BL practice of Cheiron2014 @ BL19LXU

Real-time observation of photoinduced valence-transitions in mixedvalence compounds by means of hard X-ray photoelectron spectroscopy
Ashish Chainani (chainani@spring8.or.jp) & Masaki Oura (oura@spring8.or.jp)
Research Infrastructure Group, Advanced Photon Technology Division,
RIKEN SPring-8 Center
ABSTRACT :It is well known that hard X-ray photoelectron spectroscopy (HAXPES) is one of themost powerful methods to investigate the bulk electronic structure of condensed matter. In
order to extend its capability for studying transient electronic states in the time-scale fromfemtosecond to subnanosecond regimes, we have been developing a time-resolved
HAXPES system at the ultrashort high-brilliance X-ray sources, such as the 27m-longundulator beamline BL19LXU of SPring-8 and the X-ray free electron laser facility
SACLA.
In the practical course, participants will learn the principle of HAXPES and measure thecore-level photoelectron as well as Auger electron spectra of target material. Furthermore,with the use of femtosecond optical laser, we will try to perform the real-time observationof photoinduced valence-transitions in mixed-valence compounds by HAXPES.
Participants will learn how we can realize time-resolved HAXPES to study the ultrafast
dynamics of electronic structure in condensed matter.
1. Introduction
1.1 Photoelectron Spectroscopy (PES)
Photoelectron Spectroscopy (PES) is one of the most important techniques to study theelectronic structure of materials [1]. The electronic structure of a material describes theenergies of electrons in a material. The energy of electrons in an isolated atom get
modified when the atoms get arranged into a solid or a liquid, or even a gaseous molecule,but they can still be described in terms of their energy levels in an atom. The type ofelectrons (s, p, d, f), the number of a particular type of electron in an atom, and the energy
the electrons possess in a material, determine the chemical and physical properties of amaterial. For example, the energy distribution of electrons of a particular atom existing in
an insulator (such as copper atoms in cupric oxide, CuO) and in a metal (such as copper
atoms in copper) are different, although they originate from the same type of atoms,
namely, copper atoms. This difference in energy distribution is directly related to thevalency of copper in copper metal (Cu0) as compared to its valency in insulating cupricoxide (Cu2+). The electronic energy levels of a material can also depend on very small
amount of changes in its chemical composition. This is appropriately described by thechange in properties of an intrinsic semiconductor obtained upon doping very small
amount of holes and electrons by chemical substitution. The best example is p-type (hole)or n-type (electron) doping in silicon achieved by site-substitution of boron or phosphorus
atoms, respectively. The precise control of doping holes and electrons in various
semiconductors such as Si, Ge, GaAs, GaN, etc. is the basis of manufacturing a variety ofelectronic devices, from rectifier diodes and light emitting diodes (LEDs) to state of the art
integrated circuits (ICs) like microprocessors, memory chips, etc. Another important topic
in materials science is phase transitions : a material can undergo a transformation in its
geometric structure and/or properties as a function of a physical parameter such astemperature, pressure, etc. The competition in the elastic energy of a material versus its
electronic energy often leads to a phase transition as a function of a physical parameter.
Accordingly, the elucidation of the electron energies of a material is a major field of study
in present day research.
1.2 Salient Features of PES
PES is a primary tool to study the electronic structure of materials – solid, liquid or gas.
It has been used to study the electronic structure of solids extensively over the last 50
years and today, there exists a large database of the electronic structure of materials as
determined by photoelectron spectroscopy. The earliest record of experiments describing
photo- electrons (that is, photon induced emission of electrons, also called photoemission)is due to the German physicist, Heinrich Hertz (1856–1894) as part of his studies on thenature of electro-magnetic radiation and the experimental discovery of the photoelectriceffect [2]. From a series of experiments, Hertz showed that the absorption of electromagnetic radiation ( = photon) by a material placed near a spark gap, directly affected thecurrent-voltage ( = electric) characteristics of a spark gap : the first illustration of photoelectric effect. Since the flow of current across a spark gap is due to electrons, it meantthat the electro-magnetic radiation affected the electrons in the material. Historically, theseexperiments were done before the discovery of the electron by J. J. Thomson in 1899 andthe photo- electric effect experiments could not be reconciled with the classical or wave
nature of electro-magnetic radiation. It was later explained by Albert Einstein [3], basedon the quantum nature of electro-magnetic radiation, which describes light (electromagnetic radiation) as consisting of photons and every photon has a specific frequency νand energy hν. The Nobel Prize citation of Albert Einstein reads “for his services to thedevelopment of Theoretical Physics, and especially for his discovery of the law of thephotoelectric effect”. The photoelectric effect is described by the equation,hν = KE + BE + Φ(1)where, hν is the known incident photon energy, KE ( = EK ) is the measured kinetic energyof the emitted electron, BE ( = EB ) is the binding energy of the electron, and Φ is the
known work function of the material under study. This equation tells us that if the incident
photon energy is more than the sum of the binding energy of an electron and the work

