FEL

Asymmetric Reactions of Amino-Acids and Their Precursor Molecules by
Circularly Polarized Light from Free Electron Laser
P. K. Sarker1, T. Kaneko1, K. Kobayashi1, J. Takahashi2, H. Mita3, M. Adachi4, H. Zen4,
M. Hosaka5 and M. Katoh4
1
Yokohama National University, Yokohama 240-8501, Japan
2
NTT Microsystem Integration Laboratories, Atsugi 243-0198, Japan
3
Fukuoka Institute of Technology, Fukuoka 811-0295, Japan
4
UVSOR Facility, Institute for Molecular Science, Oakazaki 444-8585, Japan
5
Nagoya University, Nagoya 464-8601, Japan

Introduction
The origin of homochirality in terrestrial biologicalmolecules (dominant L-body amino acids and D-bodysugars) remains an unresolved important problem inthe study for the origins of life. One of the most
attractive hypotheses for the origin of homochirality
is nominated as “Cosmic Scenario”; some chiral
impulses from asymmetric excitation sources in space
triggered asymmetric reactions on the surfaces ofsuch space materials as meteorites or interstellar dusts.
According to this scenario, the enantiomeric excesses
in terrestrial amino acids can be advocated thatasymmetric reactions of complex organic molecules
including amino acid precursors were induced bycircularly polarized light (CPL) from synchrotron
radiation (SR) source in space prior to the existenceof terrestrial life. Recently, a wide-field and deepnear-infrared circularly polarized light has been
observed in the Orion nebula, where massive stars
and many low-mass stars are forming [1]. Thisobservation result strongly support the “Cosmic
Scenario” with CPL in space for the origin ofhomochirality in terrestrial bioorganic compounds.
Presently, we are conducting verification groundexperiments for the extraterrestrial scenario by using
CPL from SR facilities as a simulating polarizedenergy source. We have already reported theexperimental results of asymmetric reactions in solid
films of racemic mixtures of amino acids by
irradiating them with CPL in ultraviolet (UV) region
from free electron laser (FEL) of UVSOR [2]. The
emergence of optical anisotropy in the irradiatedfilms was detected by measurements of circulardichroism (CD) spectra. These results have suggested
that the UV-CPL irradiation caused not only
preferential photolysis between enantiomers but also
asymmetric reactions including conformation changes
or some other construct distortions. We have carriedout UV-CPL irradiation experiments for aqueous
solution of amino acids (DL-isovaline andDL-histidine) and for the films of the precursor
molecules of amino acids, such as hydantoins(five-membered heterocyclic molecules).

Experimental
Aquesous solution of IVal, His, or copper complexof His (pH = 3, 7 or 11 in all cases) was irradiatedwith CPL-UV at 215 nm from an FEL of UVSOR-II(IMS, Japan). Amino acids were determined byion-exchange
HPLC(Shimadzu
LC-10A).
Enantiomers of amino acids were separated by HPLC
with a chiral column (Sumichiral OA-50000).
Vacuum-evaporated thin film of hydantoin (5,5-dihydrogen; Fig. 1) was
also
irradiatedwith
CPL-UV: hydantoin is anachiral molecule and a
precursor of glycine. Afterthe irradiation, opticalanisotropy of the resulting
thin films were observedwith circular dichroism
Fig. 1. Hydantoin(CD) spectra.
Results and Discussion
After R-CPL irradiation of DL-isovaline aqueous
solution, small D-excess was observed, and the
L-CPL irradiation caused L-excess. Amino acidanalysis showed that the major amino acid productfrom isovaline was alanine.
Achiral hydantoin has presented apparent opticalanisotropy in the CD spectra of the irradiated films,
suggesting that asymmetric reactions peculiar to theheterocyclic ring molecules can induce chiralconstructions in the starting achiral molecules.
These results suggest that UV-CPL played an
important role to produce homochirality of terrestrial
amino acids from their precursor molecules.
Additional experiments for both chiral and achiralhydantoins and analyses including both identificationof reaction product molecules and theoretical
calculation of photo-induced constructions areimportant for the solution of the origins of biologicalhomochirality.
[1] T. Fukue et al., Orig. Life Evol. Biosph. 40 (2010)335.
[2] J. Takahashi et al., Int. J. Mol. Sci. 10 (2009)
3044.

