Chemical Physics 447 (2015) 71–75

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Chemical Physics
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Ultraviolet photoelectron spectra of Ce2@C80 and La2@C80
Takafumi Miyazaki a,⇑, Sosuke Okita a, Tomona Ohta a, Hajime Yagi a, Ryohei Sumii b,c, Haruya Okimoto d,
Yasuhiro Ito d, Hisanori Shinohara d, Shojun Hino aaGraduate School of Science and Engineering, Ehime University, Matsuyama 790-8577, Japan
Institutes for Molecular Science, Okazaki 444-858, Japanc
Research Center for Materials Science, Nagoya University, Chikusa-ku, Nagoya 464-8602, Japand
Graduate School of Science, Nagoya University, Chikusa-ku, Nagoya 464-8602, Japanba r t i c l e

i n f oArticle history:
Received 27 August 2014
In ﬁnal form 3 December 2014
Available online 11 December 2014
Keywords:
Endohedral fullerenes
La and Ce atoms encapsulated C80 cage
Electronic structure
Ultraviolet photoelectron spectroscopy
X-ray photoelectron spectroscopya b s t r a c t
Ultraviolet photoelectron spectra (UPS) of C80-Ih cage endohedral fullerenes, La2@C80 and Ce2@C80 were
measured using a synchrotron radiation light source. The spectral onset energy of La2@C80 and
Ce2@C80 is around 0.8–0.9 eV, which is smaller than that of empty C80-Ih. The UPS of these endohedral
fullerenes are almost identical and are discussed with an aid of density functional theory (DFT) calculation. Simulation spectra calculated with using the results of the DFT calculations on an optimized structure starting from D3d geometry reproduces the UPS of La2@C80 and Ce2@C80 very well, which supports
the theoretically proposed structure.
Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction
Fullerene cages often encapsulate metal atoms and C82 cage
seems to be the most favorable cage in which metal atoms are
entrapped [1]. However, other cages also encapsulate metal atoms
although their production yield from direct current arc dischargeof metal oxide/carbon composite rods is not so high compared withthat of C82 endohedral fullerenes. Among other cage endohedral
fullerenes, C80 endohedral fullerenes attract attention because of
the reasons such as (a) empty Ih-C80 is not stable and cannot be isolated [2] but it becomes stable upon encapsulation of metal atoms[3], (b) Ih-C80 has the same symmetry as C60 [4] which exhibits
many interesting solid state properties like superconductivity [5]and ferromagnetism [6]. Although early stage X-ray diffraction
analysis combined with maximum entropy method (MEM) [7] suggested Ih-La2@C80 cage, theoretical calculations [4,8–12] revealed
that metal atoms entrapped Ih-C80 did not retain the original symmetry and encapsulation of metal atoms induced the degradationof symmetry. Crystallographic structure of a La2@C80 adduct also
supported this conclusion that the encapsulation of metal atoms
is favorable for the stability of the fullerene cage [13].
Early theoretical calculation on La2@C80 suggested thatentrapped two La atoms rotated in the D2h-C80 cage [8]. X-ray crystallographic analysis of its adduct, La2@C80 (CH2)(C6H5)3N, revealed⇑ Corresponding author. Fax: +81 89 927 9942.
E-mail address: miyazaki@eng.ehime-u.ac.jp (T. Miyazaki).
http://dx.doi.org/10.1016/j.chemphys.2014.12.004
0301-0104/Ó 2014 Elsevier B.V. All rights reserved.

