BL2B2

Photofragmentation of C60 in Valence Ionization

J. Koua, T. Moria, Y. Haruyamab, Y. Kubozonob and K. Mitsukea,cDepartment of Vacuum UV Photo-Science, The Institute for Molecular Science, Myodaiji,Okazaki 444-8585, Japanb
Department of Chemistry, Faculty of Science, Okayama University, Okayama 700-8530,
Japanc
Graduate University for Advanced Studies, Myodaiji, Okazaki 444-8585, Japana= 39.7±1, 44.7±1, and 51±1 eV for C58+, C56+, andC54+, respectively. These large internal energies arefar beyond the adiabatic energies (E0) for dissociationof C60-2n+ĺC60-2n-2++C2 (n = 0,1,2,…)……(1).
We applied the RRKM model to estimate theinternal energies of the parent ions. The detail of thecalculation is the same as that described in Ref [2].
Fig. 2 shows yield distribution of C58+, C56+, and C54+
as a function of the internal energies of the parentions. We adopted E0 of 7.06, 6.78, and 6.60 eV for n=1, 2, and 3 in the reaction (1), respectively[2]. Theinternal energies for appearance thresholds of C58+,
C56+, and C54+ are 40.7, 47.3, and 51.7 eV,respectively. These values are in good agreement withthe observed Eimax. However, all curves in thecalculation lie in a range from 40 to 70 eV and do not
reproduce the long tails in the higher energy side.
Minimum photon energy for the excitation to thisinternal energy range is 44.3 d hQ d 74.3 eV.
Therefore, the long tails observed at hQ t 90 eV
indicate that photoelectron takes away a part of theexcitation energy as a kinetic energy and leads to a
production of the parent ion with an internal energy
from 40 to 70 eV.

It is well-known that dissociative ionization of C60
results in fragment ions with even numbered carbon
atoms (C60-2mq+, m=1,2,…). Experiments of laser
multiphoton ionization[1] or electron impactionization[2] have shown that these ions are produced
via thermal fragmentation of C60 parent ions in highly
excited vibrational states. Single photon excitation
with a tunable synchrotron radiation is expected to be
helpful in measuring the appearance energies of thefragment ions and investigating redistribution ofinternal energy of the parent ions, since one canestimate the magnitude of the vibronic energy
initially deposited in the C60 system. However, nophotoion yield curves for production of the fragment
ions have been reported. As part of our recent studies
on photoionization of fullerenes[3], we havemeasured the photoion yield curves of the fragment
ions produced from C60 at hQ = 25 -150 eV toinvestigate mechanism of the photofragmentation ofC60.
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Fig. 1. Photofragment ion yields.
Yield curves for singly charged fragment ions areshown in Fig. 1. As the fragment ions become smaller,
their appearance photon energies (AE) shift to higher
energy side and curves rise more gently towards thepeaks. All curves have long tails and a yield ratio of
C58+: C56+: C54+ keeps a constant value ofapproximately 4: 3: 1 at hQ t 90 eV. The maximum
internal energy (Eimax) of the parent ion produced via
an excitation with the appearance photon energy isfound by
Eimax = AE + Ev – Ip.
Here Ev = 3.3 eV means the initial vibrational energy
deposited in C60 at a heating temperature of 680 K
applied to produce a C60 molecular beam and Ip = 7.6
eV the ionization potential of C60. We obtained Eimax㪇㪅㪇㪊㪇㪋㪇㪌㪇㪍㪇㪎㪇㪏㪇㪠㫅㫋㪼㫉㫅㪸㫃㩷㪼㫅㪼㫉㪾㫐㩷㫆㪽㩷㫇㪸㫉㪼㫅㫋㩷㪚㪍㪇㪂㩷㫀㫆㫅㩷㩿㪼㪭㪀Fig.2 Distribution of fragment ions calculatedwith the RRKM model.
[1] P. Wurz and K. R. Lykke, J. Phys. Chem. 96, 10129(1992).
[2] R. Wörgötter, B. Dünser, P. Scheier, T. D. Märk, M.
Foltin, C. E. Klots, J. Laskin, and C. Lifshitz,M. J.
Chem. Phys. 104, 1225 (1996).
[3] J. Kou, T. Mori, S. V.K. Kumar, Y. Haruyama, Y.
Kubozono, and K. Mitsuke, J. Chem. Phys. 120, 6005
(2004).