53

BL2B

Effect of the Magic Number “60” on the Photodissociation Process of C70

1

H. Katayanagi1,2 and K. Mitsuke1,2Dept. of Photo-Molecular Science, Institute for Molecular Science, Okazaki 444-8585, Japan
2
Graduate University for Advanced Studies, Okazaki 444-8585, Japan

We have developed a photofragment imaging
spectrometer suitable for synchrotron radiation (SR)excitation of gaseous molecules of refractorymaterials [1]. Using this apparatus we have observed
the scattering distributions of the fragments producedby the photodissociation of C60 such as C60-2n2+ [2].
We found that the kinetic energy release (KER) of thereaction step to produce C502+ fragments was smaller
than those of the other reaction steps. This shows that
the C502+ fragment is more stable than the otherC60-2n2+ fragments. The relative stability of thefragments can thus be obtained. In the present study,
we apply this method to observe fragments producedby the photodissociation of the higher fullerene, C70,
which decays into C60 fragments through stepwise C2
emission.
The experiments were performed at BL2B in
UVSOR. The experimental procedure is almost
identical to that of the photofragment imaging of C60[2].
Figure 1 shows two-dimensional (2D) maps oftime-of-flight (TOF, t) and arrival position along ydirection of ion signals on a position sensitive
detector (PSD). The y-direction on the figure isorthogonal to the path of the parent C70 beam. The
conspicuous three spots in Fig. 1 (a) are ascribablefrom left to right to the parent ions of C703+, C702+ and
C70+, respectively. These spots have narrow
y-distributions since the C70 neutral beam is wellcollimated. In Figs. 1 (b) and (c) subtle stripes withmuch broader y-distributions are seen on the left ofthe parent ions’ spots; these stripes are assigned to thefragments produced by successive ejection of C2 units
from the parent ions. The broader y-distributions of
the stripes indicate larger velocity, or higher
translational temperatures of the fragments acquiredby the dissociation than those of the parent ions.
Figure 2 shows the translational temperatures, T, ofthe fragments determined from the y-distributions of
the fragments on Fig. 1. The smaller fragments havethe higher temperatures. We can regard the gap
between curves of n = 4 and 5 are smaller than theother gaps between the curves. This means that thetemperature increase between C622+ (n = 4) and C602+(n = 5) is exceptionally smaller than those between
other fragments. The small increase in the C602+
production process reflects small KER in this reaction
step. This is partly because of relative stability ofC602+ fragment having the magic number of 60. In
addition, there might be another reaction pathway to
produce C602+ such as one-step two-fragment fission.

Fig. 1. (a-c) y-t maps of parent and fragment ionsproduced by the photodissociation of C70. Excitation
photon energies are shown in each panel. (d) TOF
profile obtained from (c) by integration with respect
to y. Assignments of peaks are shown in the panel.

Fig. 2. Excitation photon energy dependence oftranslational temperatures of parent and fragment ionsproduced at n-th reaction step to produce C70-2n2+
evaluated from y-t maps shown in Fig. 1 (a-c). Error
bars show deviations among experimental runs on
different days.
[1] Md. S. I. Prodhan et al., Chem. Phys. Lett. 469(2009) 19.
[2] H. Katayanagi and K Mitsuke, J. Chem. Phys. 133(2010) 081101.

