BL practice

BL17SU : Electronic State Observation of Solute Molecules under Ambient
Conditions

Takashi Tokushima, Yuka Horikawa (RIKEN/SPring-8)
INTRODUCTION
Knowledge of the electronic structure of materials is quite important for understanding theproperties of materials. Especially, valence electrons play important roles in electric, magnetic
and chemical properties. Therefore, valence orbitals/bands of gaseous/solid materials have beencommonly investigated by means of photoemission spectroscopy (PES). However, an ordinary
PES is not suitable for the studies of liquid materials since PES is a photon-in electron-out
experiment which detects photoelectrons in vacuum condition. Recent advancements in the
production of ultra thin films in the order of 100-200nm thickness, which is almost transparent

Molecular orbitalsof water~few fsec

Absorption

Emission

Energy2b2

Intensity= Density of states

4a 1

-

Unoccupied

Soft x-ray
absorption

Occupied

1b1
Soft x-ray
emission

3a 1

Valence

ay
x-r
ft
So

1b2

y )
-ra ont x issi
f
SoEm
(

2a 1

Core
1a 1

Fig. 1 Schematic energy diagrams of soft x-ray emission and absorption spectroscopy. Relation
between spectral feature and molecular orbitals of water molecule is shown as an example.
1

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
photon-out experiment observes the valence electronic structure through energy spectra ofemitted photons, can be used as an alternative method of PES for liquid samples. The firstobservation of XES for liquid water was reported in 2002[2]. Subsequently, 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 and emission spectroscopy. Xray absorption spectroscopy (XAS) measures the absorption coefficient of electron transition
from core to unoccupied orbitals [20]. Therefore, XAS provided information on unoccupied
states. Since XAS process generates unstable core hole in the system, subsequent relaxation
processes occur within a core-hole lifetime (a few fsecs for light elements). Major relaxation
pathways is Auger electron emission which is related to Auger electron spectroscopy. One of
the minor relaxation pathways is the x-ray emission. XES spectra reflect occupied valence
electronic states via electronic transitions between core and valence orbitals.

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SELECTIVE XES OBSERVATION OF SOLUTION SAMPLES
Fig.2A shows the binding energy of core orbits up to 2200 eV (Data were taken from thetable of electron binding energies in the "X-ray Data Booklet"[21]). The binding energy of acore orbital is largely different depending on elements. Core level spectroscopies have thus theunique capability to observeshows XES spectra of aqueous
Fe(III) chloride, which containshydrogen, oxygen, iron andchlorine as elements. Owing todifference of binding energy in

Binding energy [eV]elements separately. Fig.2B

2200
2000
1800
1600
1400
1200
1000
800
600
400
200
0

A

observed at different energy
region.
In general, many solutions
contain same element in solute

3s
0

B

5

consist of limited elements

10

15
20
25
Atomic number

O (H2O)

30

35

Fe3+

Lα,Lβ
Fe3d→2pKα
O2p→1sx3

FeCl3 aq.

1 mol/L

500

600
Emission energy [eV]and solvent molecules.
Especially, organic molecules

Iron

2p

Intensity [arb. units]iron of Fe(III)Cl3 were1s

Oxygencore orbitals, X-ray emission
spectra of oxygen in H2O and2s

Elastic
scattering(722eV)

700

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.

such as carbon, nitrogen andoxygen atoms. In this case, element selectivity of XES is not enough to specify one sort ofmolecule. Hence, selectivity beyond element selectivity is needed for the research of solution
samples using XES.
Fig. 3A shows the O1s XAS spectrum of liquid acetone, liquid N,N-dimethylformamide(DMF), liquid acetic acid, aqueous acetic acid and liquid water. It might not be hard to find theconnection between molecular structure around oxygen and the peak structure of XAS. The O1s3

XAS spectrum of liquid acetone shows an intense peak at 531.3eV. This peak structure is also
appeared in XAS spectra of DMF with small energy shift. Interestingly, energy positions of thefirst peak correspond well to the first peak of acetic acid.
has no C=O type oxygen,

On the other hand, liquid water thatthen the O1s XAS spectrum shows no peak structure corresponding

to the first peak of acetic acid.

