3
Molecular
spectroscopy
and dynamics

53

BL2B

Formation Mechanism of Multilply-Charged Fullerene Ions
K. Mitsuke, H. Katayanagi, C. Huang, B.P. Kafle, M.S.I. Prodhan, H. Yagi
Division of Chemical. Dynamics and Grad. Univ. Adv. Studies, Inst. for Molec. Sci.,
Myodaiji, Okazaki 444-8585, Japan.
Formation mechanism of multiply-charged
fullerene ions was discussed in terms ofvalence-electron excitation to antibonding unoccupied
orbitals and/or spherical standing waves inside thecavity of fullerene. This excitation should be followedby Spectator Auger processes and transmission of theexcess electronic energy among enormous vibrational
degrees of freedom.
Colavita et al. have calculated the partial
photoionization cross sections for valence electrons
as a function of h using local density approximation
Hamiltonian over an extended energy range. Hence
we will discuss the formation mechanism of C602+
from C60 with the help of their results. Analogous
mechanisms may hold for the formation of C702+ andC842+ from C70 and C84, respectively. The partial
photoionization cross sections of Colavita et al. werecharacterized by the presence of many resonances up
to about 40 eV above the respective photoionizationthresholds. Since the binding energies of five deepest
valence orbitals of C60 range from -24.2 to -27.5 eV,
one can expect that, below h  65 eV, theseresonances should participate in the formation of
C602+. Among the reported partial cross sections thecurve of photoionization of the 3gg and 5hg orbitals
demonstrates pronounced broad resonances at h =37 and 50 eV. These resonances are considered togive rise to the peaks at 35 and 50 eV on the crosssection curve for double photoionization of C60 if wetake account of the broadening of the theoretical
30
25

peaks by many electron or vibrational effects.
Colavita et al. ascribed the resonances at h = 37 and
50 eV to promotion of the 3gg and/or 5hg electron tospherical standing waves inside the cavity of fullerene
which have high angular momentum. The energies ofthe final orbitals are calculated from the binding
energies (-22.42 and -22.89 eV) of the 3gg and 5hg
orbitals to be  14 and  27 eV, corresponding to theresonances at h = 37 and 50 eV, respectively.
We can propose a possible pathway for theformation of C602+, identifying the dominant featuresof the C602+ cross section curve with the above cavity
resonances involving spherical standing waves (seeFig. 1). Let’s assume that the cavity-resonance state isformed by promotion of the 3gg or 5hg electron when
C60 absorbs a 50 eV photon. Then spectator Auger
ionization leads to C60+* in which the excited electron
is still trapped inside the cavity. This excited state
undergoes multiple conversion to nearby cavity- or
shape-resonance states by transmitting a part of theelectronic energy to numerous vibrational modes.
Such conversion may occur consecutively until the
excited electron is stabilized down to one of low-lying
unoccupied orbitals of antibonding * or * characternear the threshold. Thereby the excess energy of at
most  27 eV is dissipated in the fullerene cage.
Finally C602+ is produced by tunneling of the electronin the antibonding orbital. We believe this mechanism
is most plausible below h  65 eV for the formationof C602+.

cavityresonance

Energy level / eV

20
15
10

mostly *

5
0
-5
-10

mostly *

6hu

-15
-20
-25

3gg,5hg

-30

C60*

+

C60 *

+

C60 *

2+

C60

Fig. 1 Schematic picture illustrating the decay mechanism of the cavity-resonance state formed by excitationof the 3gg electron of C60. Vibrationally-excited C602+ can be produced by spectator Auger ionization, multiple
internal conversion with intramolecular vibrational redistribution, and tunneling of the excited electron.

54

BL2B

Partial Cross Sections for Single and Multiple Photoionization of C60
B.P. Kafle, H. Katayanagi, H. Yagi, M.S.I. Prodhan, C. Huang, K. Mitsuke
Division of Chemical Dynamics and Grad. Univ. Adv. Studies, Inst. for Molec. Sci.,
Myodaiji, Okazaki 444-8585, JapanThe fine structures observed in Kou(C60+)
reappeared in the present (C60+) curve: two peaks at
26 and 34 eV and flat area ranging 40 – 50 eV. The 34
eV peak was also discernible on the (C602+) curve asa shoulder. These structures originate from ionization
via the shape resonances as single-electron excitation
to vacant orbitals. The curve of (C603+) has an onset
around 40 eV and steadily increases with h. This
onset energy is consistent with the publishedionization potential of 35.6 [4] or 39.8 eV [5] for theformation of C603+ from C60.

