BL practice

BL17SU : Soft X-ray spectroscopy of solution under atmospheric pressure
Takashi Tokushima (RIKEN SPring-8 center)
INTRODUCTION
Knowledge of an electronic structure is quite important for understanding the properties of materials.
Especially, valence electrons play important roles in electric, magnetic and chemical properties.
Therefore, valence orbitals/bands of gaseous/solid materials have been commonly and extemsively
investigated by means of photoemission spectroscopy (PES). Electronic structure is also important formaterial in liquid satate. Large variety of phenmenone in liquid satate have some connection tovariation and dynamics of intermolecular interactions. The origin of intermolecular interactions, suchas dipole-dipole interactions and hydrogen bonds, have closely related to the electronic structure ofmolecule. Hence, information on the valence electornic states of a molecule in liquids is important forunderstanding of liquids.
However, an ordinary PES is not suitable for the studies of liquid materials since PES is a photonin electron-out experiment which detects photoelectrons in vacuum condition. In SPring-8 BL17SU,
we have use x-ray emission and absorption spectroscopy as an alternative method. Recent
advancements in the production of ultra thin films in the order of 100-200nm thickness, which isalmost transparent for soft X-rays, enables us to use the thin films as a vacuum tight window for x-ray
spectroscopy. Hence the x-ray emission spectroscopy (XES) [1], which is a kind of photon-in photonout experiment observes the valence electronic structure through energy spectra of emitted photons,
can be used as an alternative methodof PES for liquid samples. The firstobservation of XES for liquid water
was reported in 2002[2]. In recent ten
years, liquids and solutions have been
actively studied using XES [3-19].
Fig.1 shows schematic energy
diagrams of x-ray absorption andemission spectroscopy. If incident
energy of x-rays is enough to excite
core electrons, one can observe
electronic transitions between coreandvalenceabsorption.

orbitals

as

x-ray

emission and absorption spectroscopy. Relation between

X-ray

absorption

spectral feature and molecular orbitals of water molecule

spectroscopy (XAS) measures the1

Fig. 1 Schematic energy diagrams of soft x-ray

is shown as an example.

absorption coefficient of electron transition from core to unoccupied orbitals and provided informationon unoccupied states [20]. Since XAS process generates unstable core hole in the system, subsequentrelaxation processes occur within a core-hole lifetime (a few fs for light elements). Major relaxation
pathways of the core excited state is Auger electron emission. Energy analysis Auger electron provide
information on occupied state and is known as Auger electron spectroscopy. One of the minor
relaxation pathways is the x-ray emission. Since x-ray emission is energy release of excess energy due
to electronic transitions between core and valence orbitals, XES spectra reflect occupied valence
electronic states. While Auger electron spectroscopy reqire vacuum condition to detect electrons foranalysing electron kinetic energy, XES is possible to observe emitted soft x-ray photons from sample
under atomos pheric pressure through thin film window.
In this text, I’ll explaine some of basic of x-ray emission study of solution sample.

SELECTIVE XES OBSERVATION OF MOLECULE IN SOLUTION SAMPLES
Fig.2A shows the binding energy of core orbits up to 2200 eV (Data were taken from the table ofelectron binding energies in the "X-ray Data Booklet"[21]). The binding energy of a core orbital islargely different depending on elements. Core level spectroscopies have thus the unique capability toobserve elements separately. Fig.2B shows XES spectra of aqueous Fe(III) chloride, which containshydrogen, oxygen, iron and chlorine aselements. Owing to difference of binding
energy in core orbitals, X-ray emission
spectra of oxygen in H2O and iron of
Fe(III)Cl3 were observed at different
energy region.
In general, many solutions contain sameelement in solute and solvent molecules.
Especially, organic molecules consist oflimited elements such as carbon, nitrogenand oxygen atoms. In this case, element
selectivity of XES is not enough to specify
one sort of molecule. Hence, selectivity
beyond element selectivity is needed for the
research of solution samples using XES.

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Fig. 2 (A) binding energies of core orbitals up to
2200 eV for the elements of atomic number less than
36. (B) XES spectra of aqueous Fe(III) chloride.

Fig. 3A shows the O1s XAS
spectrum ofliquidacetone,liquid N,N-dimethylformamide(DMF),

liquidaceticacid,

aqueous acetic acid and liquidwater. It might not be hard to findtheconnection

molecular

structure

betweenaroundoxygen and the peak structure of
XAS. The O1s XAS spectrum ofliquid acetone shows an intense
peak at 531.3eV. This peakstructure is also appeared in XAS
spectra of DMF anc acetic acidwith small energy shift.

On theFig. 3 (A) XAS spectra of acetone, DMF, acetic acid, aqueous
acetic acid and water. Molecular structures of each molecule
are depicted on right side of the figure. (B) Schematic drawingof selective excitation of XES.

other hand, liquid water that has
no C=O type oxygen, then the O1s XAS spectrum shows no peak structure corresponding to the firstpeak of acetic acid.
Based on XES experiments using BL17SU, the first peak and the second peak in O1s XAS spectrumof acetic acid were assigned to selective excitations of the OC=O 1s and OOH 1s respectively [5, 8]. Bytuning excitation energy to such a specific peak corresponding to molecular structure, one can selectmolecule based on molecular structure as depicted in Fig. 3B.
Fig.4A shows an example of OC=O selective observation for aqueous acetic acid in acid-baseequilibrium. In order to perform a selective observation of XES for acetic acid molecules in solvent
water, excitation energy was tuned to the OC=O 1s → π* resonance excitation, which is absent in water.
As shown in Fig.4A, pronounced changes of XES spectra depend on pH is observed. Since aciddissociation constant, pKa, of acetic acid in aqueous solution is known to be 4.756 at room temperature,acetic acid molecules exist in the neutral form in the acidic solution with pH 0.29. In contrast, for thebasic solution (pH 11.44), the anionic form of acetic acid should be dominant species. In thetheoretically calculated XES spectra of neutral and anionic acetic acid plotted below experimental
spectra in Fig.4A, all peak structures in the experiment are well reproduced. This result indicatesthat the selective excitation of XES successfully extracts the occupied valence electronic structure of
the acetic acid molecules in aqueous solution.