function of a material, on absorbing the photon, the electron will get emitted with a finite
kinetic energy. The important features of the photoelectric effect are :(1) The rate at which electrons are emitted is directly proportional to the intensity of theincident light, for a given material and frequency of incident radiation.
(2) For every material, there exists a certain minimum frequency of incident radiation
below which electrons cannot be emitted by the photoelectric effect, and this minimum frequency is called the threshold frequency.
(3) For incident radiation above the threshold frequency, the maximum kinetic energy ofthe emitted electron depends on the frequency of incident radiation, but not on theintensity of the incident radiation.
These features were a direct proof of the particle nature of electro-magnetic radiation andplayed a vital role in establishing the wave-particle duality principle in quantum
mechanics. The equation (1) above is a statement of the law of conservation of energy andimplicitly includes the law of conservation of momentum.
1.3 Technique of PES
The photoelectron “spectroscopy” experiment involves measuring the intensity of photo-emitted electrons from the core levels andvalence band of a material, as
a function of kinetic energy ofthe emitted electrons. Figure
1 shows a schematic of thephotoelectron process.
Pioneering work on thescience and technique ofphotoelectron spectroscopy
Figure 1. A schematic diagram showing the energy
for the study of electronic
ofelectrons in a material and the process of thestructure [4], or “ESCA” (anphotoelectric effect.
acronym for electron spectroscopy for chemical analysis), was carried out by the group of Kai Siegbahn (1918–2007) at
Uppsala University in Sweden. He was awarded the Nobel Prize for “his contribution to
the development of high-resolution electron spectroscopy” in 1981. The schematic of the
instrumentation required for carrying out photoelectron spectroscopy is shown in Fig. 2.
Since we have to measure electron kinetic energy, the experiment has to be done in ultrahigh vacuum (UHV), because electrons have a very short mean free path in air [5,6].
Typically, one needs a vacuum of better than 10–6 mbar (= 10–4 Pascal), in the case ofexperiment for materials science, it is typically in the range of 10-8 Pa. Further, theelectrons also have a short inelastic mean free path inside a solid [5-7]. The inelastic mean
free path (IMFP) of an electron inside a solid is defined as the average distance travelledby an electron before making an inelastic collision. Since an inelastic collision results in achange of its energy, it leads to a loss of the signal intensity that is being measured. So thetechnique can be extremely surface sensitive and probes typically a few angstroms toabout 20Å of the surface with conventional laboratory equipment of the type shown in Fig.
2.
For a material with an IMFP, λ, the measured intensity I is given by,
I = Io exp ( –d / λ )(2)where, Io is the intensity without any inelastic losses and d is the distance from the surface
from which the electrons are emitted. The IMFP λ depends strongly on the kinetic energyof the electrons, and also the specific properties of the material under investigation.
However, a general empirical curve, well-known as the universal curve for IMFP ofelectrons [5,6], shows that the IMFP has large values ( > 50 – 100 Å) for very low kinetic
energies ( < 10 eV ), a broad minimum of 5 – 10 Å for kinetic energies between 30 – 100 eV,
and then a monotonic increase for kinetic energies above 100 eV. Typically, the IMFP isagain about 50 – 150 Å at energies of 6 – 8 keV. The surface sensitivity is thus determinedby the depth perpendicular to the surface from which the electrons escape, and is definedas the mean escape depth σ = λ cos α, where λ is the IMFP and α is the emission angle ofthe electrons measured with respect to the surface normal. The technique is thus most bulk
sensitive at normal emission, and as the emission angle increases, it becomes more surface
sensitive.
The general photoelectron spectroscopy instrument consists of stainless steel chambers
which are pumped by a combination of UHV turbo-pumps, backed by rotary/diaphragmpumps. A typical stand-alone instrument (Fig. 2) consists of three chambers : (1) the mainchamber or analysis chamber has an electron energy analyzer, a photon source, and sample
manipulator which facilitates sample cooling and heating, (2) a sample preparationchamber in which the sample surface is prepared and characterized inside the UHV (thischamber usually has a sample cleaver or, sample annealing and ion-etching facilities, etc.)
and (3) a fast entry load-lock or sample bank chamber, which alone is opened and exposed
to air, for introducing the samples into the preparation and analysis chambers. Thischamber is necessary for minimizing the time required to achieve UHV for experiments to
be done in the preparation and analysis chambers. The spectroscopy experiments usually
use a hemispherical electron energy analyzer for measuring the kinetic energies ofelectrons, which is achieved by applying a fixed voltage to the hemispherical plates of theanalyzer for one particular kinetic energy. The energy analysis is computer controlled andthe spectroscopy experiment scans over the kinetic energy of the measured electrons to
obtain a spectrum: intensity as a function of kinetic energy. The spectrum can be readilypresented as a function of binding energy by knowing the work function or calibrating theenergy scale with a standard database of core-level energies [8]. PhotoelectronFigure 2. Schematic of a photoelectron spectrometer showing various components :(1) main analysis chamber for experiments(2) sample preparation chamber for in-vacuum cleaning and characterization(3) load lock chamber(4) hemispherical electron analyzer(5a-c) ultra-high vacuum pumps (6a,b) X-ray and ultraviolet photon source(7) Detector(8) Instrument control panel(9) Computer(10) Sample manipulator
spectroscopy is often done using a fixed photon energy, but it can be done with a photon
energy of a few eV (e.g. 6 or 7 eV) or even with few keV (e.g. 6 or 8 keV). It is also done
as a function of tunable photon energy obtained from a synchrotron in the special case ofresonant photoelectron spectroscopy.
1.4 Nomenclature
The following table lists the various names and acronyms used in the literature forphotoelectron spectroscopy, depending on the energy of photons used for the experiment.
Also listed are names and acronyms of the complementary techniques of electron
spectroscopy :Photon energy
Ultraviolet (UV) photons : He I (21.2 eV),He II (40.8 eV)

Name
Ultraviolet Photoelectron
Spectroscopy

Acronym
UVPES or UPS

Laboratory X-rays : Mg Kα (1253.6 eV),Al Kα (1486.6 eV)
Lasers : 4th harmonic of Ti-sapphire laser(5.95 eV),frequency-tripled Nd:YVO4 laser(6.99 eV)Tunable UV and soft X-ray photons(30 – 1500 eV) using grating
monochromators at a synchrotron
Hard X-rays obtained at a synchrotron using
crystal monochromators (6 – 10 keV)
Complementary techniques :Inverse photoelectron spectroscopy
Bremsstrahlung isochromat spectroscopy
X-ray Absorption spectroscopy