Partial cross sections for the photoion formationfrom C60 were determined from the yields of singly-,
doubly- and triply-charged ions which were measured
by mass spectrometry combined with tunable
synchrotron radiation at h = 25 – 120 eV.
Open circles of Fig. 1 show the relative partial
cross sections (C60+) and (C602+) for the formationof C60+ and C602+ from C60. Each partial cross section
includes the contribution of not only the parent but
fragment ions produced by the C2-loss processes, sothat it is equal to the cross section involving all theionic species in a particular charge state. The solid
curves in Fig. 1 show partial cross sections for theformation of C60+ and C602+ reported by Kou et al. [1],which will be referred to hereafter as Kou(C60+) andKou(C602+), respectively. The Kou(C60+) is normalizedto (C60+) at h = 100 eV. The (C602+) curve makes abroad maximum at 50 eV above which it
monotonically descends, in consistent withKou(C602+) [1]. Nonetheless, (C602+) is about one
third as large as Kou(C602+) at all photon energies,
because, in the present study, we have taken into
account the difference in the detection efficiency of
C60+ and C602+. The (C60+) and Kou(C60+) curves
agree well between 60 and 120 eV. With decreasingh the former curve starts to deviate upwards fromthe latter curve and the ratio of (C60+) to Kou(C60+)
reaches 2.5 at 25 eV. This discrepancy can be
accounted for by that the Kou(C60+) curve has a
discontinuity between the h regions covered bygrating G2 and that by G3. The contamination of the
2nd-order light of G2 at h = 40–50 eV brings about
the discontinuity. In the present study correction ofthe (C60z+) has been performed carefully by
estimating the percentage of the 2nd-order light.
Reliability of the present (C60z+) in Fig. 1 can be
confirmed by comparing with previously publisheddata of the total photoionization cross section T of
C60. In Fig. 2 open circles indicate the sum of (C60+),
(C602+), and (C603+), which is approximately
equivalent to T, while the dotted curve representtheT curve observed by Reinköster et al. [2] anddashed curve does the total photoabsorption crosssection calculated by Colavita et al. [3] These three
curves are normalized at h = 100 eV. Our curve is ina fair agreement with the published curves over awhole energy range, which provides tangible
evidence for the validity of our data analysis.
Neglecting the m/z-dependence of the detection
efficiency resulted in overestimation of Kou(C602+)
and eventually rendered the sum of cross sections,
Kou(C60+)+Kou(C602+) enhanced above 50 eV.

400

Relative cross section / arb. units

350+

C60, present data
2+

C60 , present data

300+

C60, Ref. [1]250

2+C60 , Ref. [1]

200
150
100
50
0
30

40

50

60

70

80

90

100

110

120

Photon energy / eV

Fig.1 Partial cross sections for single anddouble photoionization of C60.
400

total ions, present data
total ions, Ref. [1]experimental, Ref.[2]theoretical, Ref.[3]60  (C), by Henke

Total cross section / arb. units

350
300
250
200
150
100
50
0
30

40

50

60

70

80

90

100

110

120

Photon energy / eV

Fig.2 Total photoionization cross section of
C60. Comparison between the present andprevious data in the literature is made.
[1] Kou et al., J. Chem. Phys. 120 (2004) 6005.
[2] Reinköster et al., J. Phys. B, 37 (2004) 2135.
[3] P. Colavita, et al., Phys. Chem. Chem. Phys. 3(2001) 4481.
[4] P. Scheier et al., Int. J. Mass Spectrom. Ion Proc.
138 (1994) 77.
[5] Juranic et al., Phys. Rev. A 73 (2006) 04271.

