BL2B

Fragmentation of C70 in Valence Ionization
H. Katayanagi1,2, B. P. Kafle2, T. Mori1, J. Kou1, K. Mitsuke1,2, Y. Kubozono3
1
Department of Vacuum UV Photo-Science, Institute for Molecular Science,
Okazaki 444-8585, Japan
2
Graduate University for Advanced Studies, Okazaki 444-8585, Japan3
Department of Chemistry, Okayama University, Okayama 700-8530, Japan
increasing n. The kinetic shift in C70z+ is found to be
37 eV. In addition, the results of preliminary RRKM
calculations in photofragment size of n < 5 areconsistent with the experimental results. The
mechanism to produce photofragments of n < 5 from
C70 is, therefore, the same as that from C60. Whereas
the values of AE(n, z) for n > 5 stay almost constant.
This means that another mechanism should be
invoked to explain the production of C60-2n z+ in thephotofragment size of n  5. To elucidate themechanism, we plan to perform a photoelectron
photoion coincidence and a photofragment
momentum spectroscopy to clarify energy
partitionings during the dissociative photoionizationof fullerenes.

In the electron impact [1] and photo [2] ionization
processes of the C60 molecules, fragment ions such as
C60-2nz+ (n = 1, 2 …; z = 1, 2 …) appear when theexcess energies exceed those thermochemical
thresholds of dissociative ionization by 30 – 40 eV.
This is because C60 has a large number of internal
degrees of freedom and does not have specificallyweak bond to rupture owing to its highly symmetricalstructure. This energy storage in the fullerenes is thus
referred to as the “kinetic shift”.
In our previous study [3], the fragment ion yield
spectra of C60 in dissociative ionization by thesynchrotron
radiation
have
been
recorded
experimentally. From the spectra, we evaluated theappearance energies, AE(n, z), which corresponds tothe onset energies of ion yield spectra. The
experimental values of AE(n, z) were well consistent
with those obtained by our RRKM calculations. The
values of AE(n, z) exceeded their thermochemical
thresholds by 30 – 33 eV and increased by 5 – 6 eVwith increasing n. From these results, we concluded
that the photofragmentation of C60z+ was governed by
the mechanism of internal conversion of theelectronically excited states of C60z+, statistical
redistribution of the excess energy among a numberof vibrational modes, and sequential ejection of theC2 units.
Is this mechanism also valid in C70? The C70 is astable fullerene next larger in size to C60. The kinetic
shift can be either smaller or larger than that of C60,
since the C70 molecules have less symmetry while
they have a larger number of degrees of freedom thanC60. To answer the question, we measured thephotofragment ion yield spectra using time-of-flightmass spectrometer coupled with a monochromator at
the beamline 2B in UVSOR.
Photofragment ion yields of C70-2n2+ divided by theyield of the parent ion, C702+, are shown in Fig. 1.
Corresponding triply charged ion yields are also
shown in Fig. 2. Note that singly charged
photofragment ions were scarcely detected in wholeenergy range of the measurement. At the first glance,
the behavior of the photofragment yield curves for n 5 (i. e. production of smaller fragment than C62z+) isdifferent from that for n < 5. This fact implies the
carbon number of 60 is a magic number also in thephotofragmentation processes.
This difference appears more clearly in energies at
onsets of the yield curves, namely AE(n, z). Both indoubly and in triply charged ions, the values of AE(n,z) for n < 5 increase by approximately 7 eV with

C2+
68

0.15

C2+
66

2+
Ratio (C2+70-2n / C70 )

C2+
64
C2+
62
C2+
60

0.10

C2+
58
C2+
56
C2+
54

0.05

0.00

60

80

100

120

Photon energy / eV

Fig. 1 Doubly charged photofragment ion yields(divided by the parent ion yield).
C3+
68
C3+
66

0.3
3+
Ratio (C3+70-2n / C70 )

C3+
64
C3+
62
C3+
60

0.2

C3+
58

0.1

0.0
70

90

110

Photon energy / eV

Fig. 2 Triply charged photofragment ion yields(divided by the parent ion yield).
[1] R. Wörgötter, B. Dünser, T. D. Märk, M. Foltin, C.
E. Klots, J. Laskin and C. Lifshitz, J. Chem. Phys.
104 (1996) 1225.
[2] P. Wurz and K. R. Lykke, J. Phys. Chem. 96(1992) 10129.
[3] J. Kou, T. Mori, Y. Kubozono and K. Mitsuke,
Phys. Chem. Chem. Phys. 7 (2005) 119.

50

BL2B

Photoion Yield Curves of Pr@C82 in the Range 100 – 150 eV
H. Katayanagi1,2, B. P. Kafle2, T. Mori1, J. Kou1,
Y. Takabayashi3, E. Kuwabara3, Y. Kubozono3, K. Mitsuke1,2
1
Department of Vacuum UV Photo-Science, Institute for Molecular Science,
Okazaki 444-8585, Japan
2
Graduate University for Advanced Studies, Okazaki 444-8585, Japan3
Department of Chemistry, Okayama University, Okayama 700-8530, Japanhigher-energy shifted and its widths is slightly
narrower than that of Pr atoms. This could be
described that the interference effect on the spectrum
for Pr@C82 induced by the fullerene cage is different
from that for Ce@C82. We plan to make theoretical
calculations necessary for more quantitative analysisof the interference effect.