54

BL2B

Mass-Analyzed Velocity Map Imaging of Thermal Photofragments from C60
1

H. Katayanagi1,2 and K. Mitsuke1,2Dept. of Photo-Molecular Science, Institute for Molecular Science, Okazaki 444-8585, Japan
2
Graduate University for Advanced Studies, Okazaki 444-8585, Japan
steps increase with increasing hQ, reflecting statistical
redistribution of the excess energy in the transition
state, whereas that in the fifth step leading to C502+ was
exceptionally small. This might reflect C50 has a cage
structure and the number 50 is a magic number
although C50 cannot satisfy the isolated pentagon rule.
In addition, knowledge on the energy partitioning canbe obtained from the KER curves. We can estimate
the internal energies, Eint, of parent C602+ ions fromthe excitation photon energy, ionization potential andthermal energy of the parent ion. The dashed line in
Fig. 2 shows the internal energy divided by the
degree of freedom, Eint / F. The KER values of n=1
agree with the dashed line. This means that theinternal energy partitioned to one degree of freedom
is dissipated to the reaction coordinate of C2 emission
among the vibrational degrees of freedom.

Dissociation mechanisms of fullerenes have been
extensively studied since the fullerenes are systems
which
have
well-defined,
highly-symmetricmolecular structure despite their large number ofdegrees of freedom, F (e.g. C60, F = 174). The
knowledge on the dissociation mechanisms of suchlarge F systems provides a severe benchmark forvarious statistical theories of reaction dynamics. In
the present study, the velocity distributions of thefragments produced by dissociative photoionization of
C60 have been measured in the extreme UV region for
the first time, using a flight-time resolved velocity map
imaging technique combined with a high-temperature
molecular beam and synchrotron radiation [1]. Fromthe velocity distributions, detailed properties
reflecting the dissociation mechanism can be obtainedsuch as kinetic energy release (KER) of dissociationand partitioning of internal energies of reactants
among the degrees of freedom.
The experiments were performed at BL2B in
UVSOR. Fullerene (C60) powder was loaded in aquartz tube and heated up by an electric heater ataround 700-800 K in vacuum. The C60 vapor passed
through two apertures and reached the ionization
region, where the C60 molecular beam (x axis)
intersected the monochromatized synchrotron
radiation (y axis) at right angles. Ions produced at theionization region were extracted by a velocity map
imaging electrode assembly and projected along z
axis on to a position sensitive detector (PSD) of 40
mm in diameter and 375 mm away from theionization region. Photolectrons were extracted to the
opposite direction to the ions and detected by amicrochannel plate (MCP) detector. Time of flight(TOF, t) and arrival position (x, y) of ions on the PSD
were recorded using the signal of photoelectrons as astart trigger. Three-dimensional (3D) lists of data, (x,y, t), were thus obtained.
Integration of the 3D listdata with respect to x, 2D
y-t distributions were obtained (not shown here, see
Ref. 1). Further integration with respect to y gives us
ordinary TOF profiles. Distributions along y-direction
on the y-t map with limited TOF ranges whichcorrespond to mass-to-charge ratios (m/z) of eachfragment were then extracted. The y-distribution ofeach fragment is 1D projection of 3D velocity
distribution and is converted to translationaltemperature.
Figure 1 shows the translational temperatures ofthe fragments. From the temperatures, average KER
for respective reaction steps were obtained assuming
stepwise C2 emission and shown in Fig. 2.
In Fig. 2, thevalues of KER in the first to fourth

Fig. 1. Excitation photon energy dependence oftranslational temperatures of the fragments (C60-2n2+).

Fig. 2. Excitation photon energy dependence of total
average KER generated at the n-th reaction step to
produce C60-2n2+. Error bars show the uncertainty
caused in the fitting procedure. Dashed line indicatesthe value of Eint / F in the first reaction step.
[1] H. Katayanagi and K Mitsuke, J. Chem. Phys. 133(2010) 081101.