55

BL2B

High Resolution Photoelectron Spectroscopy of Gaseous C60
H. Yagi, C. Huang, S. I. Prodhan, B. P. Kafle, H. Katayanagi, K. Mitsuke
Division of Chemical Dynamics and Grad. Univ. Adv. Studies,
Institute for Molecular Science,Okazaki 444-8585 Japan
C60 has many degenerate bands in the 7-40 eV
ionization energy range. To clarify its electronic
structure, photoelectron spectroscopy (PES) studies
of solid C60 have been made by several groups.
However, the PES studies of gaseous C60 are few andthe energy resolution was about 100 meV at the best,
which was not enough to resolve closely spaced
bands in detail. We are developing an apparatus of
high-resolution angle-resolved PES. Our goal is tocarry out PES of various kinds of gaseous fullerenes
with a total energy resolution of ~20 meV.
At the beginning, we performed He I (h=21.2 eV)PES of free C60 molecules. The experimental setup is
illustrated in Fig. 1. Powder of C60 was loaded in anickel sample holder, which was attached to a conicalnozzle, and heated up to 800 K by a cartridge heater.
The fullerene vapor was discharged from the nozzle
and ionized by He I radiation. Electrons ejected atright angles to the direction of the photon beam wereanalyzed by the electron energy analyzer (AC-902,
comstock). The flux of the fullerene beam wasmeasured by a thickness monitor (XTM/2, Inficon).
When the oven temperature was ~800 K, the mass
deposition rate was ~16 ng/s. From these values, thenumber density of C60 at the photoionization region isestimated to be 1.2 × 1016 molecules/m3. The totalenergy resolution of this apparatus at the pass energyof 3 eV is estimated to be ~50 meV from the peakwidth of O2 spectra.
Figure 2 shows the photoelectron spectrum of C60
from which the background is already subtracted. The
sharp peak at 12.62 eV is attributed to the watercontained in the sample powder. Results andexperimental conditions of other groups [1,2] are also
shown for comparison. Although our data have large
uncertainties above ~14 eV due to the large
background at low electron kinetic energies, the peakstructures below ~14 eV is reconciled well with thoseof the precedence research. However, in order toelucidate the fine electronic structure such as the
Jahn-Teller splitting and the vibrational progressionsof C60+ cations, a better signal to noise ratio is needed.
We are now trying to improve S/N ratio by setting theoven unit closer to the ionization region and coating
the inner wall of the analyzer by aerodag (a colloidal
suspension of fine graphite particles).
As the next step, we plan to carry out PES study at
BL2B in UVSOR using a Scienta SES-100 electron
spectrometer, which will give higher intensity andbetter energy resolution. Using BL2B, the partial
photoionization cross sections and anisotropy
parameters of each band may be determined in thephoton energy range of 25-150 eV. The partial crosssections of the HOMO and HOMO-1 orbitals werereported between 18 and 130 eV photon energies [3],but those of other orbitals are still unknown because
their peaks are closely spaced and unresolved.

Fig. 1 Apparatus for the photoelectron spectroscopyof gaseous fullerenes. The direction of the photon
beam is normal to the paper.

Fig. 2 Photoelectron spectra of C60. The results ofother groups are also plotted for comparison [1,2]. is the photoelectron sampling angle relative to thephoton beam.
[1] T. Liebsch et al., J. Electron Spectrosc. Relat.
Phenom. 79 (1996) 419.
[2] D. Lichtenberger et al., Chem. Phys. Lett. 176(1991) 203.
[3] A. Rüdel et al., Phys. Rev. Lett. 89 (2002)
125503.

56

BL2B

Refinements on the Estimation Method of Photoabsorption Cross Sectionsof Metallofullerenes
H. Katayanagi1,2, B. P. Kafle2, T. Mori1, J. Kou1,
Y. Takabayashi3, E. Kuwahara3, 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, Japan
Endohedral metallofullerenes (M@Cn; M: metal, n= 60, 70, …) have a unique structure in which one ormore metal atoms are encapsulated in a “nano-space”
inside a fullerene cage. We carried out a systematicinvestigation of their absolute photoabsorption crosssections which are expected to be of fundamental
importance in various contexts of science andapplication of metallofullerenes.
We have reported photoabsorption cross sections ofseveral metallofullerenes (Ce@C82 [1], Dy@C82 [2]and Pr@C82 [3]) in the photon energy range available
with a monochromator at BL2B in UVSOR. We
obtained the values by means of comparison between
normalized count rates for the metallofullerenes andstandard samples of which cross sections are wellknown. While the standard samples have much lower
masses than those of the metallofullerenes, we did notcorrect the detection efficiency dependence of thetime-of-flight (TOF) mass spectrometer on sample
masses and charges at the time. Although the values
obtained by our previous study might serve as the firstestimate of the cross sections of metallofullerenes, werealized that it was a crude assumption. Accordingly
we started an attempt to refine these cross sections.
In the present study, in order to compensate a massdependence of the detection efficiency we adopted anempirical formula which has been proposed by
Twerenbold et al. [4]. The formula gives relative
detection efficiency of a Microchannel plate (MCP)
detector, which mainly governs the overall detection
efficiency of the TOF mass spectrometer. We verified
the formula using rare gases (He, Ne, Ar, Kr and Xe)and confirmed that the formula is applicable to ourexperimental setup.
We applied the formula to our on-going study toevaluate the photoabsorption cross sections of
Pr@C82 at the outset. In this study, we found out inthe photoabsorption spectra that the giant dipole
resonance of praseodymium atoms survives
regardless of the encapsulation. The refined values for
Pr@C82 at the on-resonance (130 eV) and
off-resonance (110 eV) photon energies are shown in
Table 1. In this evaluation we used C60 as a standard
sample. The cross section at 110 eV agrees well withthe cross section (33.5 Mb) 82 times as large as that ofa carbon atom (C). In the previous evaluation
without the efficiency correction, the off-resonancevalue was only approximately 60% of 82(C).