Ion intensity / arb. units

Endohedral metallofullerenes (Mm@Cn) haveattracted attention because of their possibilities ofapplications toward materials of novel functionalities[1]. The metallofullerenes which have a structure of
M@C82 have been used in studies on theirgeometrical and electronic structures, because of theiravailability. Their, however, had been no
experimental work on the optical properties of gas
phase M@C82 in the vacuum UV and soft X-ray
regions, until our report of the photoion yield curvesof Ce@C82 was published in 2005 [2].
In our work on Ce@C82, we found a giant
resonance peak which had been predicted
theoretically by Wendin and Wästburg [3]. This peakoriginates from 4d – 4f dipole resonance of the Ce
atom. Wendin et al. found that the calculated peakconsists of a broad atomic component and an
oscillation induced by a fullerene cage. Observed
photoion yield curves of Ce@C82 had structures andthus found to be affected by the interference. In thepresent study, we measured the photoion yield curvesof metallofullerenes containing Pr which is next to
the Ce in the periodic table of the elements.
The Pr@C82 beam was generated in vacuum by
sublimation of the solid sample, which wassynthesized by the procedure described elsewhere [4].
The typical temperature of a sample holder was 780
K. The photoion yield was measured by means of thetime-of -flight (TOF) mass spectrometry in thephoton energy range 100 – 150 eV.
Figure 1 shows TOF mass spectrum of Pr@C82z+ (z= 1, 2) produced by the synchrotron radiation (124eV). Peaks of the parent ions, Pr@C82+ and Pr@C822+,
are clearly observed. Small peaks of C60+ and C602+
can also be recognized. The intensities of them are,however, insignificant.
Time-of-flight mass spectra of other photon
energies were recorded in the same manner.
Intensities of ion signals in the spectra werenormalized by the photon flux. We thus extracted theyield curve of Pr@C822+ from the normalized spectra.
The yield curve of Pr@C822+ is shown in Fig. 2. The
photoabsorption spectrum of Pr atoms [5] is also
shown in Fig. 2. The yield curve of Pr@C822+ has asimilar behavior to the photoabsorption spectra of Pr
atoms. This fact leads us to conclude that 4d – 4f
giant resonance does not collapse in Pr@C82, as wellas in Ce@C82.
Closer inspection reveals characteristics of Pr@C82.
The peak position of Pr@C822+ spectrum is

1.5
Pr@C2+
82

1.0
C2+
60 ?

Pr@C+82

C+60?

0.5

0.0

8

12

16

20

Time of flight / Ps

Fig. 1 TOF mass spectrum of Pr@C82z+ at thephoton energy of 124 eV.

Intensity / arb. units

0.4
Pr@C82

0.2

Pr

0.0
100

110

120

130

140

150

TOF / ch (4ns)Fig. 2 Photoion yield curve of Pr@C822+ (Filledsquare). Error bars indicate 1V of values inexperimental runs repeated five times. Dashed curveshows absorption spectrum of Pr atoms [5].
[1] H. Shinohara, Rep. Prog. Phys. 63 (2000) 843.
[2] K. Mitsuke, T. Mori, J. Kou Y. Haruyama and Y.
Kubozono, J. Chem. Phys. 122 (2005) 064304.
[3] G. Wendin and B. Wästberg, Phys. Rev. B 48
(1993) 14764.
[4] K. Shibata, Y. Rikiishi, T. Hosokawa et al. Phys.
Rev. B 68 (2003) 094104.
[5] M. Richter, M. Mayer, M. Pahler, T. Prescher, E. v.
Raven, B. Sonntag and H.-E. Wetzel, Phys. Rev. A 40
(1989) 7007.

51

BL2B

Photofragmentation of Fullerenes in the Extreme UV: Ion Yield Curves for
the Fragment Ions
K. Mitsuke1,2, J. Kou2, T. Mori2, Y. Kubozono3
1
The Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan.
2
Graduate University for Advanced Studies
3
Okayama University, Okayama 700-8530, JapanThe C60 molecule with a compact structure of atruncated icosahedron exhibits an exceptionally highstability. However, mass spectrometric studies of C60
in combination with laser multiphoton ionization,
heavy-ion excitation, or electron impact ionization
have revealed that appreciable fragmentation occurs
when the system gains energy in excitation andionization processes. Decomposition of C60z+ (z t 1)
primarily formed is known to lead to fragment ionswith even numbered carbon atoms (C60-2nz+, n=1,2,…) via sequential loss of C2 units from C60z+ ions inhigh-vibrational states. In contrast, experimental
studies of single photon excitation are very limited[1-4]. Recent results on the relative cross section of
the fullerene fragments from C60 show that theappearance photon energies of C60-2n+ and C60-2n2+ are
higher by 30 - 40 eV than the thermochemical
thresholds for dissociative photoionization of C60[2-4]. In a limited case of C60 + hv o C58+ + C2 + eYoo et al. [1] have discussed this large kinetic shift interms of unimolecular decay modelled byquasiequilibrium theory. However, they could not
determine the theoretical appearance energy with agood accuracy, since a reliable value for the binding
energy of C60+ was not available at that time. Since
then there has been no effort to apply
quasiequilibrium
theorytodissociative
photoionization of C60 and to clarify thefragmentation mechanism of the fullerene ions towhich large excess energy is initially deposited. In thepresent work we will discuss the detailed mechanismsof unimolecular reaction induced by equilibration of
the excess energy among many density of states ofthe fullerene, by using the yield curves for the
fragment ions obtained in a wide photon energy
range.
All the measurements have been carried out at thebending magnet beamline BL2B constructed in the
UVSOR synchrotron radiation facility in Okazaki,
equipped with an 18 m spherical grating
monochromator of Dragon type. The Experimental
set-up for photoionization mass spectrometry of C60
has been described in details elsewhere [5,6]. Briefly
a molecular beam of C60 was produced by heating thesample powder to approximately 680 K.
Monochromatized synchrotron radiation was focused
onto the C60 beam. The produced photoions wereextracted by a pulsed electric field, mass-separated bya time-of-flight (TOF) mass spectrometer, anddetected with a microchannel plate electron multiplier.