A

OC=O O1s XAS
Acetone

C

O

CH3

H

DMF

B

CH 3

C
O

CH 3

N

CH3

Intensity (arb. unit)

Aceticacid

Selective Excitation

CH 3
O

C
O

H

Aqueous acetic acid
H O

OO-H

530

water

H

540
550
560
Photon energy [eV]Fig. 3 (A) XAS spectra of acetone, DMF, acetic acid, aqueous acetic acid andwater. Molecular structures of each molecule are depicted on right side of the
figure. (B) Schematic drawing of selective excitation of XES.

Based on XES experiments using BL17SU, the first peak and the second peak in O1s XAS
spectrum of acetic acid were assigned to selective excitations of the OC=O 1s and OOH 1s
respectively [5, 8]. By tuning excitation energy to such a specific peak corresponding tomolecular structure, one can select molecule 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 in4

solvent water, excitation energy was tuned

A

O 1s XES

to the OC=O 1s → π* resonance excitation,

Acetic acid aq. 2M
pH 0.29

which is absent in water.

CH3COOH
Exp.

changes of XES spectra depend on pH isobserved. Since acid dissociation constant,pKa, of acetic acid in aqueous solution is

Intensity (arb. unit)

As shown in Fig.4A, pronounced

MO Calc

pH 11.44

CH3COOExp.

known to be 4.756 at room temperature,

MO Calc515

acetic acid molecules exist in the neutral
form in the acidic solution with pH 0.29. In

the anionic form of acetic acid should bedominant species. In the theoretically

A pH 11.44
B pH 6.30
C pH 5.71
D pH 5.20
E pH 4.73
F pH 4.30
G pH 3.73
H pH 3.28
I pH 0.29

515

calculated XES spectra of neutral andanionic acetic acid plotted below
experimental spectra in Fig.4A, all peakstructures in the experiment are well
reproduced.

This result indicates that the525

O 1s XES

Intensity (arb. unit)

contrast, for the basic solution (pH 11.44),

B

520
Emission energy [eV]520
Emission energy [eV]525

Fig. 4 (A) XES spectra of aqueous acetic acid inacidic and basic solution. Calculated XES spectra
based on DFT calculation is plotted in lower partof the plot. The both calculated spectra were
shifted by −1.2eV to fit the experimental peakfeature with highest intensity. (B) pH dependence

selective excitation of XES successfully extracts the occupied valence electronic structure of theacetic acid molecules in aqueous solution.
Since acetic acid is a sort of acid, dissociation of hydrogen in OH structure carboxyl groupoccurs in aqueous solution. However, acetic acid is classified as weak acids. Deprotonation
occurs as the pH of its solution decreases as shown in the pH evolution of the valence electronic
structure probed by XES (see Fig.4B). Importantly, there are five cross points, i.e. isoemissive
points, in XES spectra of the pH evolution. Isoemissive points indicate the spectra consist of theoverlapping signals of two components; two components are neutral and anionic forms of acetic
acid in this case. XES is also useful as analytical tool to monitor the ratio of neutral and anionicforms based on the data in Fig.4B (see ref. [5] for detail).

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Such kind of technique of selective observation is useful for the study of chemical reaction.
Because reactants and products in chemical reactions always contain the same elements,
electronic state observation using XES is quite difficult only with element selectivity.

OUTLINE OF THE SOFT X-RAY UNDULATOR BEAMLINE BL17SU
AND THE EXPERIMENTAL APPARATUS
Soft x-ray beamline BL17SU [22] is constructed to advance the spectroscopic studies formainly 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]. This 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 lightof 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 high resolution and highly stabilized monochromator [22, 27] and several endstations. The available energy is ranged between 300 and 1800 eV. Typical resolving power

Fig. 5 Schematic drawing of soft x-ray emission spectrometer HEPA 2.5
6

E/ΔE of the monochromator is higher than 10,000 and the photon flux is of the order of 1011
photons/s.
End-stations at BL17SU beamline are actively used for studies of surface adsorbates, liquids,
solutions and solids using PES, XAS, XES spectroscopies and soft x-ray diffraction. In thecourse, the participants will use the end station for liquids and solutions, which is located at thebranch-a. The apparatus consists of the XES spectrometer, which named High Efficiency
Photon Analyzer (HEPA) [28, 29](see Fig.5), and the main chamber equipped with the compact
flange-mount liquid flow-cell which is designed for the studies of liquids under ambientconditions. Fig.6 shows liquid flow cell which utilizes ultra-thin film of SiN (Silicon nitride) or
SiC (Silicon carbide) with 150nm thick as a window to separate vacuum and sample underatmosphere.