Moreover we measured the phoion yield spectra of
Xe in order to use Xe as another standard sample. The
cross sections of Pr@C82 using Xe as a standard
agreed with those using C60 as a standard. Thisagreement shows that the efficiency dependence isfairly corrected.
Without correcting the detection efficiency, we hadcome to the conclusion that the giant resonance was
weakened by the fullerene cage in Pr@C82 as well as
in Ce@C82. In order to ascertain it, we again
estimated an enhancement by the giant resonancefrom the difference between cross sections at 110 and
130 eV. The enhancement was thus 15 Mb, which
agreed well with the previous value (16.5 Mb [3]).
Similar enhancement by the giant resonance was also
observed in Ce@C82 [1] and its value was found to be
14.3 Mb. The refined value corroborated ourstatements that the giant resonance survives but issuppressed by the encapsulation and that it is not
specific to Ce@C82 but almost universal amongmetallofullerenes.
In the present study, we confirmed that the absolutevalues obtained by this method are reliable.
Refinements of cross sections for othermetallofullerenes as well as higher fullerenes such as
C70 and C84 are also in progress using several
standards and the Twerenbold’s formula. Accurate
determination of cross sections of fullerenes willallow us to assign structures appear in thephotoabsorption spectra which arise from molecularnature of fullerenes.
Table 1 Refined cross sections of Pr@C82 at thephoton energy of 110 and 130 eV. All cross sections
are in Mb. Numbers in parentheses show estimatederrors.
Photon energy

cross section
Pr@C82+
Pr@C822+110 eV (off-resonance) 22 (11) 15 (4)130 eV (on-resonance) 28 (11) 24 (7)Total
37 (12)52 (13)[1] K. Mitsuke et al., J. Chem. Phys. 122 (2005) 064304.
[2] K. Mitsuke et al., Int. J. Mass Spectrosc. 243 (2005) 121.
[3] H. Katayanagi et al., UVSOR Activity Report 32 (2006)
51.
[4] D. Twerenbold et al., Proteomics 1 (2001) 66.

57

BL2B

Partial Cross Sections for Single and Multiple Photoionization of C84
C. Huang1, B.P. Kafle1, H. Katayanagi1, M.S.I. Prodhan1, H. Yagi1,
Y. Kubozono2, K. Mitsuke1
1
Division of Chemical Dynamics and Grad. Univ. Adv. Studies, Inst. for Molec. Sci., Myodaiji,
Okazaki 444-8585, Japan
2
Dept. of Chem., Okayama Univ., Okayama 700-8530, Japan
gradually. Over the whole h range (C842+) remains
almost flat or slightly declines.
Figures 2 and 3 show the ratio between (C842+) and(C84+) and that between (C843+) and (C84+). The
ratio (C843+)/(C84+) has a positive dependence on thephoton energy and takes 0.16 at 100 eV. In contrast the(C842+)/(C84+) curve appears to reach a maximum
value of 1.0 around 80 eV and thereafter slowly
decreases. The ratios of the partial cross sections arecompared with those for C60 and C70 evaluated at h ~100 eV. All these ratios can be arranged in descendingorder as(C702+)/(C70+)>(C842+)/(C84+)>(C602+)/(C60+)
and(C703+)/(C70+)>(C843+)/(C84+)>(C603+)/(C60+).

Partial cross sections for the photoion formationfrom C84 were determined from the yields of singly-,
doubly- and triply-charged ions which were measured
by mass spectrometry combined with tunable
synchrotron radiation at h = 50 – 120 eV.
The soot containing C84 was prepared by anarc-discharge of carbon graphite composite rods with
Eu2O3. The C84 sample was purified by highperformance liquid chromatography with toluene aseluent, after a Soxhlet-extraction of the soot. The
laser-desorption mass spectrum of the purified sample
exhibited only a single peak for C84, and the purified
C84 sample contains only isomer with D2 symmetry.
About 5 mg of C84 was produced with purity better
than 99 % from 10 g of the soot.
Figure 1 shows the spectrum taken at h = 100 eV,
the oven temperature T = 413 C, and mass deposition
rate D of 0.2 ng s-1. Peaks of the parent ions C84z+ areclearly observed. The shoulder in the smaller m/z sideof the main peak of C842+ is ascribed to C822+ and otherfragments produced by the C2-loss processes from
C842+. We calculated the relative partial cross sections
(C84z+) (z = 1 - 3) for the formation of C84z+ from C84
in the gas phase that was calculated usingabs(C84z+)  R(C84z+) m v 2 /  D C84z+ R(C84z+) T /  D C84z+(C84z+),where  is the photon flux of synchrotron radiation,and (C84z+) is relative detection efficiency of the MCP.
The net count rate R(C84z+) of the ion signal wascomputed by integrating the signal counts over eachmass peak and then subtracting a uniform background.
With increasing photon energy (C84+) shows a slow
decrease, while the (C843+) curve tends to increase2++