In order to normalize the ion counts the fluxes of themolecular and light beams were monitored
throughout the measurement by a silicon photodiodeand a crystal-oscillator surface thickness monitor,
respectively.
Taking TOF mass spectra with scanning themonochromator we could measure the yield curves
for C60-2nz+ from C60 as a function of hv. For instance,
Figure 1 illustrates the curves for C60-2n+. The curves
for C60-2nz+ are considered to provide fractional
abundances of C60-2nz+ within a 25 Ps after ionization,
at least around the onset region for each ion. Withdecreasing the size of 60-2n the appearance photon
energies AE(n,z) for a given z shift to higher hv
positions and the curves rise more gently towards apeak. Values of AE(n,z) are found to be higher by 30– 33 eV than the thermochemical thresholds fordissociative photoionization of C60 leading to C60-2nz+.
Reinköster et al. have found similar large kinetic
shifts [2] but their appearance hv positions for z = 1
and 2 are higher by 2 – 8 eV than our values,
probably due to their insufficient statistics. On thecontrary, the appearance energies measured inelectron impact ionization [7] show good agreementwith our values.
7+(u 1/30)

C58+

AE(1) = 44r1 eV

C56+

AE(2) = 49r1 eV

C54+

AE(3) = 56r1 eV

C60

Ion yield / arb.units

6
5
4
3
2
1
0
40

60

80

100

120

140

160

Photon energy / eV

Fig. 1 Yield curves of C60-2n+ ions (n = 1 - 3)
obtained from time-of-flight mass spectra.
[1] R.K. Yoo et al., J. Chem. Phys. 96 (1992) 911.
[2] A. Reinköster et al., J. Phys. B 370 (2004) 2135.
[3] J. Kou et al., J. Electron Spectrosc. Relat.
Phenom. 144-147 (2005) 247.
[4] J. Kou et al., Phys. Chem. Chem. Phys. 7 (2005)
119.
[5] J. Kou et al., Chem. Phys. Lett. 374 (2003) 1.
[6] T. Mori et al., Rev. Sci. Instrum. 74 (2003) 3769.
[7] P. Scheier et al., Int. J. Mass Spectrom. 138(1994) 7.

52

BL2B

Photofragmentation of Fullerenes in the Extreme UV: Remarkably Large
Kinetic Shifts of the Appearance Energies of the Fragment Ions
K. Mitsuke1,2, J. Kou1, T. Mori1, Y. Kubozono3
1
The Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan.
2
Graduate University for Advanced Studies
3
Okayama University, Okayama 700-8530, Japan
We have measured the yield curves for C60-2nz+
from C60 as a function of the photon energy hv to
study the mechanisms and kinetics of theunimolecular reaction of parent ions C60z+ in
high-vibrational states. Typical spectra for C60-2n2+ are
given in Fig. 1. These curves are considered toprovide fractional abundances of C60-2n2+ within a 25
Ps after ionization, at least around the onset region foreach ion.
When C60 is photoionized at the appearance photon
energies, namely at hv = AE(n,z), we are able to write
the maximum internal energy Emax(n,z) initiallytransmitted to C60z+ as(1)
Emax(n,z) = AE(n,z) + Ev – IP(z)by assuming the kinetic energy of the emitted
photoelectron to be zero. Here, Ev denotes thevibrational energy of C60, and IP(z) is the ionization
potential of C60 for the formation of C60z+ (see Fig. 2).
In this study, Ev = 3.3 eV is assumed and IP(z) takenfrom the literature (7.6, 19.0, and 35.6 eV for z = 1, 2,
and 3, respectively). We have evaluated Emax(n,z)
from eq. (1) and listed the resultant values in Table 1
for C60-2nz+. It is expected that Emax(n,z) is nearly equal
to the upper limit of the internal energies of the
primary C60z+ above which C60-2n+2z+ fragments cannot
escape from further dissociating into C60-2nz+ + C2.
Table 1 reveals two trends in the dependences ofEmax(n,z) on n and z of the fragments. First, thedifference Emax(n+1,z)  Emax(n,z) depends weakly onn, varying from 5 to 9 eV. This may reflect relatively
weak n-dependences [1,2] of the binding energies forthe unimolecular reaction:(2)
C60-2n+2z+ (n t 1) o C60-2nz+ + C2
Second, Emax(n,z) for a given n is almost unchanged
irrespective of z. This remarkable finding, together
with the prominent kinetic shifts, implies that internalconversion of the electronically excited states of C60z+
results in redistribution of the excess energy among
the vibrational degrees of freedom followed bysuccessive ejection of the C2 units, as demonstrated
schematically in Fig. 2.
7

Ion yield / arb.units

2+

C58

6

2+

C56

5

2+

C54

2+

C52

4

2+

C50

3
2
1
0
40

60

80

100

120

140

Photon energy / eV

Fig. 1 Yield curves of C60-2n2+ ions (n = 1 - 5)
obtained from time-of-flight mass spectra.

e-

C60z+

C2

C58z+

C2

C60-2n+2z+

C60-2nz+
C2
Kn = 0.03Knmax

AE(n,z)Internal
energy H

Emax(n,z) =AE(n,z)+Ev-IP(z)

BE(1)C60z+ (Ground state)
C60 (T = 680K)
C60 (Ground state,

Ev=3.3 eVIP(z)BE(n) : Binding energy of C60-2n+2z+ fordissociation into C60-2nz+ + C2

T = 0)

Fig. 2 Scheme of the energetics of the C2 loss indissociative photoionization of C60.
[1] M. Foltin et al., J. Chem. Phys. 98 (1993) 9624.
[2] R. Wörgötter et al., J. Chem. Phys.104 (1996)
1225.