43

BL2B2Absolute Photoabsorption Cross Section of C60

T. Mori, J. Kou, Y. Haruyama*, Y. Kubozono*, K. Mitsuke
Department of Vacuum UV photo-science, Institute for Molecular Science, Okazaki 444-8585
Japan*Department of Chemistry, Faculty of Science, Okayama University, Okayama 700-8530
Japan
Relative photoabsorption cross sections of C60 havebeen reported by several groups. Most of the authors
focused their attention to photon energies below 25
eV or those around the carbon 1s edge (~ 280 eV).
Furthermore, only a few experiments have been madeon the absolute photoabsorption cross section Vabs ofC60. The Vabs curve can be estimated by directabsorption of synchrotron radiation in a cell filledwith C60 vapor. Jaensch and Kamke measured Vabs byusing this method in the vacuum UV region [1]. They
were confronted with several serious problems. First,
the available energy range is crucially restricted by
the material of the windows of the cell. Goodtransmission should be secured for the windows up tothe highest photon energy of interest even at highambient temperatures. Second, knowing the sample
number density is prerequisite for calculating Vabs,
though the density is markedly unstable at hightemperatures. With these difficulties, there has been
no report on Vabs at photon energies above 24.5 eV,
where only relative absorption cross sections havebeen determined from the yields of positive ionsproduced from C60 [2,3].GIn this report, we attempt to obtain the Vabs curve of
C60 from 25 to 180 eV by means of mass
spectrometry. For this purpose, several essential
devices have been exploited in combination with ahigh-temperature source of gaseous fullerenes: agrazing-incidence monochromator, a conical nozzle, aquartz-oscillator thickness monitor, and an efficient
time-of-flight (TOF) mass spectrometer. The
thickness monitor was used for the measurement of
the C60 density. The detail of the apparatus has been
described elsewhere [4].
We obtained the Vabs curve by following equation.

nozzle with a throat diameter of 0.5 mm and adiverging angle of 7.2 degree. The distance between
the throat of the nozzle and the central point of theionization region was set to 34.8 mm. The numberdensity at the photoionization region is obtainablefrom the sublimation rate, beam size there (18.5 mm2),
and the average of vertical velocity v of the C60
beam at the oven temperature T:v

1 28k B  T / S  m

Where kB is the Boltzmann constant and m is themass of C60. Assuming that the sublimation rate was 5ng s-1 at T = 743.15 K, the number density is
estimated from v = 74 m s-1 to be 0.765×109
molecules cm-3.
The sensitivity of our spectrometer is derived fromthe signal counts of Kr whose absolute
photoionization cross section is given in the literature[5].
The obtained cross section of C60 is shown in Fig. 1.
The Vabs reported by Kamke were found to agree wellwith our data at 24 eV. The reliability of the Kamke’s
data appears to be degraded to some extent at hQ>25eV.
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Vabs = (Ion count rate)/{(Photon flux)·(The sampledensity)·(Interaction length)·(Sensitivity of thespectrometer)·(Residence time)·(Pulse frequency)}㩷㪚㫉㫆㫊㫊㩷㫊㪼㪺㫋㫀㫆㫅㩷㪆㩷㪤㪹㪏㪇㪇㪇㪇㪌㪇㪈㪇㪇㪈㪌㪇㪧㪿㫆㫋㫆㫅㩷㪼㫅㪼㫉㪾㫐㩷㪆㪼㪭A pulse voltage rising from the ground level to+100 V was applied to the ion repeller electrode as
start trigger for the TOF measurement. The durationand frequency of this pulse voltage were 5 Ps and 5kHz, respectively. Ions were drawn through a central
hole 10 mm in diameter of the grounded extractor
electrode. Photoions were detected with amicrochannel plate (MCP) electron multiplier. The
Photon flux was measured by a photodiode.
The vapor of the fullerene was discharged from theFig. 1

Absolute cross section of C60[1] R. Jaensch and W. Kamke, Mol. Mat. 13, 143(2000).
[2] I. V. Hertel et al. Phys. Rev. Lett. 68, 784 (1992)[3] J. Kou et al. Chem. Phys. Lett. 374, 1 (2003)[4] T. Mori et al. Rev. Sci. Inst., 74(8) 3769 (2003)[5] J. A. R. Samson and W. C. Stolte, J. Electron
Spectrosc. Relat. Phenom. 123, 265 (2002).

44

BL3B

Autoionization and Neutral Dissociation of Superexcited HI Studied byTwo-Dimensional Photoelectron Spectroscopy