Ratio of cross sections, (C84 ) / (C84)2.0

Raw data
curve optimized by
the least-squares fitting

1.5

1.0

0.5

0.0

50

70

80

90

Photon energy / eV

100

110

120

Fig. 2 Ratio between the cross sections fordouble and single photoionization.

0.4

18

Raw data
curve optimized by
the least-squares fitting

12

3+

C84

+

o

h = 100 eV, 413 C
AT=200s Dep.Rate = 0.2ng/s

9

0.3

0.2

Ratio of cross sections, (C84 ) / (C84)

15

3+

2+

C84
Ion intensity / arb. units

60

+

C84

6

0.1

0.0

-0.1

3

-0.2

50

60

70

80

90

100

110

120

Photon energy(eV)

0
20

25

30

35

40

45

50

Time of flight / sFig. 1

Fig. 3 Ratio between the cross sections for triple
and single photoionization.

Time-of-flight mass spectrum of C84.

58

BL4B

Observation of Correlation Between Electron Emission andDissocciation Following S 2p Excitation of OCS
T. Kaneyasu1, M. Ito1,2, Y. Hikosaka1, E. Shigemasa1
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585 Japan
2
Graduate School of Science and Technology, Niigata University, Niigata-shi 950-21 Japan1

dissociation limits of S+(4S) + CO(X1), S+(2D) +CO(X1) and S+(2P) + CO(X1) are energeticallyaccessible, and the limits are indicated by diagonallines, neglecting the internal energy of CO fragment.
The S+ kinetic energy distributions are spread even to
0 eV, which are due to the vibrational and/or
rotational excitations of the neutral CO fragment. The
A and B states dissociate into S+(4S, 2D) + CO, while
the C state decays into S+(2D, 2P) + CO. The present
analysis demonstrates the direct observation of thecorrelation between the resonant Auger emission andmolecular dissociation.

Coincidence detection and correlation analysis among
the particles ejected during the relaxation processes ofcore-excited molecules bring a deeper insight into themolecular inner-shell processes. When both an Auger
electron and fragment ions are analyzed in energy,
one can give access to the state-to-state dissociation
dynamics of the core-excited molecules. We haveobserved correlation between resonant Auger
emission and subsequent molecular dissociation
following S 2p3/2  * excitation of OCS molecules.
In contrast to the dipole-allowed direct photoemission
from the valence shells, it is expected to observespin-forbidden states as the Auger final states [1], but
their dissociation dynamics has not been revealed.
The experiment was performed on BL4B using theelectron-ion coincidence spectrometer [2]. The
monochromator bandwidth was set to Eph/Eph~1000
at h=164 eV. The electrons ejected at 54.7° withrespect to the electric vector of the light wereanalyzed in energy by the double toroidal electron
analyzer, while ions were extracted from theinteraction region into the momentum spectrometerby a pulsed electric field according to the electron
detection. Arrival positions on the detector andtime-of-flights (TOFs) of ions were recorded forevery event. The pass energy of the DTA was set to
50 eV for observing resonant Auger electrons in abinding energy range from 14.5 to 19 eV. The
resolving power of the DTA was estimated to be
E/E~50. The coincidence data set was recorded ath=164.3 eV, corresponding to S 2p3/2  *
excitation.
Figure 1 shows a correlation between the resonant
Auger final states and the TOFs of the fragments ions.
Note that false coincidence events are subtracted. Dueto the limited energy resolution in the present setup,the spin-forbidden states are hardly discernible in theelectron spectrum. The two bands observed areassigned to the overlap of the A2 and B2 states,
and the C2 state as noticed in the spectrum. The A
and B states are energetically capable to dissociate
into S+ + CO, while the C state can decay into boththe S+ + CO and S + CO+ limits. This is roughly
confirmed in Fig. 1(a), although the CO+ fragment ishardly seen in the C state, which may arise from its
low branching ratio [3] and the imperfection of thefalse coincidence subtraction.
We have deduced kinetic energy of the S+ fragments
formed through the dissociations from A, B and C
states. In Fig. 2, the energy correlation between theAuger final states and S+ ions is plotted. Three

19

(c)

(a)