Table 1 Upper limit Emax(n,z) of the internal energies (eV) of the primary C60z+ ions above
which C60-2n+2z+ fragments cannot escape from further dissociating into C60-2nz+ + C2 (nt1).
C60-2n+2z+
1
2
3
4
5
a
e

C60z+
C58z+
C56z+
C54z+
C52z+

C60-2nz+
C58z+
C56z+
C54z+
C52z+
C50z+

BE(n)

z=1
39.7r1
44.7r1
51.7r1-e-eObserved,a Emax(n,z)
z=239.3r1
44.3r1
50.3r1
58.3r164.3r1z=3
40.7r1
49.7r1-e-e-eObtained by using eq. (1) from the appearance photon energies for the formation of C60-2nz+ from C60.
C60-2nz+ fragment ions are not detectable.

53

BL2B

Photofragmentation of Fullerenes in the Extreme UV:
Analysis on the Critical Internal Energies
K. Mitsuke1,2, J. Kou1, T. Mori1
1
The Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan.
2
Graduate University for Advanced Studies
0.03Knmax. The values of ERRKM(n) determined from
Fig. 1 are listed in the seventh and eighth columns ofTable 1. For the formation of C58z+, C56z+, and C54z+
ERRKM(n) are 40.9, 47.0, and 52.8 eV, respectively, in
excellent agreement with the observed maximum
internal energies Emax(1,z), Emax(2,z), and Emax(3,z),
respectively. These results are the manifestation of thevalidity of the present statistical treatment: large
amounts of the internal energy of C60z+ is equilibrated
among the vibrational degrees of freedom, andsubsequent fragmentation proceeds through reaction(1) via a transition state with the activation energy ofE0n.

We have employed the RRKM theory to derive thecurves of fractional abundance (breakdown graphs)
for C60z+ and C60-2nz+ ions (n = 1 – 5) as a function of
the internal energy of the primary C60z+. The
microcanonical rate constant kn(H) for reaction
C60-2n+2z+ (n t 1) o C60-2nz+ + C2
(1)
of C60-2n+2z+ having internal excitation energy H can begiven by [1-3]k n (H)D G * (H  E0n )h N (H )(2)

Here, D is the reaction path degeneracy, E0n is thecritical activation energy for reaction (1), G*(H-E0n) is
the number of states for the transition state (activatedcomplex), and N(H) is the density of states of C60-2n+2z+.
For n t 2 the H value of C60-2n+2z+ is computed under
the assumption that the energy available after
dissociation of C60-2n+4z+ o C60-2n+2z+ + C2 isstatistically partitioned between C60-2n+2z+ and C2. We
have used Haarhoff’s approximation [3] to calculate
G*(H-E0n) and N(H), assuming vibrational frequencies
of C60-2n+2z+ to be replaced by those of a neutral C60
reported by Schettino et al. [4]The critical activation energies E0n in eq. (2) aretaken from the binding energies of C60-2n+2z+ (n = 1 –5) for reaction (1) in the literature [2,5]. We adopted
the two sets of the binding energies of C60-2n+2z+, i.e.
the sets for the TS-1 and TS-3 models. The TS-3transition state is consistent with a large frequency
factor in the Arrhenius relation which has been
expected by a very large rotational partition functionof C2. The binding energies of C60+, for example, are7.06 eV and 9.2 eV for TS-1 and TS-3, respectively.
Figure 1 shows the fractional abundance Kn for
C60-2nz+ ions with the TS-3 model as a function of theinternal energy of the primary C60z+ at 25 Ps after
photoionization of C60. We defined the appearanceinternal energy ERRKM(n) for the formation of C60-2nz+
as the internal energy of C60z+ corresponding to Kn =

100

z+

Fractional abundance, Kn / %

C60

z+

C58

80

z+

C56

z+

C54

60

z+

C52

z+

C50
40

20

0
30

40

50

60

70

80

90

z+

Internal energy of the primary C60 ions / eV

Fig. 1. Fractional abundance curves of C60z+ andC60-2nz+ ions obtained by using the RRKM theory(TS-3 model).
[1] R. K. Yoo et al., J. Chem. Phys. 96 (1992) 911.
[2] R. Wörgötter et al., J. Chem. Phys. 104 (19961225.
[3] W. Forst, Theory of Unimolecular Reactions,Chap. 6, Academic, New York, (1973).
[4] V. Schettino et al., J. Chem. Phys. 101 (1994)
11079.
[5] M. Foltin et al., J. Chem. Phys. 98 (1993) 9624.

Table 1 Upper limit Emax(n,z) of the internal energies (eV) of the primary C60z+ ions above which C60-2n+2z+
fragments cannot escape from further dissociating into C60-2nz+ + C2 (nt1).
C60-2n+2z+
1
2
3
4
5

C60z+
C58z+
C56z+
C54z+
C52z+

C60-2nz+
C58z+
C56z+
C54z+
C52z+
C50z+

z=1
39.7r1
44.7r1
51.7r1-

Observed,a Emax(n,z)
z=239.3r1
44.3r1
50.3r1
58.3r164.3r1

Calculated,b ERRKM(n)z=340.7r1
49.7r1
-

TS-1c

TS-3d

40.1
47.1
52.9
58.9
65.1

40.9
47.0
52.8
59.1
65.3

Obtained from the appearance photon energies for the formation of C60-2nz+ from C60.
Appearance internal energies of C60z+ corresponding to Kn = 0.03Knmax at 25 Ps after photoionization of C60. The RRKM theory is
employed to derive the rate constant for reaction (1) from which the Kn curves can be calculated.
c
ERRKM(n) determined from a set of the binding energies for the TS-1 model was employed.
d
ERRKM(n) determined from Fig. 1. A set of the binding energies for the TS-3 model was employed.
a
b