55

BL2B

Breakdown Pathways of Doubly-Charged Perfluorocyclobutane
and cis-1,1,2,2,3,4-Hexafluorocyclobutane

K. Okada1, T. Nakashima1, M. Sakai1, A. Suemitsu1, H. Katayanagi2,3 and K. Mitsuke2,3
1
Department of Chemistry,Hiroshima University, Higashi-Hiroshima 739-8526, Japan
2
Institute for Molecular Science, Okazaki 444-8585, Japan
3
Graduate University for Advanced Studies, Okazaki 444-8585, Japan
C4F72+ ĺ C2F4+ + C2F3+,
C2F3+ ĺ CF2+ + CF.
In this way, the following breakdown pathways canbe identified for c-C4F8:(1)
c-C4F82+ ĺ C4F72+ + F ĺ C3F5+ + CF2+ + F
C2F4+ + C2F3+ + F
Ļ
C2F4+ + CF2+ + CF + F (2)C4F62+ + 2F ĺ C3F4+ + CF2+ + 2F (3)ĻC3F3+ + CF2+ + 3F (4)c-C4F82+ ĺ C3F62+ + CF2 ĺ C2F4+ + CF2+ + CF2
ĻCF3+ + CF2+ + CF2 + CF (5)etc.
Similar breakdown pathways are found for the
fragmentation of cis-c-C4H2F4. Reaction channels
deduced from the slope analysis correspond to
Reactions 1, 2, 4 and 5, with formally replacing one
or two F atoms with H atoms.

Molecular photoionization and ionic fragmentation
processes are of fundamental importance in theupper-atmospheric chemistry and plasma physics.
Perfluorocyclobutane (c-C4F8) is extensively used asa reagent for dry etching of semiconductors. However,the use of the gas in industry has atmospheric
implications for global warming. In our previous
study [1] we report yield spectra of the fragment ionsof c-C4F8 and cis-1,1,2,2,3,4-hexafluorocyclobutane(cis-c-C4H2F6), a candidate for replacing c-C4F8, in
the photon energy range of 25–170 eV. Due to theshortage of the knowledge of the photochemical
process, ion pairs originating from charge separationof the multiply-charged molecular ions were detectedin this study using a photoelectron–photoion–
photoion coincidence (PEPIPICO) technique toobtain information on the breakdown pathways.
The experiments have been performed on thebeamline BL2B at the UVSOR facility. The
experimental setup has been described in a previous
paper [1]. Electrons and ions produced by thephotoabsorption were extracted toward the opposite
directions. The detection signals of the electrons andions were fed into the start and stop pulse inputs of amulti-stop time-to-digital converter (FAST ComTec,
P7888), respectively, to record time-of-flight (TOF)
data. The data were acquired at 35.0, 40.0, 45.0, 50.0
and 90.0 eV. PEPIPICO spectra were constructedfrom the ion–ion coincidence events as maps.
Figure 1 depicts a typical PEPIPICO spectrum
obtained for c-C4F8 at the excitation energy of 45.0
eV. Several coincidence islands can be found in the
spectrum, along with ridges arising from false
coincidences. In this study we focus on the slopes of
the islands. The coordinate tells us the masses of thetwo fragment ions, while the slope gives information
about the fragmentation mechanism to produce suchions.
The most abundant fragment ion pair detected is
C2F4+/CF+. Its coincidence island has round shape,
indicating that the ion pair can be formed by somedecomposition processes. A long parallelogram ofslope –1.01 is observed for the island corresponding
to the C3F5+/CF2+ ion pair. This can be explained by
the two-body fragmentation of C4F72+. The measured
slope for the C2F4+/CF2+ coincidence island is –1.63,
which is well described by the three-body secondary
decay mechanism:

Fig. 1. Overview of a typical PEPIPICO map for
c-C4F8 acquired at 45.0 eV. The top and left panels
indicate the corresponding TOF spectrum.
[1] K. Okada, T. Nakashima, M. Sakai, A. Suemitsu,
C. Huang, H. Yagi, H. Katayanagi, K. Mitsuke and K.
Tabayashi, J. Phys. Conf. Ser., accepted.