0

2

2

C

5

7

17

10

12

16

2

B

Inensity / arb. units

Binding energy / eV

18

14

2

A

17

Inensity / arb. units

Inensity / arb. units

15

+

(b)

S
+

CO
4200

4400

4600

4800

5000

19

5200

TOF / nsFig. 1 (a) Correlation between the Auger final states
and TOF of fragments. (b) TOF spectrum offragments. (c) Resonant Auger spectrum.
19

)

1 

Binding energy / eV

18
+

2

C

S

)+
(P

CO

0

)

(X

2

1 

+

)
+ ( D
S

CO

(X

12

2

24

)

1 

17

+

S

16

2

B

)+
(S

CO

37

(X

49

4

61

Inensity / arb. units

(a)

(c)

73

2

15

A

85

0.0

0.2

0.4

0.6

0.8

1.0

2

2

1.6

1.8

A , B 
2
C
Total

(b)

1.2

1.4

2.0

Inensity / arb. units

98

Inensity / arb. units+

Kinetic energy of S / eV

Fig. 2 (a) Correlation between the Auger final states
and kinetic energies of S+ fragments. (b) Kinetic
energy distribution of S+. (c) Resonant Auger
spectrum.
[1] S. Masuda et al, J. Electron Spectrosc. Relat.
Phenom., 137-140 (2004) 351.
[2] T. Kaneyasu et al, J. Electron Spectrosc. Relat.
Phenom., in press.
[3] S. Morse et al., Int. J. Mass Spectrom., 184 (1999)
67.

59

BL4B

Three-dimensional Momentum Analyses in Ionic Fragmentations
Following Inner-Shell Photoionization
T. Kaneyasu1, M. Ito1,2, Y. Hikosaka1, E. Shigemasa1
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585 Japan
2
Graduate School of Science and Technology, Niigata University, Niigata-shi 950-21 Japan1

indicated by arrows in the distributions correspond toAuger electron-ion coincidence is a powerful methodforstudyingthedecay
dynamics
ofcore-excited/ionized molecules produced by soft
x-ray irradiation. In order to exert the full potential ofthis method, the spectrometer should be equippedwith a performance realizing analyses of vector
correlations among the momenta of all the particles
emitted. Coincidence imaging spectrometers, whichenable to measure three-dimensional momenta ofboth the electron and ions, have been widely used in
the research field of atomic and molecular science.
However it becomes difficult to observe fast Auger
electrons with a sufficient energy resolution by theimaging technique. In this respect, a conventional
electrostatic analyzer is suitable for observing the fast
Auger electrons.
We have developed an Auger electron-ion
coincidence spectrometer which consists of a double
toroidal electron analyzer (DTA) and athree-dimensional ion momentum spectrometer [1,2].
We report on the three-dimensional momentum
analysis for fragment ions following the C 1s
photoionization of CO. The momentum, energy andangular resolutions for the fragment ions have been
evaluated.
The experiment was carried out on BL4B. The DTA
pass energy was set to 200 eV for observing theAuger electrons with the binding energy range of 40 –55 eV. The pulsed electric field for ion extraction was
adjusted to ensure the detection angle of 4 sr forions with kinetic energies less than 6 eV. In thismeasurement, the arrival positions and time-of-flights(TOFs) of C+ + O+ ion-pairs were recorded incoincidence with the C 1s Auger spectrum at 334 eV
photon energy. Three-dimensional momentum vectorsof the ions P1(C+) and P2(O+) are derived from theirarrival positions and time information. The momentaof each C+ + O+ pair should obey the momentum
conservation on the two-body fragmentation: P1 +P2=0. Conversely, the values of P1 + P2 which arepractically observed reflect the momentum resolutionof the fragment ions measured by the present
spectrometer. Figure 1(a) plots the correlation
between PX1 + PX2 and PY1 + PY2. Here, the x-and yaxes are defined to be perpendicular to the TOF axis,
while the z-axis corresponds to the TOF axis which isparallel to the electric vector of the radiation. The
spot size on the map reflects the momentum
resolution. In order to evaluate the momentum
resolution, the sum-momentum distributions arederived by projecting the map onto each axis, which
are shown in Figs. 1(b) and 1(c). The peak widths

momentum resolutions 2 PX(Y). As a result wehave evaluated momentum resolutions as PX=11 au,
PY=17 au. Similarly PZ=38 au is determined fromthe PZ1+PZ2 distribution. The kinetic energy andangular resolutions are also estimated for the C+ + O+
events. Figure 2(a) represents the energy difference
3/4E1 - E2. From the energy spread in the distribution,
the energy resolution E is estimated to be 1.3 eV.
The angular distribution is analyzed in a similar way
to the energy resolution, and the relative angle, 1 +
2 - 180, distribution is shown in Fig. 2(b). The
emission angle  is measured with respect to the TOF(electric vector) axis. The angular resolution obtained
is better than 18°, which is sufficient to measure theemission angle of the fragment ions and to define
initial molecular orientations when the axial-recoil
approximation is valid.
80