54

BL3U

Site-Specific Photoionization of Free Krypton Clusters
T. Hatsui1, H. Setoyama1, N. Kosugi1, B. Wassermann2, I.L. Bradeanu3, E. Rühl1,31Institute for Molecular Science, Okazaki 444-8585 Japan
2
Institut für Experimentalphysik, Freie Universität Berlin, 14195 Berlin, Germany
3Institut für Physikalische Chemie, Universität Würzburg, 97074 Würzburg, Germany
Cluster formation is known to give the decrease inbinding energy, which is commonly interpreted interms of final state polarization screening [1]. Fromearlier near-edge spectroscopy [2] it is known that themost significant changes in electronic structure arefound in the regime of small clusters. In the present
work, we investigate Kr 3d photoionization forvariable size Kr clusters in the average size regime<N>  30.
The experimental setup consists of a continuous
supersonic jet expansion. The correlation between the
expansion conditions and <N> is taken from
Karnbach et al. [3]. The jet is intersected with a beamof linearly polarized soft X-rays from the in-vacuum
undulator beam line BL3U with an energy resolving
power E/¨E of ~2 x103 at 140 eV photon energy.
Electrons are analyzed at 55° to the electric vectorof the incident x-rays by a Scienta SES-200
electron spectrometer (path energy: 75 V).
The new features arising from cluster formation are
observed at the binding energy lower than the bareatomic peaks. The cluster spectra are obtained bysubtracting the atomic portion (Fig. 1). Subtraction of
the atomic component from the raw spectra was only
done until spectral distortions occurred because adimer component of weak intensity exists near theatomic lines. This is consistent with the dimmer
binding energy predicted by ab initio theoretical
calculations, which is so close to the bare atomic one
that the dimer components with small mixing ratio
are invisible in the raw spectra. Core ionizationenergies calculated by using a simple model in
combination with a phenomenological model, that
describes perfect and defect cluster shapes, is shownat the top of Fig. 1.
We have performed a spectral de-convolution ofthese components by using identical Voigt profiles,
which are evaluated from the atomic spectrum
observed at the same instrumental condition(Lorentzian width: 93 meV [4], Gaussian width: 163
meV). The dimer- and the edge-components show noor little size effects. When the atomic profiles areused, then the two other ones (corner and face/bulk)
do not give rise to a significantly different line shape,
as verified by the experimental spectra. This allows
us to fit the spectra with four components for eachspin-orbit component (Fig. 1). The spectrum for<N>=4 consists of three surface components besides
the dominant dimer. The smallest of these shifts isdue to the corner sites of small clusters, followed byedge-, face-/bulk-sites. We note that face- andbulk-sites cannot be clearly distinguished from eachother since their 3d ionization energies occur in thesame energy regime (Fig. 1). As <N> is increased one
observes the following systematic changes in the
spectra shown in Fig. 1. (i) The dimer component
decreases substantially in relative intensity. Any shiftis hardly observable and the different energy
positions are well observable within the error limit;(ii) Corner-sites show an increasing shift reaching up
to ~ í500 meV. The relative intensity of thiscomponent remains almost constant; (iii) Edge-sites
gain substantially intensity, which is accompanied bya slight energy shift; (iv) Face-/bulk-sites also gain
intensity and there is also a slight size dependence of
this component. The model calculations show that
substantially larger shifts occur for the bulk
component, if large clusters are formed. This is in fullagreement with recent experimental results [5]. The
present work has successfully revealed size andsite-specific Kr 3d ionization energies of considerably
smaller clusters.

Fig. 1 Kr 3d photoelectron spectra of Kr clusters (ƕ).
The sum (í) of the spectral components (í) are given.
The calculated energy regimes of different sites (a:atom, d: dimer, c: corner, e: edge, f: face, b: bulk) are
shown at the top.
[1] O. Björneholm et al., J. Chem. Phys. 104 (1996)
1846.
[2] O. Björneholm et al., Phys. Rev. Lett. 74 (1995)
3017. A. Knop et al., Phys. Rev. Lett. 80 (1998) 2302.
[3] R. Karnbach et al., Rev. Sci. Instrum. 64 (1993)
2838.
[4] M. Jurvansuu et al., Phys. Rev. A64 (2001)
012502.
[5] M. Tchaplyguine et al., J. Chem. Phys. 120 (2004)
345.

55

BL3U

Double Photoionisation processes in SF6
P. Lablanquie1, Y. Hikosaka1, E. Shigemasa1, T. Aoto2, K. Ito2
1
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585 Japan2
Photon Factory, Institute of Materials structure Science, Tsukuba 305-0801 Japan
spectrum. Its measurement in coincidence with theS2p3/2 photoelectron (top) allows us to filter out theother S2p1/2 spin-orbit Auger components, and to
define precisely the final M2+ state and its binding
energy (= photon energy minus the energetic electronenergy). It is observed that the M2+ states populated
by Auger decay are excited states, with the final 2holes probably localized on the S atom, at the vicinityof the S2p hole. But experiment also reveals that M2+states of lower binding energies can be accessed, withthe 2 holes lying on F atoms [4]. These states are not
populated by S2p Auger decay, and they can also be
formed below S2p threshold as revealed in Fig. 2.
Comparison with the non coincident photoelectron
spectrum (in red) shows similar intensity profiles as
the valence states of 40-45 eV binding energies,
which are attributed to F2s hole states. The process isthus formation of F2s holes that decay rapidly
through ICD mechanism [4]. This situation is specificto the SF6 molecule, as usually intensity profiles aremarkedly different in photoelectron spectra and in e/e
coincidence curves, as observed for N2 in Fig. 2(bottom).