X-ray Photoelectron
Spectroscopy
Laser Photoelectron
Spectroscopy

XPS

Resonant Photoelectron
Spectroscopy

Resonant-PES

Laser-PES

Hard X-ray
Photoelectron
Spectroscopy

HAXPES

IPES
BIS
XAS

A further classification involves the names/nomenclature used to describe specificenergy levels that are measured in a material, as known from the energy levels of theelectronic states present in the atoms constituting the material. The nomenclature follows
the well-known classification for hydrogenic atoms such as s, p, d, f electrons occupying
the K, L, M, N, etc. shells in an atom [9]. The table below lists a more detailed
classification for electron energy levels from the K-shell to N-shell :K-shell
1s2
K
2
L1
L-shell2s
62p : 2p1/2, 2p3/2
L2, L32
M-shell3s
M1
63p : 3p1/2, 3p3/2
M2 , M3103d: 3d3/2, 3d5/2
M4 , M5
2
N-shell4s
N1
64p : 4p1/2, 4p3/2
N2, N310
4d
N4, N5: 4d3/2, 4d5/2
14
4f: 4f5/2, 4f7/2
N6, N7
The above classification is fully described in references 5 and 9. Photoelectron spectroscopy measures the occupied electronic states of a material and describes the core (orinner)-levels and valence (outermost) levels of a material. A related technique tophotoelectron spectroscopy is Auger electron spectroscopy [1,5,6]. When an electron isemitted from a core level, it results in an excited state with a core hole. This hole can getfilled by a higher lying electron and the energy of this transition is given out in the form ofa photon (resulting in X-ray emission), or the excess energy can be used in a non-radiative
process to remove another electron. The second process, which results in the emission ofan electron is called the Auger effect, thus giving the name Auger electron spectroscopy.
Later, we will discuss three related or complementary electron spectroscopies of theunoccupied and occupied electron states of a material, which go by the names of inverse
photoelectron spectroscopy [10], X-ray absorption spectroscopy [11], resonant photoelectron spectroscopy [12], respectively. The method of inverse photoelectron spectroscopy (IPES) is a valuable probe of the unoccupied density of states of a material, basedon the inverse process of photoelectric effect [12] i.e. absorption of an electron andemission of a photon. However, it is a very time consuming experiment and is not very
popular because the intensities are very low due to the low transition probability for the
inverse photoelectric process. The experiments of X-ray absorption spectroscopy andresonant photoelectron spectroscopy require comparatively very little time, but require atunability of the incident photon energies.
1.5 Applications
In the following, we describe the major applications of photoelectron spectroscopy.
1.5.1 Core-level photoelectron spectroscopy
Since atoms of each element possess a unique set of electron energy levels, theforemost application of photoelectron spectroscopy is to identify the elements present in amaterial, from a measurement of its core level spectra. Using tabulated atomic sensitivity
factors Si for specific core levels (or atomic cross-sections) [13], one can also quantify theratio of elements present in a sample by measuring the core-level spectra of the different
elements present in a sample. The atomic fraction Fβ of a constituent element β in asample is given by,
Fβ = (Iβ/Sβ) / Σ (Ii/Si)(3)
where, the sum is over a specific core level of each element i present in the sample and Ii
are their respective intensities.
1.5.2 Identification of chemical or valence state
The chemical state of an element can be determined because the core-level binding
energies of elements depends on their valency. e.g. it is a routine experiment to identify
and distinguish [8] a metal and its oxide such as aluminum metal (Al0) and aluminum(Al3+) oxide, or copper metal (Cu0) and cupric (Cu2+) oxide. In more complex situations, it
is possible to distinguish between multiple oxidation states using core-level spectroscopy.
This is best exemplified by Si 2p core-level spectra which showed that features due to Si0(elemental Silicon), Si1+, Si2+, Si3+ and Si4+ are systematically shifted to higher binding
energies, depending on the valence state [14]. A schematic figure is shown in Figure 3,
plotted as a function of relative binding energy. The absolute binding energy for elemental

silicon 2p core level is about 99 eV, andevery higher multiple oxidation state isabout 1 eV higher binding energy, with
the Si4+occurring at about 103 eV. Thus,
even suboxides can be identified andquantified using photoelectron spectroscopy and lead to a description of local
bonding configurations in materials. At ageneral level, core-level photoelectron
spectroscopy applications in surface
science involve the determination ofphysisorption or chemisorption of molecules, as determined from the binding
energies and symmetry of molecules
adsorbed on surfaces.

Figure 3. Schematic showing multiple
oxidation states of silicon, as measured in
Si 2p core-levels as measured from anoxidized Silicon surface, using X-ray
photoelectron spectroscopy.

1.5.3 Line shape analysis
A detailed study of core-level line- shapes using high energy resolution can reveal
material properties : a symmetric line shape for insulators, or an asymmetric line shape formetals. The symmetric lineshape (Figure 4) is described by a Voigt function : aconvolution of a Gaussian function (representing the experimental energy resolution) witha Lorentzian function (representing the lifetime of the energy level). The lifetime (∆t) of a
specific core level is related to the intrinsic energy width (∆E) of the core level reflecting
the Heisenberg’s uncertainty principle, and is given by, ∆E・∆t ≧ħ ; ħ = h/2π where h is the
Planck’s constant.
Metals show an asymmetric lineshape that is well-described by a
Doniach-Sunjic line shape [15], which isdue to the excitations of electron-holepairs at the Fermi level of a metal. Since
these electron-hole excitations haveinfinitesimally small energies and occur
when the electron is travelling within thesolid, they result in a loss of kinetic
energy of the emitted electron. This loss Figure 4: Comparison of a Gaussianin kinetic energy causes a broadening on function (dashed line) and Lorentzian
the low kinetic energy (high binding function (dotted line) of the same fullenergy) side of an otherwise symmetric width at half maximum (FWHM = 1 eV),core level, and is a characteristic of and its convolution, a Voigt function
metals. Core-level lineshapes also carry (dash-dot line).