(c)

60

1

(a)

97
129

0

160

-20

FWHM
24 au

Coincidence counts

65

20

192

-40
224

-60

256
3

x10

-80

0

5

10

Coincidence
counts

x10

3

(b)

FWHM
15 au

15
10
5

-80

-60

-40

-20

0

20

40

60

0

80

Coincidence counts

Sum-momentum PY1+PY2 / au

33

40

Sum-momentum PX1+PX2 / au

Fig. 1 (a) Contour plot of correlations between thesum-momenta PX1+PX2 and PY1+PY2. Projections of
the intensities on the contour plot to (b) x-axis, and to(c) y-axis.
(b)

-10

Intensity / arb. units

Intens ity / arb. units

(a)

-5

0

3/4 E1 - E2 / eV

5

10

-60

-40

-20
0
20
40
1+2 - 180 / d egree60

Fig. 2 (a) Energy difference distribution and (b)relative emission angle distribution for the C+ + O+
dissociation events.
[1] T. Kaneyasu et al, AIP conf. proc. 978, (2007)
1793.
[2] T. Kaneyasu et al, J. Electron Spectrosc. Relat.
Phenom., in press.

60

BL4B

Molecular Deformation of Core-Excited OCS Probed by a Momentum
Imaging Method
M. Ito1,2, T. Kaneyasu1, Y. Hikosaka1, K. Soejima2, E. Shigemasa1
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585 Japan
2
Graduate School of Science and Technology, Niigata University, Niigata 950-2181 Japan
1

Interaction between molecules and soft X-rays ischaracterized by excitation and ionization of the coreelectrons. Core-excited molecules decay mostly via
Auger electron emission, leading to multiply-charged
ion states. The subsequent dissociation of the ion
states often results in ionic fragmentations. The
emission directions of the fragment ions reflect themolecular structures at the moment of dissociation.
An ion momentum correlation measurement,
therefore, enables us to determine the molecular
structures at the dissociation, and further to discuss
the molecular deformations of core-excited states.
In this work, we have studied molecular
deformations of core-excited OCS, with an ion
momentum spectrometer [1] followed by a position
sensitive detector. Here, fragment ions were extracted
from an interaction region by a DC electric field, andthe momentum vector of each fragment ion wasdetermined from the arrival position on the detector.
We have accumulated a multi-ion coincidence dataset
at a photon energy of 288 eV which corresponds to
the C1s  π* resonance. Only the triple coincidences
due to the three-body dissociation of OCS3+ wereused for the present analysis, because this choiceenables us to determine directly the momentum
vectors of all the fragment ions. The triple
coincidences associated with the formation of
O++C++S+ can be deduced from the coincidence
dataset, by using information on time-of-flight (TOF)
differences between the three ion fragments. Fig.1
shows a two-dimensional plot of the numbers of thetriple coincidence events, as a function of TOF
difference between first hit and second hit(horizontal) and that between first hit and third hit(vertical). The triple coincidences for O++C++S+ canbe identified in the marked area.
Figure 2 shows momentum correlations among
three ionic fragments included in the triple
coincidences. Here, the relative momentum vectors of
C+ and S+ are plotted, with normalizing the O+
momentum vectors to (X,Y)=(-1,0). One can see in
Fig. 2 that angles between O+ and S+ fragments are
preferably around 150 degree, while the neutral
ground state of OCS has a linear geometry. Thisobservation implies that the molecular geometry is
deformed after the inner-shell photoexcitation to OCS(C1s-1π*). Here, a significant molecular deformation
is not expected on the OCS3+ potential energy
surfaces in general, due to the fast dissociation whichapproximates to the Coulomb explosion. Therefore,
the molecular deformation seems to be occurred at
OCS (C1s-1π*). The degeneration of OCS (C1s-1π*) is

Intensity(arb.units)

dissolved by the Renner-Teller effect, and one of the
OCS (C1s-1π*) states has a bent geometry. The
observed molecular deformation shows a goodcorrespondence to the Renner-Teller distortion at
OCS (C1s-1π*).

Time of Flight Difference
betwee n first hit and third hit(nsec )

1200

1000

800

O++C++S+

600

400

O++C++S2+

200

0
0

200

600 Inte nsity

400(arb.units)Time of Fligt Differnce
between first hit a nd second hit(nse c)

Fig. 1 Time-of-flight difference between first hit
and second hit, and those between first hit and third
hit. The marked area with a square shows triple
coincidences associated with formation of O++C++S+.