Double photoionisation, that is removal of twovalence electrons from a molecule by a single VUVphoton can usually proceed by two different paths,
that form similar dication final states:(A)ĺ M+ + e1=InnerShell ĺ
hQ + M———ĺ
M2+ + e1 + e2 (B)(A) is the Auger effect where an inner shell electron
is first extracted and subsequent relaxation leads toemission of a secondary Auger electron. Valence
double ionisation, (B) is often improperly referred toas ‘direct double photoionisation’, but includes in factlarge contribution of indirect, sequential paths. It wasrecently demonstrated [1] that these indirect paths
take surprisingly long time, with similar time scales
for electron emission and nuclear motion, up to thepoint that the intermediate states often dissociate fullyprior to emission of the second electron. We present
here evidence of a different process in SF6 molecules:
an intense valence singly charged intermediate stateautoionizes very efficiently through ICD (Inter
Atomic Coulombic Decay [2]) prior to nuclear
motion.
Experiment was performed on the new BL3U
undulator line with the coincidence experimental
setup developed at PF by Hikosaka et al [3].
Investigation of double photoionisation requires
detection in coincidence of both emitted electrons, inorder to define the reaction paths, which often means
low signal. The trick in our experiment is to impose
one of the two electrons to be of zero energy, anddetect it with a threshold electron analyzer.
Advantages are high collection-efficiency (on 4Ssr)and potentially high energy-resolution.
Fig. 1 (bottom) displays the SF6 L2,3VV Auger

Fig. 2 Black: threshold electron / energetic electron
coincidences measured with 80eV photons, andcompared to the noncoincident photoelectron
spectrum (red). Top: SF6 , Bottom: N2.
[1] J. Eland et al., Chem Phys 290 (2003) 27 and ref
included.
[2] C. Buth et al., J. Chem. Phys. 119 (2003) 10575.
[3] Y. Hikosaka et al., Meas. Sci. Technol. 11 (2000)
1697.
[4] R. Feifel et al., submitted to J. Chem. Phys.

Fig. 1
Bottom: SF6 Auger spectrum. Top:
coincidence Auger spectrum measured at 2p3/2
threshold.

56

BL3U&4B

Sub-Natural Linewidth Auger Electron Spectroscopy: Application to
the Sulfur 2s Hole Decay in OCS
Y. Hikosaka1, P. Lablanquie1, E. Shigemasa1, T. Aoto2, K. Ito2
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585 Japan2
Photon Factory, Institute of Materials structure Science, Tsukuba 305-0801 Japan
1

Inner-shell photoionization and the subsequentAuger decay give dication states as the final product.
Since the kinetic energy of the Auger electron
corresponds to the transition energy from thecore-hole state to the dication state, spectroscopic
information on the dication states is obtained by the
analysis of individual Auger transitions. Auger
spectroscopy has been, therefore, one of the mostuseful tools for investigating dication states.
However, the analysis of Auger spectra has been
restricted mainly for the following two reasons.
First, the spectral features become complicated when
the spectrum consists of the superposition of Augerlines associated with various core-hole states andtheir satellites. Second, the spectral resolution for an
Auger line is limited by the natural width of the initialcore-hole state.
Auger lines associated with a core-hole state can be
distinguished by coincidence detection with thecorresponding photoelectrons. Thus the difficulties
resulting from the first restriction above can beremovedby
Photoelectron-Auger
electroncoincidence
spectroscopy.
Furthermore,
thiscoincidence spectroscopy also enables the second
restriction to be overcome and achieve a spectral
resolution better than the natural width of the initialcore-hole state. This sub-natural linewidth regime
results from the complete definition of the dication
states, brought about by the coincidences.
Detection of threshold photoelectrons, i.e. electrons
with near-zero kinetic energy, takes advantage of thehigh energy-resolution (few meV) and high (4S)collection-efficiency that can be attained. Theseadvantages are carried over to the Auger electron
spectrum measured in coincidence, when we adopt
threshold photoelectrons as the coincidence
counterpart. So far, we have applied the thresholdelectron-Auger electron coincidence techniquemainly to Auger transitions of rare gases, and havegained spectroscopic information on the dication
states as well as the dynamics on the multiple
ionization processes ([1] and references therein).
In this work, we have studied sulfur 2s decay in
OCS using the threshold photoelectron-Auger
electron coincidence technique. The S 2s hole has avery short lifetime resulting from the fast L1L2,3V
Coster-Kronig decay, and the resultant wide natural
width (1.8 eV) has hidden the spectroscopic
information on the final dication states. In practice,
the Auger spectrum we measured with conventional
Auger spectroscopy shows in the kinetic energy range

20-50eV only broad structures which can be allocatedto the L1L2,3V Coster-Kronig transitions (see Fig. 1(a)),
and it is difficult to deduce the spectroscopic
information on the dications from this spectrum. In
contrast, the threshold photoelectron-Auger electron
spectrum in Fig 1b exhibits the same dication stateswith a much higher energy resolution. Here, theenergy resolutions of the Auger electron andthreshold electron analyzers are set to around 0.8 eV
and 30 meV (FWHM), respectively, and the photon
energy band width is to 0.25 eV; therefore the energy
resolution in the coincident spectrum is expected to
be around 0.85 eV and the sub-natural linewidth
regime is established by the coincidence
measurement. Each structure in the coincidence
spectrum is attributable to an electronic state with theconfiguration of (S 2p)-1(valence)-1, and their detailed
interpretations are under way.

6000

(a)

Normal Auger at 305 eV(b)

Auger in coincidence

5000
4000
3000
2000
1000
0
1000
800
600
400
200
0
20

30

40

50

Auger electron energy (eV)Fig. 1 (a) normal Auger electron spectrum measured
at 305 eV photon energy. (b) Auger electron spectrum
measuredincoincidencewiththresholdphotoelectrons at the S 2s threshold electron peak.
[1] F. Penent et al., J. Electron Spectrosc. Relat.
Phenom. 144-147 (2005) 7.