56

BL3U

Electronic Structure of Liquid Methanol Studied
by Carbon K-Edge Soft X-Ray Absorption Spectroscopy
M. Nagasaka and N. Kosugi
Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan

Methanol (CH3OH) is a liquid at room temperatureand is the simplest of the amphiphilic molecules withboth hydrophilic and hydrophobic groups. It is known
that some mixtures of methanol and water show
characteristic hydrogen bonding networks by thecluster formation [1]. The microscopic structure of
the hydrogen bonding network of liquid methanol is
not fully understood. X-ray absorption spectroscopy(XAS) is a promising method to study the local
electronic structure of the hydrogen bond in liquidwater [2] and so on. Combination of the O and C
K-edge XAS spectra of methanol is useful to reveal
the local electronic structures around C and O atoms
separately. Wilson et al. reported XAS spectra ofmethanol in the total electron yield of liquid microjetof methanol [3]. It is difficult to extract liquid XAS
spectra from the spectra of gas and liquid mixture.
Recently, we have developed a liquid cell for themeasurement of XAS in the transmission mode [4].
In the present work, we apply this transmission C
K-edge XAS technique to investigate the local
electronic structure of liquid methanol.
The experiments were performed at BL3U. The
liquid thin layer was sandwiched between two 100
nm-thick SiNx (NTT AT Co.). Thickness of the liquid
layer was optimized to be 250 nm by adjusting thehelium backpressure. The energy resolution was set tobe 0.19 eV at 280 eV. The photon energy wascalibrated by the C 1s - ʌ* peak (290.77 eV) of theCO2 gas mixture in He [5]. XAS spectra for methanol
gas mixture in He were also measured.
Figure 1 shows C K-edge XAS spectra ofmolecular (gas) and liquid methanol at 25 ºC. Twopeaks around 288 and 289.5 eV and several Rydberg
states are observed in the molecular spectra. The 288
eV and 289.5 eV peaks contain O-H and C-H
components and the 292.5 eV peak contains a ı* C-Ocomponent. Our C K-edge XAS spectrum of liquidmethanol shows a simple structure with threecontributions around 288.5, 290, and 293 eV as
shown in Fig. 1(b); on the other hand, the microjet
experiment did not give such a large spectral
difference between gas and liquid [3]. The XAS
spectra for methanol clusters [6] are similar to our
liquid spectrum. The contribution from the methanol
gas would be not completely removed in the microjet
experiments.
As shown in Fig. 1 (b), the peak around 288.5 eV
in liquid methanol is shifted to higher photon energy
compared to that of methanol gas. The energy shift(0.53 eV) would be caused by the formation of thehydrogen bonding networks between methanol

molecules. The peak around 290 eV in liquidmethanol is also shifted to the higher photon energy
compared to that of methanol gas, but the energy shift(0.20 eV) is smaller than in the first peak. This may
be explained by a dominant contribution from thehydrophilic OH component in the first band and adominant contribution from the hydrophobic CH
component in the second peak. The peak around
293 eV does not show a noticeable difference
between gas and liquid because the ı* (C-O) orbital
is not influenced by the hydrogen bonding network.

Fig. 1. Carbon K-edge XAS spectra for (a) methanol
gas and (b) liquid methanol at 25 ºC. The thickness ofliquid methanol layer is estimated to be 250 nm.
[1] C. Corsaro et al., J. Phys. Chem. B 112 (2008)
10449.
[2] Ph. Wernet et al., Science 304 (2004) 995.
[3] K. R. Wilson et al., J. Phys. Chem. B 109 (2005)
10194.
[4] M. Nagasaka et al., J. Electron Spectrosc. Relat.
Phenom. 177 (2010) 130.
[5] T. Tanaka et al., Phys. Rev. Lett. 95 (2005)
203002.
[6] Y. Tamenori et al., J. Chem. Phys. 128 (2008)
124321.