valence electron spin information. The splitting of the 3s core levels in a molecular gas isindicative of the exchange coupling between the s core levels and the valence electron spin[4] e.g. the oxygen 1s core-level spectrum of molecular oxygen consists of two peaks
separated by 1.1 eV and with an intensity ratio of 1:3 which is proportional to thedegeneracy of the states. This behaviour is understood as follows : The photoelectron of
one 1s electron results in a photohole with a spin of s = 1/2. Since molecular oxygen has atotal valence electron spin of S = 1, it couples with the s = 1/2 spin and gives a total spin J= 1/2 and 3/2. The degeneracy is given by 2J+1 and hence the intensities are in the ratio of
1:3. Water, with a valence electron spin of S = 0 does not show this exchange splitting in
the O 1s core-level spectrum. Similar behaviour is also known in the 3s, 4s and 5s
core-level spectra of transition metal and rare-earth compounds [1].
1.5.4 Valence band photoelectron spectroscopy
A direct probe of the occupied
density of states (DOS, number ofelectronic states, which is proportional
to the measured intensity of electronic
states as a function of their energy) of amaterial is provided by valence band
photoelectron spectroscopy. Thus, one
can distinguish between an intrinsic
semiconductor (or insulator) compared
to a metal, because semiconductors (orFigure 5. The Fermi-Dirac function at T = 0
insulators) have a gap in the density ofand 300 K. Note x-scale is in eV
states at the chemical potential, or
Fermi level EF, while a metal has no gap in the density of states (See Figure 1). The
Fermi level of a metal (the highest occupied energy level of a metal ) is identified as thehighest kinetic energy level that can be measured using a specific photon energy and iscalled the Fermi edge. At zero temperature, it is a sharp cut-off or step-function as
determined by the Fermi-Dirac distribution function but since any experiment is done at a
finite temperature, the Fermi level is broadened at the edge by the Fermi-Dirac function atthat temperature. The Fermi edge is often used to determine the energy resolution of aphotoelectron spectrometer, defined as the Gaussian function that needs to be convoluted
with the Fermi-Dirac function (Figure 5), which simulates the Fermi edge spectrum
accurately at the temperature of the measurement.
The present record for energy resolution in photoelectron spectroscopy of a solid is
360 micro-electron volts (µeV), obtained for the Fermi edge of gold measured at atemperature of 3 K, using a laser as a photon source [16] (Figure 6). A high energy
resolution of about 1–5 meV has been achieved by several groups in the world in the past
few years. High energy resolution photoelectron spectroscopy has played a very importantFigure 6. The highest energy resolution photoelectron spectrum (circles) of gold (Au),measured for a solid using Laser PES at a sample temperature of 2.9 K, compared with a
FD fit (full line), a Fermi-Dirac function convoluted with a Gaussian function having afull width at half maximum of 360 micro-eV. Note x-scale is in milli-eV (from Ref. 16).
role in the study of phase transitions including superconductivity, metal-insulatortransitions, Kondo materials, etc. and defines the frontline of research in the electronic
structure of novel materials.
1.5.5 Angle-resolved photoelectron spectroscopy
In angle-resolved photoelectron spectroscopy (ARPES) experiments, in addition tomeasuring the kinetic energies of the emitted electrons, one measures the spectra as afunction of emission angle with respect to the surface normal, in terms of the polar andazimuth angles. Before we describe the ARPES technique, it is useful to consider thephotoemission process as involving the initial state with energy, Ei = –BE (negative of thebinding energy of the electron) and the final state of the electron after emission as a statewith an energy Ef = KE + Φ. This implies Ef = Ei + hν, from the photoelectric equation.
Applying the law of conservation of momentum to the electrons in a crystalline solid withinitial state energy and momentum as (Ei,ki) and the final state to be described by (Ef,kf),
the electron emitted from the solid has the measured momentum Kf = kf + G where G is areciprocal lattice vector. Under the assumption of a free electron final state, with kf
resolved into the in-plane momentum k║ and out of plane momentum k⊥, the KE ofemitted electron is
EK = ( ħ2 / 2m )( k║2 + k⊥2 )(4)then,k║ = [( 2m / ħ2 ∗ EK )]1/2 sinθ(5)where, θ is the emission angle measured with respect to the surface normal, andk⊥= [( 2m/ħ2 ∗ EK cos2θ + Vo )]1/2(6)
where, Vo is the inner potential, and we refer the interested reader to a review which
discusses the details of the ARPES technique extensively [17].
1.6 Complementary Techniques
1.6.1 Inverse photoelectron spectroscopy
In inverse photoelectron spectroscopy [10], one carries out the inverse experiment ofphotoelectron spectroscopy to study the unoccupied states of a material. A mono-energetic
beam of electrons is incident on a material. It occupies an available electronic state as its
final state, which is not occupied in the initial state, and emits a photon. By scanning theincident electron energy and measuring detected photons at a fixed photon energy, it ispossible to map out the unoccupied density of states of a material. In this mode, it is called
bremsstrahlung isochromat spectroscopy (BIS).
1.6.2 X-ray absorption spectroscopy
X-ray absorption spectroscopy(XAS) is a measurement of the X-rays absorbed by amaterial as a function of the incident X-ray photon energy. In particular, the intensity of
X-rays absorbed by a solid undergoes a sharp increase when the incident X-ray photon
energy is tuned across a core-level threshold energy [11]. The intensity is directly
proportional to
IXAS ∝ | < Θf | ê・r | Θi >|2 δ( Ef – Ei – hν )(7)where Θi, Θf are the initial and final states of the system, and ê・r is the dipole operator
coupling the initial state with energy Ei to the final state with energy Ef on absorption ofan X-ray photon of energy, hν. The material absorbs a photon and results in exciting anelectron to a continuum or an unoccupied state, when the energy hν equals Ef – Ei. The
dipole matrix element approximation indicates that only transitions with ∆L = ± 1, andwhich conserve spin ∆S = 0 are allowed transitions. Thus, XAS probes the site selective,
element specific, angular momentum projected density of unoccupied states of a material.
For soft X-rays (30 – 2000 eV), the quadrupole transition is very weak (usually 100 or
more times weaker than the dipole transition ) and is often neglected in discussions of soft
XAS.
1.6.3 Resonant photoelectron spectroscopy
Resonant photoelectron spectroscopy (RESPES) measures the occupied partial densityof states [12], and is thus a complementary technique to XAS. In RESPES, the photon
energy is also tuned across a core-level threshold and one measures the valence band
photoelectron spectra as a function of the incident photon energy. In this way one canprobe the angular momentum projected density of states in the occupied valence band, e.g.