57

BL7B

Velocity Imaging Spectrometer for Negative Fragment Ions
Y. Hikosaka, E. Shigemasa
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585 Japan

Negative ion mass spectrometry working withsynchrotron radiation source has been extensively
used in investigation of the ion-pair formation, where
negative ion detection achieves a basic discrimination
against abundant photoionization processes associatedwith the formations of photoelectrons and positiveions. The negative ion yield curves have provided
spectroscopic information on the superexcited states.
Onsets in the curves often match the thermochemical
thresholds for different ion-pair dissociation channels,
from which one could deduce the states of the positive
and negative ions formed. Ordinary mass spectrometry,however, rarely has the ability to determine kinetic
energies and angular distributions of the fragment ions,though such information brings about further insights
into the spectroscopy of the superexcited states and the
dynamical property on the ion-pair dissociation.
We have developed a negative ion imaging
spectrometer [1] designed to be used in combination
with an ordinary synchrotron radiation source. The
powerfulness of the imaging technique has beenexerted on ion and photoelectron observations in the
study of laser chemical dynamics. The advantages of
the imaging technique applied to the study of theion-pair formation process are (i) high collection
efficiency (4S-sr. solid angle), and (ii) observation of
the velocity vector distributions of fragment ions. The
first useful information derived from the velocity
vector distributions is the angular distribution withrespect to the electric vector of the radiation. The
angular distribution may imply the molecularorientation induced by photoexcitation, where theorientation reflects the symmetries of initially-formed
superexcited states. The kinetic energy distribution of
the negative fragment ions is the second useful
information from the velocity vector distributions on
image. Internal states of negative and positive
fragments are accessible from the kinetic energy
distribution. The electronic and ro-vibrational
populations of the final fragments closely relate to themolecular states concerning the reaction dynamics.
The present negative ion imaging spectrometer isillustrated in Fig. 1. We have installed the imaging
spectrometer on the beamline BL7B. The
monochromatized synchrotron radiation has been
introduced into the middle point between the plates P1
and P2 perpendicularly to the plane of Fig. 1. The
polarization axis of light was positioned
perpendicularly to the axis of the flight tube.
Negatively-charged
particles
produced
after
photoabsorption, i.e. electrons and negative ions, can beextracted by the three-element electrostatic lens into theflight tube, and projected onto the position-sensitive
detector terminating the tube. For observation ofnegative ion images, we have applied a strong magnetic
field in the ionization region, in order to suppress
electrons’ arrivals to the detector.
Usefulness of the present method for observing
negative ions is clearly demonstrated on the images
from O2 measured at 21.06 eV photon energy
with/without a strong magnetic field. The image
observed without any extra magnetic field (Fig. 2(a)) is
fully occupied by the electrons due mainly to theformations of O2+(b46g- and B26g-). In contrast, only asingle ring due to O- fragments emerges in Fig. 2(b) as
we applied the strong magnetic field. The single ringcorresponds to the formation of O-(2P)+O+(2D). The
intensity distribution along the circumstance of thering shows that O- fragments are preferably emitted
along the electric vector. This fact implies that thephotoexcitation to 6u- symmetry state(s) is dominant at
this photon energy.
S
N
P1

P2

P3

gas
+

PSD

S
N

i

ik

k

hi

Fig. 1 A schematic view of the negative ion imaging
spectrometer.
(a)

(b)

E

Fig. 2 Raw images from O2 measured with 21.06 eV
photons (a) without any extra magnetic field and (b)with a strong magnetic filed (~15 mT flux around theionization region). Each image is plotted on an
individual linear scale. The electric vector of the lightlies along the vertical axis.
[1] Y. Hikosaka and E. Shigemasa, J. Electron
Spectrosc. Relat. Phenom. 148 (2005) 5.

58

BL7B

Negative Fragment Ion Formation from N2O Studied by
Velocity Imaging Spectroscopy
Y. Hikosaka, E. Shigemasa
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585 Japan
shapes of the potential energy surfaces on which theion-pair dissociation occurs. The images measured at
non-resonant energies show weak preferences for the
O- emission in the perpendicular direction to theelectric vector. In striking contrast, at the two resonant
energies the O- fragments are preferably emitted along
the electric vector (E=0.5 at the lower resonance andE=0.7 at the higher one). This fact is a direct evidence
that initial photoexciation to the two Rydberg states
results from parallel transition, i.e. that the two
Rydberg states have Ȉ symmetry, inconsistent withsome previous interpretations inferred mainly from thequantum defects of the Rydberg states. The
assignments we propose for these two Rydberg series
are given in the figure.

Formation of a pair of positive and negativefragment ions following molecular photoexcitation iscommonly observed in the vacuum ultraviolet region,
although in general this process occupies only a small
fraction in the total cross section. The cross section of
the ion-pair formation is often enhanced at
superexcited states which are mostly Rydberg states
converging to upper-lying ion states. The
enhancements result from the interactions between the
superexcited states and the ion-pair states correlating
directly to ion-pair dissociation limits, and theinteraction strength relates to the electronic properties
and the potential energy surfaces of the relevant states.
Accordingly the superexcited states indiscernable in
the total cross section may be exhibited on the ion-pairformation; observation of the ion-pair formation,
therefore, can be utilized as a specific probe forsuperexcited states.
We have developed a negative ion imaging
spectrometer which uses magnetic field to filter out
electron signals [1]. The present method enables us toperform imaging observation of negative ions, for the
first time, by the use of ordinary synchrotron radiation
which offers properties of wide tunability and linear
polarization. We have applied the new method to
ion-pair dissociation in N2O, which proves anddemonstrates the powerfulness of the present method
in study on ion-pair dissociation dynamics.
Measurements were made at beamline BL7B.
Formation of O- fragments from N2O is most extensivein the 18-20 eV photon energy range [2]. Figure 1
shows a total yield curve of the imaging detection. The
spectral features on the yield curve are essentiallyequivalent to those on the O- yield curve obtained by aquadrupole mass spectrometer [2], though thebackground due to remaining electrons is anticipatedin the present curve. Five Rydberg series converging toN2O+(C2Ȉ+) have been located in this photon energy
range; though the assignments of the Rydberg series
have been discussed, they are still not definitelydetermined. Two of the Rydberg series are remarkably
exhibited in Fig. 1.
Negative ion images measured at five photon
energies, two of which correspond to Rydberg peaks,
are displayed in the inlets of Fig. 1. No clear ring canbe seen on all the images, implying that the Ofragments do not have some distinct kinetic energies
but widely-distributed energies. Here, the kinetic
energy distributions of the O- fragments reflect theinternal energy distributions of counterpart N2+
fragments, and accordingly the counterpart N2+
fragments are excited ro-vibrationally rather than only
electronically. The excitation closely relates to the60