57

BL3U

Hydration Structures of Li Cations in LiCl Aqueous Solutions Studied by
Oxygen K-Edge Soft X-Ray Absorption Spectroscopy
M. Nagasaka1, T. Hatsui2 and N. Kosugi1
Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan
2
XFEL Project Head Office, RIKEN, Sayo-cho, Hyogo 679-5148, Japan
1

Macroscopic properties of aqueous solutions, suchas viscosity, boiling point, and freezing point, areinfluenced by the interaction between ions and watermolecules because the hydrogen bonding networks
are formed or broken by the interaction of the ions. In
aqueous salt solutions, the cation is near the oxygensite of water molecules, whereas the anion is near thehydrogen site. The interaction of the anion with water
has been studied by using the OH stretch vibration
mode in the Raman spectroscopy [1]. On the other
hand, the hydration of the cation has not been studied
in detail by vibrational spectroscopies because theOH vibration is insensitive to the cation. Recently, theelectronic structure of liquid water has been studiedby oxygen K-edge X-ray absorption spectroscopy(XAS) [2]. The pre-edge peak of liquid water (535eV) corresponds to the transition from the oxygen 1s
electron to the 4a1* unoccupied orbital, which ismainly distributed on the oxygen atom and isdominated by the short-range interaction. The energy
shift of the pre-edge peak would be sensitive to thenearest neighbor cation coordination to the water
oxygen. In this work, we have studied the hydration
shell of Li cations in the LiCl aqueous solutions byusing oxygen K-edge XAS.
The experiments were performed at BL3U. The
details of the liquid cell for the XAS measurements inthe transmission mode were described previously [3].
The liquid thin layer was sandwiched between two
100 nm-thick SiNx (NTT AT Co.). The thickness of
the liquid layer was optimized from 50 to 1000 nm bychanging the He backpressure [3]. The photon energy
was calibrated by the O 1s - ʌ* peak (530.8 eV) of
the O2 gas mixture in He.
Figure 1 shows O K-edge XAS spectra for the LiClaqueous solutions with different concentrations at 25ºC. As the concentration increases, the pre-edge peakis gradually shifted to the higher photon energy. As
shown in the inset, the isosbestic points wereobserved in the pre-edge region, which suggests theexistence of two components in the pre-edge region
dominated by the short-range interaction. We alsomeasured the O K-edge XAS for different cations andanions (NaCl, KCl, NaBr, and NaI); the energy shiftof the pre-edge region is dependent on the alkali ion
but is not on the halide ion. This indicates that thetwo components in the spectra (Fig. 1) are derived
from the bulk water and the Li+-solvating water.
In order to extract the contribution of the
Li+-solvating water from the spectra, the numbers of
the bulk and solvating water molecules were

calculated, assuming the coordination number ofwater molecules to Li+ is 4 [4]. In the XAS spectra
for the nearest neighbor water solvating the different
cations, the pre-edge peak in the Li ion shows a larger
energy shift than in the Na and K ions. The neutron
diffraction studies showed that the bond between the
Li cation and water is 0.190 nm, which is shorter thanthose of the Na and K ions [4]. This indicates that thepre-edge peak shift arises from the nearest-neighborinteraction of the cation with the oxygen site ofsolvating water molecules and the hydration to the Liion is stronger than the Na and K ions.
We also measured O K-edge XAS for the LiClaqueous solutions with different temperatures (4, 25,
and 60 ºC); the pre-edge peak corresponding to the
Li+-solvating water is not shifted irrespective of thetemperature. This agrees that the interaction of the Li
ion with liquid water is strong enough to keep thehydration shells even in different temperatures.