when the incident photon energy is tuned across the 2p (3d) core level of a 3d transition
metal (4f rare-earth) element or compound, the 3d (4f) partial density of states in thevalence band can be identified. The process resulting in resonant photoelectron emission
involves the absorption of a photon and creation of a core hole (like XAS), and this state
undergoes a de-excitation, with the core hole getting filled up and the emission of anelectron from the valence band. The remarkable increase in intensity often leading toso-called giant resonances are described by the Fano effect [18]. The experiments of XAS
and RESPES are done across particular core-level thresholds and thus necessarily require
synchrotron radiation.
1.7 Synchrotron Radiation
Synchrotron radiation [19] is electromagnetic radiation generated by the accelerationof relativistic (near the speed of light) charged particles, e.g. electrons, through magnetic
fields. The acceleration is first done in a synchrotron, a cyclic particle accelerator in which,the electric field used for accelerating the particles and the magnetic field used for turning
the particles for circulation, are synchronized to the particle beam. The high speed electron
beam achieves a final energy that is in the GeV (giga-electron-volt) range, and it is stored
in the UHV ring in a closed loop. Since the electrons are forced to travel in a closed loop
by strong applied magnetic fields, they undergo acceleration and thus emit electromagnetic radiation which includes frequencies from radio waves, infra-red, visible,
ultraviolet and X-rays. The synchrotron radiation sources are the brightest known sourcesof X-rays, which can be linearly or circularly polarized. The electromagnetic radiation canbe further intensified by using insertion devices, called wigglers or undulators (composedof an array of north and south pole magnets). The storage ring has openings through which
the radiation exits the storage ring and which are subsequently monochromatized using
grating- or crystal-monochromator to obtain a specific energy X-ray beam. Figure 7(upper panel) shows a typical layout of the setup for the HAXPES experiment, based on a
crystal monochromator with X-ray optics, and the inserted photograph at the right-upper
corner shows the apparatus for Hard X-ray ( 6 – 8 keV ) Photoelectron Spectroscopy(HAXPES) [20]. Several recent HAXPES studies have now established that photoelectron
spectroscopy can indeed probe the bulk sensitive electronic structure of solids in a reliable
way [20]. The beamline X-ray optics is necessary for obtaining a controlled size beam ofthe order of micron, which is focussed, collimated and delivers a high photon flux.
Typical applications of synchrotron radiation are :Diffraction, single crystalline and powder, of inorganic and organic materials,
biomaterials (including proteins), etc.
X-ray scattering : small angle, magnetic, inelastic, etc.
Tomography and imaging
Photolithography

X-ray absorption
X-ray photoelectron spectroscopy
The ability of synchrotron radiation sources to provide highly focused, high photon fluxes
over a wide energy range, which can be monochromatized to obtain a tunable photon
source, plays a major role in present day spectroscopy. In the last 10 years since the adventof the third generation synchrotron sources, typically, a 100- to 1000-times increase inphoton flux, with an energy and spatial resolution better than laboratory sources has
revolutionized spectroscopy, and also scattering, diffraction and imaging techniques.

Figure 7. Typical layout of the setup for the HAXPES experiment. Inserted
photograph at the right-upper corner shows the HAXPES apparatus installed in theexperimental hutch of BL29XU.

1.8 Some important keywords for the BL practiceIn our group, we are studying various kinds of materials. Especially, we are interested
in studying the strongly correlated electron systems. In order to understand the spectroscopic study for the strongly correlated electron system, there are several important
keywords as followings ;Free electron metal/model ( ≡ Fermi gas),
Nearly-free electron model,
Band gap,
Semiconductor,
Correlated metal ( ≡ Fermi liquid),
Correlated insulator,
Metal-insulator and Mixed valence transitions,Superconductor,… etc.
In the following, these important keywords are briefly described.
Free electron metal : A metal is said to be a free electron metal if it is described by Drude
theory of metals combined with Sommerfeld theory of conduction.
Drude model assumptions(i) A metal consists of electrons which are free to move and positive ion cores which
are immobile. Between collisions, neglect the interaction of a given electron withother(a) electrons (Independent electron approximation), and(b) ions (Free electron approximation).
(ii) Collisions are instantaneous events that abruptly change velocity of electrons. No
electron-electron scattering but a simple mechanical picture.
(iii) Electron undergoes a collision with a probability per unit time 1/τ, τ is called
relaxation time(iv) Electrons achieve thermal equilibrium through collisions.
A free electron metal means the total energy of the system consists only of kinetic energyof the electrons and the assumptions means the potential energy is zero. A free electron
metal is also called a Fermi gas.
Real metals which are well-explained as free-electron metals are the Alkali Metals :Lithium (Li), Sodium(Na), etc.
The dispersion relation (the relation of the energy of an electron with its momentum k iscalled the dispersion relation) of electrons for a free electron metal isE(k) = ħ2k2/2me
where, ħ = h/2π (h is the Planck’s constant), and me is the mass of the electron. The energyof an electron with respect to its momentum is a parabola.
Successes of Free electron model

