BL practice

BL17SU : Electronic State Observation of Solute Molecules under AmbientConditions

Takashi Tokushima (RIKEN/SPring-8)
1. Introduction
Knowledge of the electronic structure of materials is quite important for understanding theproperties of materials. Especially, valence electrons play important roles in electric, magneticand chemical properties. Therefore, valence orbitals/bands of gaseous/solid materials have beencommonly investigated by means of photoemission spectroscopy (PES). However, an ordinary
PES is unsuited for studying the electronic structure 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, whichis almost 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 ofphoton-in photon-out experiment observes the valence electronic structure through energy
spectra of emitted photons, can be used as an alternative method of PES for liquid samples. The

Molecular orbitalsof water
Absorption

Emission

Energy~few fsec

2b2

Intensity= Density of states

4a1-Unoccupied

Soft x-ray
absorption

Occupied

1b1

ayx-r
ft
So

1b2
CoreSoft x-rayemission

3a1

Valencey )-ra ont x issi
fSoEm
(2a11a1

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

first observation of XES for liquid water was reported in 2002[2]. Subsequently, in recent tenyears, liquids and solutions have been actively studied using XES spectroscopy [3-19].
Fig.1 shows schematic energy diagrams of x-ray absorption and emission spectroscopy. X-ray
absorption spectroscopy (XAS) measures the absorption coefficient of electron transition from core tounoccupied 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 (few femto secs for light elements). One of the relaxation pathways is the x-ray emission. XES
spectra reflect occupied valence electronic states via electronic transitions between core and valenceorbitals.

2. Selective XES observation of 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

elementsseparately.

aqueous Fe(III) chloride, whichcontains hydrogen, oxygen, ironand chlorine as elements. Owing
to difference of binding energy incore orbitals, X-ray emission

Binding energy [eV]Fig.2B shows XES spectra of2200
2000
1800
1600
1400
1200
1000
800
600
4002000

A

2s1sIron

Oxygen2p3s0

spectra of oxygen in H2O and iron

51015
2025
Atomic number

30

35of Fe(III)Cl3 were observed at

In general, many solutions
contain same element in soluteandsolvent

molecules.

BO (H2O)

Intensity [arb. units]different energy region.

Fe3+Lα,LβFe3d→2pKα
O2p→1sx3FeCl3 aq.
1 mol/L

Elasticscattering(722eV)

Especially, organic molecules
500

consist of limited elements suchas carbon, nitrogen and oxygen
atoms. In this case, element

600
Emission energy [eV]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.

2

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.
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 the connection
between molecular structure around oxygen and the peak structure of XAS. The O1s XAS spectrumof 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 the first peak correspondwell to the first peak of acetic acid.

On the other hand, liquid water that has no C=O type oxygen,then the O1s XAS spectrum shows no peak structure corresponding to the first peak of acetic acid.

Fig. 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
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 to molecular structure, one
can select molecule based on molecular structure as depicted in Fig. 3A.
Fig.4A shows an example of OC=O selective observation for aqueous acetic acid in acid-base3equilibrium. In order to perform a selectiveobservationofXES

molecules in solventfor

aceticacid

water, excitationenergy was tuned to the OC=O 1s → π*resonance excitation, which is absent inwater.
As shown in Fig.4A, pronounced changesof XES spectra depend on pH is observed.
Since acid dissociation 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 the basic
solution (pH 11.44), the anionic form of acetic
acid should be dominant species. In thetheoretically calculated XES spectra of neutral
andanionicaceticacid

plottedbelow

experimental spectra in Fig.4A, all peakstructures
reproduced.

inthe

experimentareFig. 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 dependenceof XES spectra.

wellThis result indicates that the selective excitation of XES successfully extracts theoccupied valence electronic structure of the acetic 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 structureprobed 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 the overlapping signalsof two components; two components are neutral and anionic forms of acetic acid in this case. XES isalso useful as analytical tool to monitor the ratio of neutral and anionic forms based on the data inFig.4B (see ref. [5] for detail).
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, electronic4

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

3. 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. Anovel 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 orpseudo-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
end-stations. The available energy is ranged between 300 and 1800 eV. Typical resolving power
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], and the main chamber equipped with the compact flange-mountliquid flow-cell whichisdesignedforVacuum

thestudies of liquids underambient

Atomosphere

To emission spectrometerconditions.
To photodiodefor absprption
measurement

Fig.3 shows liquid flow
cellICF114
flangewhich

utilizes

ultra-thin film of SiN

Thin film
window(150nm thick)

Sampleintet/outlet

Incident
beam(Silicon nitride) or SiC(Siliconcarbide)with150nm thick as a window

Fig. 5 Schematic drawing of liquid flow cell for soft x-ray spectroscopy

to separate vacuum andsample under atmosphere.

54. 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 andatmospheric 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 aplan of the experiment at present.
4.1 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.
4.2 Calibration of the beamline monochromator
Calibration of the beamline monochromator will be performed by using a hemisphericalelectron energy analyzer (SES-2002, VG-SCIENTA) installed at the different station ona-branch. The absolute energy of monochromatized x-ray will be evaluated by PES
measurement of Au 4f electrons using 1st and 2nd order x-ray of beamline monochromator. The
relationship between photon energy and grating rotation of the beamline monochromator will beobtained based on the results of PES measurement.
4.3 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, excitationenergy for selective excitation will be determined, and Selective XES observation of solute
molecule will be performed.
4.4 Data analysis and discussions
After finishing 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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