Fig. 1. Oxygen K-edge XAS spectra for the purewater and the LiCl aqueous solutions with different
concentrations at 25 ºC. The spectral differences
between aq. LiCl and bulk water are also shown. The
inset shows isosbestic points (dashed lines) in the
pre-edge region.
[1] J. D. Smith et al., J. Am. Chem. Soc. 129 (2007)
13847.
[2] L.-Å. Näslund et al., J. Phys. Chem. A 109 (2005)
5995.
[3] M. Nagasaka et al., J. Electron Spectrosc. Relat.
Phenom. 177 (2010) 130.
[4] N. Ohtomo and K. Arakawa, Bull. Chem. Soc. Jpn.
52 (1979) 2755.

58

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Structures of Small Mixed Argon-Nitrogen Clusters Studied by Soft X-Ray
Photoelectron Spectroscopy
M. Nagasaka1, E. Serdaroglu2, R. Flesch2, E. Rühl2 and N. Kosugi1
1
Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan2
Physikalische Chemie, Freie Universität Berlin, Takustr. 3, D-14195 Berlin, Germany

The structure of heterogeneous clusters isdependent on the cluster size and composition. The
structure of small clusters is very important from theviewpoint of the cluster formation mechanism. The
cluster structures have been extensively investigatedby theoretical simulations [1]; on the other hand,
there are few experiments to study the structures ofsmall mixed clusters because of several experimental
difficulties. Recently, we have measured X-ray
photoelectron spectroscopy (XPS) of small
homogeneous Kr and Xe clusters [2, 3] and small
mixed Kr-Xe clusters. Different binding energy shifts
in the Kr 3d and Xe 4d edges, which are observed indifferent sites of the Kr-Xe mixed clusters. These
arise from the two different induced polarization
effects of surrounding Kr and Xe atoms. In thepresent study, we have investigated the structures ofsmall mixed Ar-N2 clusters of different composition
by analyzing the core level shifts. N2 is a linear
molecule. Therefore, the core level shift of mixed
Ar-N2 clusters would be influenced by anisotropicinteractions of the N2 molecules.
The experiments were performed at BL3U. The
mixed Ar-N2 clusters were formed in a supersonic
expansion of the gas mixtures containing an Ar
mixing ratio between 10 % and 40 %. The nozzle
temperature and gas pressure were 163 K and 0.5
MPa, respectively. The average size of thehomogeneous Ar clusters becomes 200, as estimatedfrom the expansion conditions.
Figure 1 shows the Ar 2p3/2 XPS spectra for the Ar
cluster and the mixed Ar-N2 clusters of different
composition. The surface and bulk sites of theclusters are distinguished by a fitting procedure, as
shown in Table 1. The cluster size of 200 corresponds
to icosahedral multilayer structures with 4 or 5 layers.
The intensity ratio of the surface and bulk sites
observed for the Ar 2p3/2 XPS spectra of the pure(100 %) Ar200 cluster is consistent with suchicosahedral multilayer structures. On the other hand,in the expansion containing 10 % Ar, the intensity ofthe surface sites is smaller than that of the bulk sites.
This intensity ratio suggests a core-shell structure,
where the Ar aggregates are located in the bulk andare covered by a N2 shell. We have roughly estimatedthe composition of N2 bound in clusters from thepartial pressure of N2, and confirmed that thesurface-to-bulk ratio of Ar is consistent with acore-shell structure, where Ar is covered by the N2.
These core-shell structures are also observed in
expansions containing 20 % and 40 % Ar.

Fig. 1. Ar 2p3/2 XPS spectra of mixed Ar-N2 clusters
prepared from different mixing ratio of Ar. The
surface and bulk sites of the mixed clusters are
obtained from a fitting procedure.
Table 1. Binding energy shifts in the Ar 2p3/2 regime
at different sites of the mixed clusters relative to theatomic value in eV. The values in parentheses
correspond to the relative intensities of the Ar atoms.
Ar (%)
100
40
20
10

Surface-0.62 (124)-0.52 (79)-0.45 (32)-0.41 (11)Bulk
-0.90 (76)-0.79 (57)
-0.72 (30)-0.64 (17)[1] For example, J. W. Hewage and F. G. Amar, J.
Chem. Phys. 119 (2003) 9021.
[2] T. Hatsui et al., J. Chem. Phys. 123 (2005)
154304.
[3] M. Nagasaka et al., J. Electron Spectrosc. Relat.
Phenom. 183 (2011) 29.