(1) Ohm’s law : electrical conductivity of metals(2) Thermal conductivity of metals
Failures of Free electron model(1) Specific heat of metals
The experimentally measured specific heat C often behaves as
C/T = γm + AT2, where γ and A are constants, but the value of γm does not matchthe value of γ expected from the free electron model. The ratio of γm / γ defines
the thermal mass mth of an electron compared to the free electron mass me.
i.e. γm / γ = mth / me(2) Hall effect of metals
Hall coefficient is always negative and constant, RH = 1/ne, for a free electron
metal, but experiments show +ve and –ve values of RH , and it depends on
temperature(3) Existence of insulators and semiconductors
Nearly-free electron model : This model means the potential energy is not zero, but finite,and has the periodicity of the ions making up the lattice. In this case, we have now relaxed
the “Free electron approximation”, i.e. the electrons are not totally free to move, but nearly
free to move.
It explains• Band gap : The band gap is the gap in the electron energy levels of electrons in asolid. The band gap defines the existence of semiconductors and insulators, andcorresponds to the lowest electronic energy excitation possible in a semiconductor
or insulator. Examples include Si, Ge, GaAs, etc.
• Effective mass : In a solid described by the nearly free-electron model, thecurvature of the parabola defining the electronic dispersion gets changed comparedto the case of the free electron metal, and the change is attributed to a change in themass of the electron, i.e. it has an effective mass meff, which is different from thefree electron mass me, due to the potential energy of the system.
• Electrons and holes : the curvature of the dispersion relation can be positive or
negative and hence the carriers can be electrons or holes (= missing electrons
which behave as a positive carrier).
If the importance of the potential energy in a solid keeps on increasing, the electronic
properties get further and further away from a free electron model as well as the nearly
free-electron model. In such cases, the electrons influence the motion of each other and theelectrons are not independent of other electrons anymore. This means the “Independentelectron approximation” is also not valid and such metals are also called correlated
metals. If such correlations increase to such a value that the electrons do not allow otherelectrons to move at all, then we land up with a correlated insulator.
The cases where the electrons are strongly correlated but have not yet become
insulating, lead to heavy effective masses of electrons and they are also called heavy
electron systems or heavy fermion systems (electrons are fermions). Sometimes, suchmaterials exhibit a condition called mixed valence transition: in particular conditions oftemperature, or pressure they undergo a change in valency of their atoms i.e. the same typeof atoms can exist with two different valencies in the solid at the same time e.g. YbInCu4,
EuNi2[Ge1-xSix]2. Another interesting case is when, as a function of temperature orpressure or doping, such materials undergo a metal to insulator transition e.g. VO2,
V2O3, etc. All such cases put together are called strongly correlated electron systems,
because their properties cannot be explained by the free electron or nearly-free electron
models of solids. The famous high-temperature cuprate superconductors (copper oxides
such as La2-xSrxCuO4, Nd2-xCexCuO4) or the recently discovered iron-arsenide/selenide/telluride based materials (such as BaFe2As2, FeSe1-xTex) are also considered tobelong to this class of materials.
2. Experimental
2.1 Brief description of the 27m-long undulator beamline BL19LXU
2.1.1 Overview
RIKEN SR Physics beamline, BL19LXU, has been constructed in 2000 [21] to utilize ahigh-brilliance X-ray beam generated from a 27m-long undulator [22]. In the optics hutch,a Si(111) double crystal monochromator and a double mirror system have been installedwith standard transport-channel components in 2000 and 2004, respectively. Threeexperimental hutches of EH1 to EH3 are connected to the optics hutch in tandem as shownin Fig. 8.
The commissioning of beamline has been started in October 2000. In the spring of 2001,
the extension of to another experimental hutch EH4, which is located at an extension
building of annex West around 130 m from the source (see Fig. 8), has been completed.
The research and developments have been focused on the experiments which require
high-brilliance X-ray beam, such as X-ray free electron laser applications.

Fig. 8Layout of RIKEN SR Physics beamline BL19LXU

2.1.2 Light source and optics
BL19LXU is mainly characterized
by the long undulator [22] shown in
Fig. 9. The total length of the undulator
magnet is 25 m with a magnetic period
length of 32 mm (781 periods) and amaximum K value of 1.77 at 12 mm
gap. A photon energy range from 7.2 to18 keV is covered by the fundamental
radiation of the undulator. Although thetotal radiation power reaches 35 kW at
a maximum, most of the off-axis power Fig. 9 Photograph of 27m-long undulator
is removed in the front-end (FE)section [23]. The FE-slit picks up the on-axis portion of the undulator radiation with apower of about 500 W. The radiation passing through the FE-section is monochromatizedby a SPring-8 standard type Si(111) double crystal monochromator (DCM). As the first
crystal of DCM receives the undulator power, the liquid-nitrogen based cryogenic coolingsystem is installed to reduce the heat-load on the first crystal. In order to stabilize thethroughput of DCM, the first crystal angle can be controlled with MOSTAB feedbacksystem [24, 25]. The resultant energy resolution of X-ray beam is about 10-4. A double
mirror system can optionally be used to obtain a focused beam and to remove the higher
harmonics passing through the DCM. The locations of first and second mirrors are 46.80
m and 47.95 m from the source, respectively. The Pt-coated mirrors with a length of 400
mm can be bent mechanically with turning of the angle and the shift in height. Available
photon energy range and the flux at experimental hutches are summarized in Table 1.
Table 1. SR beam parameters at a sample position
Photon energy range
7.2～18 keV (1st), 22～37 keV (3rd) ( Si(111) )Photon energy resolution～1 x 10-4
Photon flux
～2 x 1014 photons/s ( @ 14 keV )Beam size (FWHM)
0.8 (V) x 1.5 (H) mm2 (EH2) ( without mirror )2.1.3 Experimental stations and the equipments
Four experimental hutches (EH1 - 4) are located as described above : A series ofexperimental hutches of EH1 to 3 is directly attached to the optics hutch in the storage
ring building, and EH4 is located in the annex West. The X-rays are guided from EH3 to
EH4 through 51m-long Pb-shielded vacuum ducts with the ICF152 conflate-flanges.
Vacuum ducts can be optionally attached in EH1-3, when an experiment is carried out inEH2, 3 and 4.
Beamline users should bring their own equipments into the experimental hutches inprinciple. The details of the experimental hutches are shown below.
 EH1
EH1 has a dimension of 5 (W) x 3.3 (D) x 3.3 (H) m3. An air conditioner stabilizes
room temperature within 0.1 K fluctuation. In this hutch, fundamental studies havemainly been conducted : such as X-ray correlation, nonlinear optical process, hard
X-ray photoelectron spectroscopy, photo-desorption process on the surface and so on.
 EH2
EH2 is equipped with a femtosecond pulsed laser system [26]. To avoid chirping of
the laser pulse, the whole laser system is placed inside the hutch. Synchronization of
the laser and SR X-ray pulses is achieved, with a precision of 5 ps, to conduct
laser-pump SR-probe time-resolved experiments. For example, a precise diffractometer has been installed for picoseconds time-resolved X-ray diffraction studiesof lattice dynamics in semiconductors and photochromic crystals, and of thecrystallization process in phase change materials, and so on.
 EH3
EH3 is an open hutch located at 77 m from the source. The dimension is 5 (W) x 3.3(D) x 4.5 (H) m3. Users can install their large instruments such as vacuum chambers
or cryostats by using a chain block. Up to now, various new type of experiments havebeen conducted, such as a scanning tunneling microscope with SR, an X-ray