1

1.5

3

C+

Relative momentum(arb.units)

1.0

5
6

0.5

8
10

0.0

-0.5

-1.0

-1.5
-1.5

O+
C
O
-1.0

S+

S
-0.5

0.0

0.5

1.0

1.5

Relative momentum(arb.units)

Fig. 2
Momentum correlation for three-bodydissociation of OCS3+ formed by Auger decay fromthe C 1s→π* resonance.
[1] T. Kaneyasu, Y. Hikosaka, E. Shigemasa, J.
Electron. Spectrosc. Relat. Phenom, in press.

61

BL7B

Ion Pair Formation in the Vacuum Ultraviolet Region of NO
Y. Hikosaka, T. Kaneyasu, E. Shigemasa
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585 Japan
Pair formation of positive and negative fragment
ions is one of the basic reaction processes induced byinteraction between molecules and vacuum ultraviolet
photons, though the cross section is highly
unfavorable. The cross section of the ion pairformation is often enhanced at neutral excited states,reflecting predissociation of the neutral excited states
into ion pair states. The predissociation rate relates tothe electronic properties and the potential energy
surfaces of the relevant states, and accordingly theneutral excited states which are hardly discernable in
the total photoabsorption cross sections may appear
remarkably in the cross section for the ion pairformation. Here, one can locate the neutral excitedstates without direct ionization background, because
direct ionization does not contribute to the ion pairformation. The observation of the ion pair formation
can be, therefore, a peculiar probe for neutral excitedstates.
In this work, we have studied the ion pairformation from NO by using a negative ion imaging
spectrometer [1]. The advantages of the present
imaging method [2] over ordinary mass spectrometryare (i) high collection efficiency (4-sr. solid angle),and (ii) informativeness on the velocity vector
distribution of negative fragment ions. The firstuseful information derived from the velocity vector
distribution is the angular distribution of fragment
ions with respect to the electric vector of the incident
light. The angular distribution may imply themolecular alignment induced by photoexcitation,
where the alignment reflects the symmetries of
initially-formed excited states. The kinetic energy
distribution of the negative fragment ions is thesecond useful information derived from the velocity
vector distribution, and the internal states of negativeand positive fragments are determinable from thekinetic energy distribution. The populations of the
final fragment states closely relate to the reaction
dynamics.
A negative ion yield curve of NO measured in thephoton energy region of 19-25 eV is shown in Fig. 1,
in a comparison with a positive ion yield curve. The
negative ion yield curve shows much richer structures
than the positive ion yield curve. This fact proves the
spectroscopic usefulness of negative ion detection to
the neutral excited states. Figure 2(a) shows anegative ion image measured at a photon energy of
22.01 eV, where the first two N++O- limits areaccessible. The measured image corresponds to theprojection of the three-dimensional velocity
distributions for the O- fragments, where the Ofragments associated with each N++O- limit
contributes to a circular pattern on the projection,with intensity concentrated around the circumference.
The two rings on the image in Fig. 2(a), which show
individual anisotropic intensity distributions withrespect to the electric vector, are associated with thefirst two N++O- limits. Figure 2(b) shows atransformed image using an onion-peeling procedure.
On the transformed image, obviously the two rings
corresponding to the individual N++O- limits become
clearer than those on the image in Fig. 2(a). The total
intensity of each ring gives the population of thecorresponding final N++O- pair, and the intensity
distribution along the ring shows the angular
distribution relative to the electric vector.
4

(a)

s

70000 3p
3d
NO+(c3)

50000

Intensity (arb. units)

4

p

3

d

30000

(b)
Rc

+ 3

+ 1

N ( D)+O-(2P)N ( P)+O-(2P) Rc

80000

+ 1

N ( S)+O-(2P)

Rc
?

RB

40000

RB

? ?

0
19

20

21

22

23

24

25

Photon energy (eV)Fig. 1 (a) Positive ion yield curve and (b) negative
ion yield curve of NO. Rc and RB denote Rydberg
states converging to NO+(c3) and NO+(B1),
respectively.

(a)

(b)

E

0.1 0.5 1

2

Kinetic energy (eV)Fig. 2 (a) Negative ion image measured at a photon
energy of 22.01 eV, where the first two ion pair limits
are accessible. (b) Transformed image using anonion-peeling procedure.
[1] Y. Hikosaka, T. Kaneyasu and E. Shigemasa, J.
Electron Spectrosc. Relat. Phenom. 154 (2007) 43.
[2] Y. Hikosaka and E. Shigemasa, J. Electron
Spectrosc. Relat. Phenom. 148 (2005) 5.

62