59

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Chemical Environment Effect on Doubly Charged Ion States
after Auger Decay of Ethyl Trifluoroacelate Molecules
H. Iwayama1, E. Shigemasa1 and P. Lablanquie2
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan
2
LCPMR, Université Pierre et Marie Curie, 75231 Paris Cedex 05, France

1

O1s-1 states are close to those for the C1s-1 states of
the carbon sites neighboring to the F and O atoms,
respectively. These results indicate that the doubly
charged ion states after Auger decay reflect thechemical environment near the core-shell ionizedatoms. Our results suggest that the doubly charged
ion states after Auger decay can be also used as asensitive tool for a chemical analysis.

Inner-shell photoionization of light elements isgenerally accompanied by an Auger decay process. A
photoelectron and Auger electron are thus ejected,
and a doubly charged ion is produced. The binding
energy of an inner-shell electron in a molecule
depends not only on the atomic energy level but also
its chemical environment, which gives rise to a small
shift in the energy spectrum. This is the so-calledchemical shift. Based on the observation of suchchemical shifts in X-ray photoelectron spectra, we
can qualitatively discuss chemical bond character andatomic charge in molecules.
The chemical shifts may also be reflected in thebinding energies of the doubly charged ion state after
Auger decay, because the Auger transition probability
depends on a spatial overlap between the inner- andouter-shell electron wavefunctions. In the present
work, we investigated the binding energy spectra
after Auger decay for ethyl trifluoroacelate molecules(C4H5F3O2) which have four carbon atoms in
different chemical environment. This sample wasused to demonstrate the importance of chemical shifts
in X-ray photoelectron spectra by K. Siegbahn [1].
In order to obtain the binding energy spectra of
doubly charged ion states after Auger decay, we
performed a high resolution electron spectroscopy on
the soft X-ray beamline BL6U at UVSOR. We
measured kinetic energies of photoelectrons andAuger electrons of the C1s, O1s and F1s ionizations
with a high performance hemispherical electronenergy analyzer MBS-A1, developed by the MB
Scientific AB company. The photon energies were set
at 330, 575 and 730 eV for the C1s, O1s and F1s
ionizations, respectively.
Figure 1 shows the C1s photoelectron spectrum,
where four peaks corresponding to the four different
carbon sites are clearly observed: the chemical shifts
in binding energies of the C1s-1 states are detected.
By varying the photon energy, binding energies ofF1s-1 and O1s-1 states are also determined to be 694
and 539 eV, respectively.
We measured Auger electron spectra for the F1s
and O1s ionizations. From the Auger electron spectra
and binding energies of the F1s-1 and O1s-1 states, we
obtained binding energy spectra of doubly charged
ion states after Auger decay, which are shown in Fig.
2. Here the binding energies for the CF and CO sites
were measured on the undulator beamline
PLÉIADES at SOLEIL with using a magnetic bottle
electron spectrometer. We found that the binding
energy spectra after the Auger decays of the F1s-1 and[1] U. Gelius et al., J. Electron Spectrosc. 2 (1973)
405.

Fig. 1. The chemical shifts in X-ray photoelectron
spectrum for ethyl trifluoroacelate. The energy
resolutions for the monochromator and analyzer wereboth set to 30 meV.

Fig. 2. The binding energy spectra of the doubly
charged ion states after Auger decay for the C1s, F1s
and O1s ionizations. CF and CO denote the carbon
atoms neighboring to the F and O atoms, respectively.

60