interferometer, and fundamental experiments toward X-ray free electron laser
applications.
 EH4
The hutch EH4 with a large dimension of 7 (W) x 6 (D) x 6 (H) m3 is located at annex
West around 130 m from the source. EH4 is equipped with a multi-circle
diffractometer mounting a 15 T super conducting magnet, as well as a pulsed magnet,
for studying X-ray magnetic scattering under intense magnetic field and extremely
low temperature. A set of K-B mirror has been installed for the development of
nano-focusing technique to obtain a beam size of 100 nm in practical application [27].
Equipments available for users are listed below : (1) Timing signal supply ; The 508.58
MHz radio frequency (RF) signal of the storage ring is provided as a trigger of timing
experiments. (2) Optical benches ; Optical benches are prepared for various experiments.
The dimension is 1.0 m by 2.0 m for one bench, 1.0 m by 1.2 m for two. (3) X-ray
detectors ; Ion chambers with a length of 3.3 cm and 14 cm are prepared to monitor theincident X-ray beam intensity. Scintillation counters and PIN photodiodes are alsoavailable to monitor the scattered X-ray intensity. (4) Motor control system ; A pulse
motor remote control system is available to use stepper motors inside experimental
hutches. (5) Electronic circuit for signal processing ; Electronic circuits such as a counter,a logic controller, a current amplifier, and so on, together with NIM power supplies areprepared for signal processing for various applications.
2.2 HAXPES apparatus and some miscellaneous components for the BL practice
The HAXPES apparatus mainly consists of the hemispherical electron energy analyzer
R4000-10kV (VG SCIENTA) mounted on an ultra-high-vacuum system. In addition to theanalyzer, a sample manipulator with a motorized XYZΘ stage, a flow-type He cryostat for
sample cooling, and two turbomolecular pumps are equipped on the main chamber. This
analysis system together with load-lock and preparation chambers are mounted on
position-adjustable stages. The vacuum of the main chamber is of the order of 10-8 Pa, andthe lowest sample temperature that can be achieved is about 15 K. Figure 10 shows aphotograph of HAXPES apparatus for the present BL practice. Details for the apparatus
are described in Ref. [28, 29].
In the practical course, the HAXPES apparatus is installed in EH3 together with some
additional optical components (a high-resolution Si(444) channel-cut monochromator anda horizontal refocusing mirror system) and also some diagnostic devices (some ion
chambers and an avalanche photodiode). Optical-laser beam generated from the
femtosecond laser system installed in EH2 will be guided to the target material via thelaser transport channel. Timing between the optical-laser and SR X-ray pulses can bemonitored by using a fast avalanche photodiode. Delay between these two pulses can be
adjusted using a digital delay unit.

Fig. 10 Photograph of HAXPES apparatus installed in the experimental hutch EH2 of
BL3, SACLA.
3. Experimental procedure
In this practical course, participants will be asked to proceed the steps which arebriefly described in the following subsections.
3.1 Preparation of SR-beam
We will basically use the X-ray beam of 8 keV. First, we will adjust the DCM, which
provides us the intense X-ray beam of ∆E/E ～ 1.6 x 10-4 (∆E ～ 1.3 eV), by looking at therocking curve. Reflectivity as a function of angle will be measured by using an ion
chamber (IC). We will also adjust the position of the transport-channel (TC) slits bymonitoring the intensity of X-ray beam passing through the TC-slit. The parameter of an
insertion device (ID), e.g. gap, will be set to provide the maximum output by monitoring
the readout of IC. After adjustments of the ID, DCM and TC-slit, we will adjust theSi(444) channel-cut monochromator, which will be used to provide the high-resolution
X-ray beam of ∆E ～ 60 meV. This high-resolution X-ray beam will be horizontally
focused using the refocusing mirror system. Then we will see the X-ray beam position at
the entrance of the HAXPES apparatus by using a burn-paper.