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Since acetic acid is a sort of acid,dissociation of hydrogen in OH structure
carboxyl group occurs in aqueous solution.
However, acetic acid is classified as weakacids. Deprotonation occurs as the pH of its
solution decreases as shown in the pH
evolution of the valence electronic structureprobed by XES (see Fig.4B). Importantly,
there are five cross points, i.e. isoemissive
points, in XES spectra of the pH evolution atleast. Isoemissive points indicate the spectra
consist of the overlapping signals of two
components; two components are neutral andanionic forms of acetic acid in this case. XES
is also useful as analytical tool to monitor theratio of neutral and anionic forms based on
the data in Fig.4B (see ref. [5] for detail).
Such

kind

of

selective

observation

technique is useful for the study of chemical
reaction. Because reactants and products inchemical reactions always contain the same

Fig. 4 (A) XES spectra of aqueous acetic acid inacidic and basic solution. Calculated XES spectra
based on DFT calculation is plotted in lower part ofthe plot. The both calculated spectra were shiftedby −1.2eV to fit the experimental peak feature withhighest intensity. (B) pH dependence of XES
spectra.

elements, electronic state observation using
XES is quite difficult only with element selectivity.

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THE

SOFT

X-RAY

UNDULATORBEAMLINE

BL17SU

A ND

THEEXPERIMENTAL APPARATUS
Soft x-ray beamline BL17SU [22] is constructed to advance the spectroscopic studies for mainly
solid state physics and materials science using high brilliant soft x-ray undulator. A novel insertion
device called a multi-polarization-mode undulator has been developed for BL17SU[23-26]. The
insertion device can be operated as a helical, elliptical, pseudo-linear or pseudo-vertical undulator.
Thus,

the

intense

soft

x-ray

beam

of

circularly/linearly polarized light of the soft x-ray
beam can be obtained at this beamline.
The beamline has branched lines (branch-a and-b) which can be switched by the pre-focusing
mirrors and used alternatively. Each branch has
highresolution

andhighlystabilized

monochromator [22, 27] and several end-stations.
The available energy is ranging between 300 and1800 eV. Typical resolving power E/ΔE of themonochromator is higher than 10,000 and thephoton flux is of the order of 1011 photons/s. End-

Fig. 5 Schematic drawing of opticalconfiguration of soft x-ray emission
spectrometer HEPA 2.5

stations at BL17SU beamline are actively used for studies using PES, XAS, XES spectroscopies andsoft x-ray diffraction.
In the course, the participants will use the end station for
liquids and solutions, which is located at the end of branch-a.
The apparatus consists of the XES spectrometer, which named
High Efficiency Photon Analyzer (HEPA) [28, 29], and themain chamber equipped with the compact flange-mount liquidflow-cell which is designed for the studies of liquids underambient conditions.
XES spectrometer at BL17SU is a kind of flat-field
spectrometer which consists of grating and detector (see Fig.5)
with additional optics to increase efficincy [29]. A thinned
backillumitated CCD is used as detector. To reduce thermal
noise of CCD, detector chip is cooling down around -100 °C
Fig. 6 An example of detected
emission lines of CCD image(a)and corresponding x-ray emission
spectra (b).

5by liquid nitrogen. Fig.6 shows an example of detected O1s xray emission from solid Al2O3.

Fig.7 shows liquid flow cell which
utilizes ultra-thin film with 150nm thickas a window to separate vacuum andsampleunder

atmosphere.

Liquidsample is circulate by pumping throughthe liquid flowcell. Since ultra-thin film
window is quite fragile, liquid flow cell
has an emergeny pumping out system
which detect leak of liquid into thevacuum chamber and stop sample feedby switching valve atuomatically.
A window material can be choosen
according to the element of target
material. At the moment, SiN (Silicon
nitride) and SiC (Silicon carbide) are

Fig. 7 Schematic drawing of liquid flow cell for soft xray spectroscopy

available. Since SiN window is cheap to manufacture, we have use SiN window ususally. If a target
sample contains nitrogen, nitrogen signal from the SiN window overlap on the nitrogen signal fromsamples. In this cse, SiC is choosen to observe nitrogen in the sample liquids without contribution of
nitrogen in the window material.

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EXPERIMENT PLAN OF THE BEAMLINE PRACTICAL AT BL17SU
In the course, the participants will
experience soft x-ray absorption andemission measurements of aqueous
solutions under atmospheric pressure.
The experiment plan covers standard
experimental procedure of XES studiesof liquid using selective observation of
the molecule.

Fig. 8 An overview photo of liquid flow cell setup for soft
x-ray spectroscopy

The followings are a plan of the experiment at present.
1.

Preparation of samples and setup of the liquid flow cell for soft x-ray spectroscopy

2.

X-ray absorption mesurments

3.

X-ray emission measurents

4.

Energy calibration of the beamline monochromator and HEPA spectrometer using ahemispherical electron energy analyzer

5.

Data analysis and discussions

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