that the rotation was frozen [4]. {The oxidation state of entrapped La
in both La2@C80 [8] and the adduct [13] was calculated by DFT calculation to be +3.} On the other hand, vibrational mode analysis using
DFT calculation of La2@C80 suggested D3d symmetry that is the global minimum in total energy [10]. This proposal was questioned
by recent DFT calculations [11,12] that supported D2h geometry;using relativistic basis sets could be a crucial factor to a favor of D2h.
The argument on the geometry of Ih-C80 endohedral fullerenes
did not stop at this point. Another Ih-cage endohedral fullerene,
Ce2@C80, was isolated [14] and its predicted geometry was not
D2h but D3d [12]. The reason of Ce2@C80 having D3d geometry was
attributed to speciﬁc bonding condition of Ce atoms to the cage:
they bonded to C atoms on the opposite side of C6 axis of C80.
We have been measuring ultraviolet photoelectron spectra(UPS) of endohedral fullerenes and reported the most plausible
cage geometry by a comparison of the UPS [15–17]. There is anempirical rule for endohedral C82 fullerenes that their electronic
structure is essentially governed by the cage structure (symmetry)
and the amounts of electrons transferred from the entrapped species. Thus, if one obtains analogous UPS from different endohedral
fullerenes, both their cage structure (symmetry) and the amountsof transferred electrons should be almost identical. Furthermore,
we have compared the UPS of endohedral fullerenes with theoretically obtained simulated spectra [18–22]. There is no theoreticalreasoning that Kohn–Sham orbital energies obtained from DFT calculation is compatible with Koopmans’ theorem, but Janak’s theorem is analog to Koopmans’ theorem in DFT calculation; that

T. Miyazaki et al. / Chemical Physics 447 (2015) 71–75

Kohn–Sham orbital energies correspond the highest occupied andthe lowest unoccupied molecular orbitals energy levels. Further,it has been found that simulated spectra generated from Kohn–
Sham orbital energies reproduce the UPS very well and the comparison was helpful to estimate the cage geometry. The UPS of
La2@C80 was tried to measure but any speciﬁc structure which
was considered to be characteristic to the UPS of fullerenes was
not observed [23]. Therefore, the comparison between the UPS of
La2@C80 and the simulated spectra was impossible.
We succeeded to measure the UPS of La2@C80 and Ce2@C80. In
this article their UPS will be presented and they are compared withsimulated spectra obtained from DFT calculation. Their possible
geometry will be provided to settle the D2h or D3d argument on
the Ih cage endohedral fullerenes.

Photoelectron counts

72

L

K

J I HG

F
E C
D BA
/ eV

hν =
60
55
50
45
40
35
30

2. Experimental and calculation methods

3. Results and discussion
Figs. 1 and 2 show the UPS of La2@C80 and Ce2@C80 obtainedwith the incident photon energy indicated beside each spectrum.

25
21.22

Ih-La2@C80
14 12 10

20

8

6

4

2

0

Binding Energy/eVFig. 1. Incident photon energy dependent ultraviolet photoelectron spectra of
La2@C80. Numeric beside each spectrum indicates the energy of the incident photon.
Approximate peak positions are indicated with dotted lines.

