the form of photons, and these constitute synchrotron radiation. The phenomenon is well described (quite literally) by the German word “Bremsstrahlung”.

Photons are non-charged particles, and as such they are unaffected by the magnetic field that bends the electron path. As a result, the photon beam propagates along the original trajectory of the parent electrons at the point their path was bent.

1.4.2

The Production of Synchrotron Light

A synchrotron light source (Figure 1.15) accelerates a beam of electrons by directing it around a ring of bending magnets, known as the storage ring. Figure 1.15 summarises the constituents of a typical synchrotron, navigating through the path of the beam by starting from the electron gun (a) and ending in the experimental hutch of a beamline (e), where it can be used for X-ray spectroscopy.

The path of the beam starts at the heart of the synchrotron, at the electron gun (a) which emits thermionic high-energy electrons. Those are linearly accelerated (b) directly into the booster synchrotron (c). Here, acceleration to near speed of light increases the beam energy to 3.0 GeV. The main synchrotron (d), i.e. the storage ring, receives regular injections of electrons to maintain the beam energy as well as a stable current and flux. Each bending magnet represents a corner of the ring at which insertion devices such as wigglers or undulators produce additional “Bremsstrahlung” that exits the storage ring into a beamline (e). The optics set-up inside each beamline uses collimating and focussing mirrors to immediately refocus (and filter) the synchrotron radiation for its respective experimental use.

Figure 1.15: Operating scheme of a Synchrotron Light Source: (a) electron gun produces high-energy electrons, (b) the linear accelerator and (c) booster synchrotron accelerate electrons to approx. speed of light, (d) the storage ring maintains the electron beam. Insertion devices produce synchrotron radi- ation that exits the storage ring, (e) beamlines re-focuss and filter the SR for experimental use.

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1.4.3

X-ray Absorption Spectroscopy

The Beer − Lambert law (Equation 1.1) describes the attenuation of light that has the energy E and intensity I0 by a sample of the length d. Equa-

tion 1.1 states that the intensity of transmitted light It depends on the

molar absorption coefficientμ(E) of the material as well as the sample thick-

ness.86, 87

It=I0· e−µ(E)·d (1.1)

X-ray absorption spectroscopy measures the element specific absorption coefficient of a material whose core-electrons are excited by light in the energy range of X-rays (2-35 keV). Figure 1.16 depicts a Ru K-edge XAS

Figure 1.16: A Ru K-edge X-ray absorption spectrum, showing the edge posi- tion E0and the two main spectral regions of interest: X-ray absorption near-edge

structure (XANES) around ± 30 eV of E0 and the extended X-ray absorp-

tion fine-structure (EXAFS), consisting of the higher-energy oscillations beyond E0 + 50 eV. Inset shows expansion of XANES region.

spectrum.

During the experiment, the energy of the incident X-ray beam sweeps from lower to higher values within the range of a few hundred eV around the ionisation energy of a core-electron level of the respective element. When the incident monochromatic X-ray beam has sufficient energy to eject a core electron from the K-shell, this electron is being excited to a higher orbital. At this energy, an abrupt increase in the absorption coefficient µ(E) is visible in the spectrum. This dominant feature is known as absorption edge or rising edge. The inflection point of the absorption edge is used to determ- ine the edge energy E0.88, 89 Spectral features within E0 ±50 eV are referred

spectroscopy (XANES).90 _{This region can contain useful information about}

the electronic structure of metal-ligand interactions.91 _{In particular, ligand}

K-edge XANES is an increasingly popular tool, for example in the study of proteins that contain spectroscopically silent metals,92 _{but can also be used}

for the study of ligand binding in organometallic anticancer complexes.93 The extended X-ray absorption fine-structure (EXAFS) comprise of the high-energy oscillations in the XAS spectrum beyond E0+ 50 eV. These are

visible modulations to an otherwise linearly decaying absorptivity, and are caused by electron-scattering within the coordination sphere of the central absorber whose core-electron has been ejected. The EXAFS oscillations therefore hold specific information about the atomic structure of a material that can be extracted by careful theoretical modelling of the experimental data.

Chapter 3 presents XAS studies of novel arene ruthenium TSC com- plexes. It describes the procedure and results of EXAFS fitting of their first coordination sphere, which elucidates Ru-TSC coordination in both solid state and solution.