3.2 Sample preparation
It is the custom for the ultra-high vacuum (UHV) apparatus, which is used forspectroscopic study, to put the sample into the UHV condition with special care. In thecase of our HAXPES apparatus, first we usually install the sample to the load-lockchamber ( < 10-5 Pa), then we transfer it into the preparation chamber ( < 5 x 10-8 Pa) byusing the transfer-rod. In the preparation chamber, we sometimes fracture the sample
under the UHV condition in order to get a clean surface. Finally, we transfer the sample
into the main chamber ( ～ 2 x 10-8 Pa) by using the transfer-rod. In the main chamber,sample will be cooled by the cryostat to low-temperature, < 60 K, with the use of liquid
helium. In this course, sample temperature will be controlled in the range of 15 ～ 50 K.
3.3 Adjustment of the apparatus
In order to adjust the apparatus to its optimum condition, we will introduce the X-ray
beam into the main chamber. We will monitor the X-ray beam at the sample position byusing the fluorescent material painted on the Cu holder. We will view the fluorescence byusing the CCD cameras and a TV-monitor. After marking the beam position on the monitor,
we will move the Z-axis of XYZΘ manipulator to the position of standard sample, i.e. Au
coated plate. We will measure the Au 4f photoelectron spectra, and adjust the sample
XYZΘ-manipulator and the height of the apparatus so as to maximize the counting rate of
the Au 4f photoelectron signals.
3.4 Calibration of the electron analyzer
We are using the hemispherical electron-energy analyzer R4000-10kV (VGSCIENTA). Pass-energy of 200 or 500 eV is usually used for our HAXPES measurements.
The control software, especially for the lens parameter, is well prepared, but sometimes
there exists an undesired energy offset for each pass-energy when we measure the kinetic
energy spectrum of photoelectrons emitted from the sample. Thus we need to calibrate theanalyzer to determine this energy offset. It will be performed by measuring the photoelectron spectra from the standard Au sample.
3.5 Measurements of electron spectra of today’s sample
We will measure some electron spectra of the today’s sample with wide-energy range,
narrow-energy range for each core-level, near the valence-band region and also some KLL
and/or LMM Auger transitions. Participants will be asked to guess what the today’s
sample is.
3.6 Measurements of core-level spectra as a function of temperature
The today’s sample is one of the mixed-valence compounds. We will measure thecore-level HAXPES spectra with high-temperature phase and also with low-temperaturephase by controlling the sample temperature. The core-level HAXPES spectra clearlyshow the changes in their spectral shapes as the sample temperature goes down across thetransition temperature. We will measure some spectra as a function of temperature.
3.7 Introduction of optical laser
After the confirmation of valence-transition, we will introduce the optical-laser beam
into the main chamber. The position of the laser beam will be adjusted to the marked
position determined in subsection of 3.3. Participants are asked to wear goggles for theirsafety.
3.8 Real-time observation of photoinduced valence-transition
First we adjust the power of optical-laser beam by using filters so as to enable
observing the photoinduced valence-transition. After achievement of the optimum
condition for the observation of photoinduced valence-transition, we will perform thecyclic and consecutive acquisition of photoelectron images in every few seconds with thespecial setup of the analyzer. Participants will experience a real-time observation ofphotoinduced valence-transition in mixed-valence compounds by HAXPES.
4. Discussion for the available time-resolved HAXPES.
Time-resolved (TR) spectroscopy is the special method to study the dynamic processes
in materials or chemical compounds by means of spectroscopic techniques. For thephotoelectron spectroscopy, transient processes in condensed matter are typically recorded
as photoelectron spectra by using probe-pulse after excitation by pump-laser irradiation. In
a typical experiment, both of the photon-pulses for pump and probe are ultra-short pulsedbeam. Temporal evolution of the photoelectron spectra as a function of delay-time
between pump- and probe-pulses gives us dynamic information on the transient electronic
structure of condensed matter.
Since 2012, we have been making efforts to realize TR-HAXPES to study the ultrafast
electron dynamics for materials science. Recently, we have developed a single-shot
CCD-based data acquisition system for time-resolved X-ray photoelectron spectroscopy atan X-ray free electron laser facility [30]. We have successfully demonstrated the capabilityof the system by measuring the core-level HAXPES spectra at SACLA XFEL facility andstudied the time-resolved space-charge effects on the Ti 1s and V 1s core-level
photoelectron spectra [31].
The time-structure of X-ray beam is determined by the electron bunches in the storage
ring. The each electron bunch has the pulse width of about 40 ps with full width at half
maximum (FWHM), with a repetition rate up to 509 MHz. The repetition rate can beselected by the operational mode of the storage ring with preference filling pattern.
Typical filling pattern is represented in Fig. 11. In the BL practice, the filling patternoperated is G?-mode, which is not suitable for pump-probe type time-resolved HAXPES.

Typical parameters for the filling pattern shown in Fig. 11 are summarized in Table 2.

A-modeC-modeF-modeFig. 11Mode
A-modeC-modeF-modeG-modeH-modeG-modeH-modeExamples of the filling patterns of electron bunches in the storage ring.
Table 2. Typical parameters for several bunch operational mode
Beam
SingleBunch
Total
Bunch mode
bunch injection
intervalcurrent
current interval
203 bunches
23.6 ns
30 s
145.5 ns
40 s
11 bunch train x 84
(165.2 ns period)
1/14-filling +342 ns
1.6 mA
20 s
12 bunches
100 mA
4/58-filling +83 ns
1.0 mA
20 s
53 bunches
11/29-filling +5 mA1486 ns
40 s～
1 bunchThe femtosecond laser system is composed of the mode-locked laser oscillator, and aregenerative amplifier to produce laser pulses with a pulse-duration of about 130 fs and apulse energy of about 1 mJ/pulse. The laser pulses are synchronized with the RF master
oscillator of the storage ring which controls the acceleration timing of the electron
bunches [26, 32, 33]. The synchronization and timing control system are depicted in Fig.
12 together with the present experimental setup.

Fig. 12Experimental setup for the time-resolved HAXPES at BL19LXU

In order to realize TR-HAXPES, it is important to achieve the condition of same
repetition rates for both the optical pump-laser and X-ray probe-pulse. We are usuallyusing an X-ray chopper to chop the 208 kHz X-ray pulses in H-mode to match with therepetition rate of the pump-laser, e.g. 1 kHz typically. This will enable us to accomplish acomplete pump-probe configuration scheme for the TR-HAXPES experiment.
In Fig.13, we show a time chart of the laser and the SR pulses for the pump-probe
measurement. In the figure, the time chart for the SR pulses are represented for D-mode(5-bunches + 1/7 filling). The SR pulse selected by the X-ray chopper is generated from ahigh-current electron bunch (3 mA). By adjusting the time-delay τ, we can measure thetemporal evolution of the transient electronic structure as a movie with the time resolutionof about 40 ps (pulse width of the X-ray SR).

Fig.13 Time chart of the pulsed laser, X-ray SR, and the gate of an X-ray pulse
selector, i.e. an X-ray chopper, for a snapshot measurement with a time-delay of τ
[32].
In the BL practice, we will see the recent results for the time-resolved HAXPES
experiments to investigate the ultrafast lead-lag electron dynamics of space-charge effects
on the core-level HAXPES spectra [31].

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