Photoelectron counts

The synthesis and isolation of La2@C80 is reported in Ref. [7].
Ce2@C80 was also obtained with the same method. Soot containing
M2@C80 (M = La or Ce) was produced by direct-current arc heatingof a M2O3/graphite composite rod in a He atmosphere. The detail of
preparation was described in Ref. [24]. Samples for the photoelectron measurements were prepared by vacuum sublimation of theendohedral fullerenes onto a gold-deposited molybdenum disk.
Sublimation was conducted using a resistive heating quartz crucible in a preparation vacuum chamber directly attached to a photoelectron measurement chamber. The temperature of the crucible
during La2@C80 and Ce2@C80 sublimation was about 850–870 K.
The pressure of the chamber during the deposition increased to2  10 6 Pa (base pressure before the deposition was less than
4.0  10 7 Pa). The thickness of deposited endohedral fullerenes
was tried to monitor by a quartz thickness monitor located besidethe disk, but because of the collimation of the crucible the readingof the monitor indicated several nm thickness. However, the actual
thickness of the ﬁlm might be several tens of nm, since the gold
Fermi edge was not observed after repeated sample deposition.
The UPS were measured using a photoelectron spectrometer at
BL8B2 of UVSOR (Ultraviolet Synchrotron Orbital Radiation Facility) at the Institute for Molecular Science. The resolution of thespectrometer was 110 meV. Energy calibration of the spectra wascarried out using the Fermi edge of a gold-deposited sample disk
before the UPS measurements of La2@C80 and Ce2@C80. The spectra
were referenced against the Fermi level. The base pressure of themeasurement chamber was 69.0  10 8 Pa, and the pressure during the measurement was about 6.0  10 8 Pa.
Molecular orbitals of La2@C80 and Ce2@C80 were calculated witha Gaussian 03 program module. Their geometry was optimized at
the Hartree–Fock level using the CEP-31G basis set. Poor correspondence was observed between the UPS and simulated spectra
generated by broadening the calculated Eigen values at the Hartree–Fock level with Gaussian functions of 0.2 eV full width at half
maximum. The DFT calculation was performed on the optimized
structures of La2@C80 and Ce2@C80 using the B3LYP hybrid functional to obtain the Kohn–Sham orbital energies with basis sets
Sapporo-DZP for C atoms and TK/NOSeC-V-TZP function for La
and Ce atoms [25,26]. Simulated spectra obtained by the same procedure describe above using Kohn–Sham orbital energies reproduced the UPS far better than those obtained from the Hartree–
Fock calculation. The simulated spectra in the following text are
the result of the DFT calculation.

J I HG
F

K
L

E C
D B

A
/ eV

hν =
55
50
45

40
35
30

25
21.22

Ih-Ce2@C80
14 12 10

20

8

6

4

2

0

Binding Energy / eV
Fig. 2. Incident photon energy dependent ultraviolet photoelectron spectra of
Ce2@C80. Numeric beside each spectrum indicates the energy of the incident
photon. Approximate peak positions are indicated with dotted lines.

The spectral onset of La2@C80 and Ce2@C80 was 0.77 and 0.89 eV
below the Fermi level, respectively. Except for the onset energy,
their UPS are almost identical. There are 12 structures labeled A
to L in their UPS. Among them, structure E appears as a shoulder
not explicit and structure H appears as a distinct peak in the spectra obtained with the incident photon energy larger than 30 eV.
Approximate peak positions of their structures are indicated withdotted lines. As were observed in the UPS of other fullerenes[18–23,27], the relative intensity of these structures oscillates

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T. Miyazaki et al. / Chemical Physics 447 (2015) 71–75

J I

hν= 40eV

HG

Ce2@C80

F
K

L

energies (Eigenvalues of occupied states). Scale of the simulated
spectra and Kohn–Sham orbital energies is shifted by 4.4 eV for
an easy comparison. Since the similarity between the UPS andthe simulated spectra generated from D5d geometry of M2@C80(M = La and Ce) was very poor, the structure of these endohedral
fullerenes should not be D5d geometry. Further, the formationenergy of this geometry is much larger than that of the most stableone. Therefore, we do not treat this geometry any farther in thistext. The simulated spectra generated from D3d and D2h geometry
seem to reproduce the UPS very well; the ﬁrst four structures A–
D are well reproduced and deeper structures F–J are also reproduced reasonably well. There are slight difference in Kohn–shamorbital energies (indicated by bars) of D2h-La2@C80 and D2hCe2@C80, but it is so small that the simulated spectra obtained bytheir convolution do not show any signiﬁcant difference. Only
attractive difference in the simulated spectra might be associatedwith the position of the HOMO; the HOMO of Ce2@C80 appears at
slightly shallower than the HOMO-1 or -2, whereas that of
La2@C80 locates rather close to the HOMO-1. From these data, present ﬁndings suggest that the simulated spectra obtained from D3d
geometry reproduced the UPS of Ce2@C80 and La2@C80 very well.
Hence the actual geometry of La2@C80 and Ce2@C80 might be D3d.
Fig. 6 shows calculated energy diagrams of Ce2@C80, C80 of thesame cage symmetry and the entrapped Ce atoms. Fig. 7 shows calculated energy diagrams of La2@C80, C80 of the same cage symmetry and the entrapped La atoms. Wave functions of some frontier
orbitals of Ce2@C80, La2@C80 and C80 are also depicted in the ﬁgures. The HOMO-3 and HOMO-2 wave functions of Ce2@C80 are
almost identical with the LUMO and LUMO + 1 of C80, and similar
resemblance is observed in the HOMO-4 of Ce2@C80 and the
LUMO + 2 of C80. That is, the upper three levels of Ce2@C80 are
derived from the unoccupied molecular orbitals of C80 and electrons of the entrapped Ce atoms are transferred to these levels.
Thus, the formal oxidation state of the fullerene can be described

