series of smaller dipole magnets yielding the multibend achromat lattice. Brilliance
is defined as the number of photons per 0.1% bandwidth, per second, per surface
of the source and per horizontal and vertical angular divergences of the source.
Reducing the size of the source was an efficient way to increase the brilliance.
Brilliance is a driving parameter for experiments where the photon source is
focused on the sample and in such cases, the increase of brilliance is fully related to
an increase of useful photons. It should nevertheless be kept in mind that for
experiments where the beam is not refocused on the sample, the gain in brilliance is
not accompanied by a high gain in useful photons: this is indeed what is observed
for experiments where the figure of merit is the flux at the sample position (the flux,
expressed in J m
À2 s
À1 , is the energy of all the photons crossing a surface unit per
second).
As a consequence of increasing the brilliance, the new X-ray beams of the
diffraction-limited storage rings have a much higher level of coherence since one
estimates that a source with size r and divergence r' has a 50% coherence for k
such that rr' ¼ k=4p. Not regarding the controversy between brilliance and flux,
the reduction of the emittance in the horizontal plane opens new perspectives for
experiments taking advantage of coherence such as X-ray phase-contrast imaging,
ptychography, Fourier transform holography, X-ray photon-correlation spectroscopy, coherent diffractive imaging, or fluctuation microscopy.
Chapter 1, “X-ray sources at large scale facilities”, gives a brief introduction to
the physics of storage rings with special attention to the physics of electron
accelerators and to the devices developed to produce X-rays: bending magnets,
wigglers, undulators. One can notice an introduction to the diffraction-limited
storage rings that are presently blooming in Lund (MAX-IV) or Grenoble (upgraded ESRF) and that will replace the old, common structure of conventional
storage rings. The X-ray free-electron lasers in conjunction with the self-amplified
spontaneous emission of X-rays are also presented and the characteristics of theses
new X-ray beams are described so that it is accessible to most, future potential
users. Based on the knowledge of the X-ray beams, Chap. 3, “Electronic structure
theory for X-ray absorption and photoemission spectroscopy” presents the connection between the electronic structure of matter and core-hole spectroscopies such
as X-ray absorption spectroscopy and X-ray photoemission spectroscopy. I would
like to attract the attention of the reader concerning the recently developed, state
of the art pieces of theory that go beyond the common density functional theory
(DFT). The time-dependent density functional theory, the Green's function
approach (with special care to treatment of the self-energy within the GW
approximation), or the Bethe–Salpeter equation are presented in a concise way so
that the reader interested in learning beyond the DFT can get a flavour of these
actual topics, still under development. Chapter 4, “X-ray dichroisms in spherical
tensor and Green’s function formalism”, calculates the various types of angular
dependence present in XAS. It first starts with a presentation of the multi-electronic,
many-body calculation of the X-ray absorption cross sections. Then connections are
made with the atomic multi-electronic approach as it is developed in the ligand-field
viii
Foreword
is defined as the number of photons per 0.1% bandwidth, per second, per surface
of the source and per horizontal and vertical angular divergences of the source.
Reducing the size of the source was an efficient way to increase the brilliance.
Brilliance is a driving parameter for experiments where the photon source is
focused on the sample and in such cases, the increase of brilliance is fully related to
an increase of useful photons. It should nevertheless be kept in mind that for
experiments where the beam is not refocused on the sample, the gain in brilliance is
not accompanied by a high gain in useful photons: this is indeed what is observed
for experiments where the figure of merit is the flux at the sample position (the flux,
expressed in J m
À2 s
À1 , is the energy of all the photons crossing a surface unit per
second).
As a consequence of increasing the brilliance, the new X-ray beams of the
diffraction-limited storage rings have a much higher level of coherence since one
estimates that a source with size r and divergence r' has a 50% coherence for k
such that rr' ¼ k=4p. Not regarding the controversy between brilliance and flux,
the reduction of the emittance in the horizontal plane opens new perspectives for
experiments taking advantage of coherence such as X-ray phase-contrast imaging,
ptychography, Fourier transform holography, X-ray photon-correlation spectroscopy, coherent diffractive imaging, or fluctuation microscopy.
Chapter 1, “X-ray sources at large scale facilities”, gives a brief introduction to
the physics of storage rings with special attention to the physics of electron
accelerators and to the devices developed to produce X-rays: bending magnets,
wigglers, undulators. One can notice an introduction to the diffraction-limited
storage rings that are presently blooming in Lund (MAX-IV) or Grenoble (upgraded ESRF) and that will replace the old, common structure of conventional
storage rings. The X-ray free-electron lasers in conjunction with the self-amplified
spontaneous emission of X-rays are also presented and the characteristics of theses
new X-ray beams are described so that it is accessible to most, future potential
users. Based on the knowledge of the X-ray beams, Chap. 3, “Electronic structure
theory for X-ray absorption and photoemission spectroscopy” presents the connection between the electronic structure of matter and core-hole spectroscopies such
as X-ray absorption spectroscopy and X-ray photoemission spectroscopy. I would
like to attract the attention of the reader concerning the recently developed, state
of the art pieces of theory that go beyond the common density functional theory
(DFT). The time-dependent density functional theory, the Green's function
approach (with special care to treatment of the self-energy within the GW
approximation), or the Bethe–Salpeter equation are presented in a concise way so
that the reader interested in learning beyond the DFT can get a flavour of these
actual topics, still under development. Chapter 4, “X-ray dichroisms in spherical
tensor and Green’s function formalism”, calculates the various types of angular
dependence present in XAS. It first starts with a presentation of the multi-electronic,
many-body calculation of the X-ray absorption cross sections. Then connections are
made with the atomic multi-electronic approach as it is developed in the ligand-field
viii
Foreword