Intensity (arb. units)

E
40

20

3

ndV
0
18.0

nsV

C26+

4

18.5

19.0

19.5

20.0

20.5

Photon energy (eV)Fig. 1 A total yield curve of the imaging detection
from N2O photoionization with a strong magnetic field,
which essentially corresponds to an O- yield curve.
Photon energy resolution is set to 20 meV. Energy
levels of two Rydberg series converging toN2O+(C2Ȉ+) are indicated with the assignments we
propose. Raw O- images measured at 18.10, 18.26,
18.43, 18.56 and 18.75 eV are attached. The electric
vector of the light lies along the vertical axis.
[1] Y. Hikosaka and E. Shigemasa, J. Electron
Spectrosc. Relat. Phenom. 148 (2005) 5.
[2] K. Mitsuke, S. Suzuki, T. Imamura and I. Koyano,J. Chem. Phys. 92 (1990) 6556.

59

BL7B&8B1

Anisotropic Fragment Emission on Valence Photoionization of CF4
Y. Hikosaka, E. Shigemasa
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585 JapanAs a prototype to understand photoionization
dynamics
ofhighly-symmetricalmolecules,
photoionization of CF4 in the valence region has beenstudied extensively by both experiment and theoretical
calculation. Photroabsorption spectra and electronenergy loss spectra of CF4 exhibit a few distinct
features and are occupied mainly by broad structures.
Harshbarger et al. assigned most of the structures totransitions into Rydberg orbitals [1], and theirassignments have been accepted in several subsequent
works. In contrast, Ying and Leung have ascribed thesame structures to valence states associated withtransitions to the unoccupied 5a2 and 5t2 orbitals [2].
Meanwhile, the partial photoionization crosssections and photoelectron asymmetry parameters
measured for individual CF4+ states have been themost important experimental data to be reproduced bytheoretical calculation. Although calculation results
agree, to some extent, with the experimental curves,
satisfactory interpretations are still not proposed to thephysical origin of the modulations on the experimental
curves. The most important and critical subject on theinterpretations is again the transitions from each
occupied orbital to the unoccupied 5a2 and 5t2 orbitals:
the calculation results tend to put the transitions into
the photoionization continua as shape resonances, instriking contrast to the assignments by Ying andLeung.
The present work offers new experimental
information to understand dynamics on the CF4
valence photoionization. Using the photoion velocity
imaging method we have measured in hQ=16-60 eVangular distributions of fragment ions with respect tothe electric vector of light. The photoion images
measured at hQ= 17.4, 19.9, 29.9 and 46.5 eV arepresented in Fig. 1. The photoion images correspondto projections of the initial three-dimensional photoion
velocity distributions onto the detector. Here, theobserved images are for all ion species, since thepresent imaging detection can not discriminate ion
species. One would speculate that at every photon
energy the isotropic fragment emissions result fromthe high symmetry (Td) of the CF4 neutral groundstate; however, apparent anisotropy is practicallyobserved on the fragment emissions at the lower threehQ: the photoions at these photon energies are
preferably emitted along the electric vector. The
anisotropic fragment emissions directly imply that theinitial molecular symmetry is broken on the dissciative
photoionization.
Our interpretation to the origin of the observed
anisotropies is as follows. Within the Td symmetry, thetransition from an occupied orbital to one of theunoccupied orbitals should be isotopic with respect tothe electric vector and result in a triply-degenerated
excited state with T2 symmetry. Such a degenerated
state is subject to symmetry distortion, and a T2 state
originally in Td symmetry results in an A1 and E states
after the distortion to C3v. The dipole moment for thetransition to the A1 state is parallel to the C3v axis,
while that for the transition to the E state isperpendicular to the axis. Here, the two transition
moments are not necessarily the same any more after
the symmetry distortion. Autoionization from such
distorted excited states produces similarly-distorted
CF4+ states. Since the CF4+(X, A) states have potential
energy surfaces being repulsive associated with the
C-F bond, the axis of the ion dissociation naturally
coincides with the C3v axis. Photoexcitation to the A1
and E states, therefore, promotes the positive andnegative asymmetry with respect to the electric vector,
respectively, for the resultant CF3+ fragments. The
observed anisotropy is always positive, which implies
that the parallel transition forming A1 states ispreferable to the perpendicular transitions. This may
be reasonable, because both the 5a2 and 5t2 orbitals
have the V* character and the associated transitions
have large contributions to the cross sections at themeasured photon energies.
hQ= 19.9 eV

hQ= 17.4 eVE

hQ= 29.9 eV

hQ= 46.5 eV

Fig. 1 Photoion images from CF4 photoionization at
hQ=17.4, 19.9, 29.9 and 46.5 eV. Each image is plottedon an individual linear scale.
[1] W.R. Harshbarger et al., J. Electron Spectrosc.
Relat. Phenom. 1 (1972/1973) 319.
[2] J.F. Ying and K.T. Leung, J. Chem. Phys. 100(1994) 7120.

60