VacuumICF114flangeAtomosphere

To emission spectrometer

To photodiodefor absprption
measurement

Thin film
window(150nm thick)

Sampleintet/outlet

Incident
beam

Fig. 6 Schematic drawing of liquid flow cell for soft x-ray spectroscopy

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EXPERIMENT OF THE BEAMLINE PRACTICAL AT BL17SU
In the course, the participants will experience soft x-ray absorption and emission
measurements for the samples in aqueous solutions under ambient conditions, i.e. room
temperature and atmospheric pressure. The participants will also learn the principle of soft x-ray
spectroscopy, detail of the soft X-ray beamline and the soft x-ray emission spectrometer. The
followings are a plan of the experiment at present.

PREPARATION OF SAMPLES AND SETUP OF THE LIQUID FLOW CELL
FOR SOFT X-RAY SPECTROSCOPY

At the beginning of the course, the participants will experience assembly of a liquid flow cellfor soft x-ray measurement. During the evacuation of the chamber, a preparation of the liquidsample will be carried out. Detail of soft x-ray spectroscopy including experimental apparatus
such as the beamline and HEPA spectrometer will be also explained.

CALIBRATION OF THE BEAMLINE MONOCHROMATOR

Calibration of the beamline
monochromator will be performedby using a hemispherical electronenergy analyzer (SES-2002, VGSCIENTA) installed at the different
station on a-branch. The absoluteenergy of monochromatized x-ray
will be evaluated by PES
measurement of Au 4f electronsstndusing 1 and 2 order x-ray ofFig. 7 Example of Au4f PES measurements for energy
calibration of beamline monochromator

beamline monochromator. The
8higher order (harmonics) of x-rays
reaches sample location as 2nd orderdiffraction of beamline
monochromator.

Since photon energyof 2nd order diffraction is exactly twiceof 1st order diffraction, one can obtainabsolute value of incident x-rays bymeans of PES measurements (seeFig.7).
Many grating spectrometers use theFig. 8 Example of energy calibration curve for beamline
monochromator of BL17SU

sine bar mechanism for rotating thegrating. The beamline monochromator of BL17SU also use the sine bar mechanism. Hence, linear
motion of motor and output wavelength of monochromator becomes as follows,
Wavelength

a

x

b,

where x denote location in linear motion.

Since we use a pulse motor for the linear motion, x ispulse count in this case. Photon energy is inversely related to the wavelength of photons. Hence,
we can calculate energy like Fig.8.

XAS AND XES MEASUREMENTS OF SOLUTION SAMPLES

When conditions are ready for the measurement, XAS measurement of the sample solution
will be performed by means of the total photon yield. Based on the obtained XAS spectra,
excitation energy for selective excitation will be determined, and Selective XES observation ofsolute molecule will be performed.

CALIBRATION OF THE EMISSION SPECREOMETERIn XES spectra, an elastic scattering peak which has exactly same energy of incident beam is9

normally observed. Using observed
elastic peaks, energy calibration ofemission spectrometer is possible.
Fig.9 shows elastic scattering peakfor three different energy excitation.
Differently

from

the

beamline

monochromator, relation between
energy and location on the detector is
not liner. Calibration curve and theequation of energy vs pixel number
on the detector are shown in Fig.10.

Fig. 9 Example of XES measurements for energy
calibration of emission spectrometer. Red line shows
spectra after filtering for cosmic rays and environmental
radiation. Since solid line is original data, spikes with solidline are cosmic rays or environmental radiation.

Fig. 10 Calibration curve and the equation ofenergy vs pixel number on the detector for the

DATA ANALYSIS AND DISCUSSIONS

After completing all the measurements, the spectrum for each sample will be analyzed andcompared. The participants are now able to compare and discuss the results of the XES
measurements for liquid samples.

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