Ionization Potential / eV
14

12

10

Photoelectron counts

Photoelectron counts

when the energy of the incident photon is tuned. Because of thisintensity oscillation, peak positions deviate slightly in accordancewith the incident photon energy change.
Fig. 3 shows the photoelectron spectra of La2@C80 and Ce2@C80
obtained by 40 eV incident photon energy together with the UPS of
empty C80 obtained by 60 eV energy [28]. There is a very poor correspondence between the UPS of La2@C80 and Ce2@C80, and the UPS
of C80. Only distinct resemblance among these spectra might be a
structure appeared at about 5.5 eV which is a typical structure in
the UPS of fullerenes [15–17,19–23] and a broad band between
6.0 and 9.5 eV which is due to r-electrons constituting fullerene
backbone. The UPS of La2@C80 and Ce2@C80 are almost identicaland hard to distinguish them. Structures labeled A–H of Ce2@C80
appear at deeper binding energy side by about 0.2 eV than corresponding ones of La2@C80. Their relative intensity is almost thesame. On the other hand, structures labeled I–L appear at the same
binding energy and there is a slight change in the relative intensityof structures I and J. Resemblance of the UPS means that these two
endohedral fullerenes have analogous electronic structure. Since
the electronic structure of endohedral fullerenes strongly depends
on the geometry of fullerenes, both cage and entrapped atoms, it ishighly plausible that they have the same cage structure. In order toexamine the validity of this deduction, their electronic structure
was calculated by the DFT method and Gaussian functions.
Two possible geometries of M2@C80 (M = La, Ce), D2h and D3dhave been proposed [4,8–14]. Three initial conformations of metal
atoms in Ih-C80 were adapted; (a) D5d symmetry, two metal atoms
were in the C5 axis of Ih-C80, (b) D2h symmetry, they were in the C2axis penetrating the center of hexagon rings perpendicular to themirror plane of Ih-C80 and (c) D3d symmetry, in the C3 axis penetrating the center of phenalene rings. Optimized structures retained
the same symmetry adapted as the initial geometry. Kohn–Shamorbital energies were obtained using the optimized geometry.
Kohn–Sham orbital energies and simulated spectra generated bybroadening them by Gaussian functions are shown in Fig. 4(La2@C80) and Fig. 5 (Ce2@C80). The UPS of La2@C80 and Ce2@C80
obtained with 30 eV excitation energy are also shown for comparison. The bars in Figs. 4 and 5 indicate the calculated ionization

8

6

4

Simulation spectra
D5d

D2h

E BA
DC

La2@C80

D3d

UPS of La2@C80hν= 30eVC80hν= 60 eV

10

8

6

4

2

0

Binding Energy / eV
14

12

10

8

6

4

2

0

Binding energy /eV
Fig. 3. Ultraviolet photoelectron spectra of La2@C80 and Ce2@C80 obtained with
hm = 30 eV photon together with the UPS of empty C80 obtained by 60 eV energy.

Fig. 4. The UPS of La2@C80 obtained with 30 eV excitation photon and the simulated
spectrum obtained by broadening of the Kohn–Sham orbital energies. Bars undereach simulation spectrum indicate the energy of calculated ionization potentials.
Three simulation spectra theoretically obtained from optimized geometry assuming
D3d cage symmetry. Details of geometry optimization are in the text.

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T. Miyazaki et al. / Chemical Physics 447 (2015) 71–75

Ionization Potential / eV
12

10

8

6

Photoelectron counts

14

4

D5d

D2h

D3d

UPS of Ce2@C80hν= 30eV

10

8

6

4

2

0

Fig. 7. Energy diagrams of empty C80, La2@C80 obtained by DFT calculation. The
Kohn–Sham orbital energies of frontier orbitals are inserted and the wave function
distributions of frontier orbitals are also shown. The electron conﬁguration of
La2@C80 could be (La2)6+@C680 .

The empirical rule that the electronic structure of endohedral fullerenes depends on the cage structure and the amounts of transferred electrons holds in La or Ce atoms entrapped fullerenes.

Binding Energy / eV
4. Conclusions
Fig. 5. The UPS of Ce2@C80 obtained with 30 eV excitation photon and thesimulated spectrum obtained by broadening of the Kohn–Sham orbital energies.
Bars indicate the Kohn–Sham orbital energies. Three simulation spectra theoretically obtained from optimized geometry assuming D3d cage symmetry.

The intensity of the structures appearing in the UPS of Ce2@C80
and La2@C80 oscillates in accordance with the incident photon
energy change, which means that this molecule has analogous
geometry to other fullerenes. The UPS of La2@C80 are the same as
those of Ce2@C80, both have the same electronic structure. The
UPS of Ce2@C80 and La2@C80 is well reproduced by the simulated
spectrum obtained from the geometry optimized structure using
DFT calculations, which indicates the validity of the geometry optimized structure. Comparison of the UPS with theoretically generated simulation spectra indicates that the most plausible
structure of Ce2@C80 and La2@C80 has D3d symmetry. Charge popu6
6+6
lation analysis suggest Ce6+
2 @C80 and La2 @C80 oxidation state.
Conﬂict of interest
There is no conﬂict of interest among authors.
Acknowledgments

Fig. 6. Energy diagrams of empty C80, Ce2@C80 obtained by DFT calculation. The
Kohn–Sham orbital energies of frontier orbitals are inserted and the wave function
distributions of frontier orbitals are also shown. The electron conﬁguration of
Ce2@C80 could be (Ce2)6+@C680 .

6
as Ce6+
2 @C80 . On the other hand, the HOMO-2 and HOMO-3 wave
functions of La2@C80 are almost identical with the LUMO andLUMO + 1 of C80, and similar resemblance is observed in the
HOMO-1 of La2@C80 and the LUMO + 2 of C80. That is, the upper
three levels of La2@C80 are derived from the unoccupied molecularorbitals of C80 and electrons of the entrapped La atoms are transferred to these levels. Thus, the formal oxidation state of the fuller6
ene can be described as La6+
2 @C80 . Calculated charges of entrapped
atoms are estimated by the natural population analysis (NPA).
Presently obtained NPA charge of Ce atom in Ce2@C80 is +2.21
and that of La atom in La2@C80 is +2.31. The formal oxidation stateof Ce and La atoms might be +3 and two atoms donates six electrons to the cage. The fullerene cage accepts the same amounts of6
6+
6
the electrons from the entrapped atoms, Ce6+
2 @C80 and La2 @C80 .

This study was conducted as part of a Joint Research Program of
UVSOR, Institute for Molecular Science. This work was supported
by a Grant-in-Aid for Scientiﬁc Research in the Priority Area Molecular Conductors (No. 15073203) as well as a Grant-in-Aid for Basic
Scientiﬁc Research (No. 18350068) from the Ministry of Education,
Science, Sports, and Culture, Japan. This work is also supported by a
Subsidy to Create Center of Excellence from Ehime University.
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