Y. Hikosaka and K.Mitsuke*
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585*Department of Vacuum UV Photoscience, Institute for Molecular Science, Okazaki 444-8585
summing electron counts over the whole Ek range as afunction of EhQ. A broad peak centered at EhQ ~ 12.3
eV can be seen on the curve. In analogy to HBr, thisbroad peak can be assigned to 5dS HI*(RA) state. The
2D-PES exhibits strong vibrational excitation ofHI+(X23) over an EhQ region from ~12 to 13.7 eV,which is attributable to autoionizing feature of the5dS HI*(RA) state [1]. In addition to the patterns due
to molecular ionization, vertical stripes runningparallel to the photon energy axis can be seen around'E = 13.5 - 14 eV. They are assigned as resulting
from autoionization of the atomic Rydberg states
I*[R(3P0,1)] converging to I+(3P0 or 3P 1). The overall
reaction scheme for the formation and autoionizationof the iodine atom is described as follows [1]:(1a)HI(X16+) + hQ o HI*(RA)4HI*(RA) o HI*[R( 31/2)](1b)HI*[R(431/2)] o H(2S1/2) + I*[R(3P0,1)] (1c)
I*[R(3P0,1)] o I+(3P2) + e- .
(1d)
In process (1d) the electron kinetic energy is definitelydetermined by the energy difference between atomic
energy levels of I*[R(3P0,1)] and I+(3P2), andindependent of the initial excitation energy in process(1a). As a consequence, autoionization of I*[R(3P0,1)]
should give rise to vertical stripes running parallel tothe EhQ axis on the 2D-PES.

Superexcited states, which are defined as neutralstates lying in ionization continua, occupy
considerable amounts of oscillator strength in theextreme ultraviolet region of a molecule.
Autoionization induced by interelectronic interactionis their major decay processes. However, in the case ofmolecular superexcited states with relatively narrowautoionization widths, other processes such as neutral
dissociation compete with autoionization and havesizable fractions in the decay.
Hydrogen halides are one of the classes of molecules
whose superexcited states attract considerable
attention in the literature. As for HCl and HBr, seriesof Rydberg states converging to the ionic A 26+1/2 states
appear as extensive vibrational progressions on
photoabsorption spectra. Characters of the involving
states and branching ratios have been well accounted
for from both experimental and theoretical sides. In
contrast, less information has been gained on decay
from the corresponding Rydberg states of HI, since theRydberg states do not exhibit distinct vibrational
progressions but broad and almost structurelessfeatures on photoabsorption and photoionization
spectra. Even for the superexcited states with suchbroad profiles, two-dimensional photoelectron
spectroscopy, in which the photoelectron yield isrepresented as a function of both photon energy EhQ
and electron kinetic energy Ek, may offer informationon the decay dynamics. This is because, when thephoton energy is tuned to a superexcited state, thevibrational distribution of the final ionic state on thephotoelectron spectrum is closely related to thepotential energy surface of this autoionizing state. In
addition, photoelectron spectroscopy can probe neutral
dissociation of the superexcited state if the neutralfragments undergo autoionization.
Figure 1(a) shows a two-dimensional photoelectron
spectrum (2D-PES) of HI measured in the EhQ regionof 11.10 - 14.85 eV. The EhQ region encompasses theFranck-Condon gap between the X23 and A26+1/2
states of HI+. Hence, the 2D-PES is expected to show
the features arising from the decay of the Rydberg
states HI*(RA) converging to HI+(A26+1/2). Two intense
diagonal stripes at 'E { EhQ- Ek = 10.39 and 11.05 eV
result from the formation of spin-orbit components ofthe v+ = 0 level of HI+(X23). In several EhQ regions
extensive vibrational excitation of HI+(X 23) up to v+ ~12 are noticeable. A broad diagonal pattern spreading
around 'E = 14 - 14.5 eV is ascribed to the formationof the HI+(A 26+1/2) state.
Figure 1(b) represents the curve obtained by(a)(b)

Energy difference, 'E (eV)
14.5

14.0

13.5

13.0 12.5 12.0

11.5

11.0

10.5

14.5

Photon energy, EhQ (eV)

14.0

13.5

13.05dS(n* = 2.78)

12.5

12.0

6pS(n* = 2.3)
11.501234

Electron kinetic energy, Ek (eV)Fig. 1 (a) Two-dimensional photoelectron spectrum ofHI in EhQ = 11.10 - 14.85 eV. The electron yield,
measured as a function of both EhQ and Ek, is presentedby the plots with eight tones from light to dark on alinear scale. Diagonal lines attached on the top of thisfigure denote the energy difference defined by 'E =EhQ - Ek. (b) The curve obtained by summing electron
counts over the whole range of Ek as a function of EhQ.
[1] Y. Hikosaka and K. Mitsuke, J. Chem. Phys.
submitted.

45

BL4B

A Toroidal Electron Analyzer for Observing Anisotropic Electron Emissions
Following Molecular Inner-Shell Photoabsorption
Y. Hikosaka and E. Shigemasa
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585
angle is suitable to coincidence measurements. Oneof our plans is to combine this electron analyzer with
an imaging analyzer for ion momentum observation.
The coincidence setup would enable us to observe
anisotropic electron emissions in molecular frame.

Molecular inner-shell photoabsorption processes
leading to both discrete and continuum states are
intrinsically anisotropic, since an anisotropic
orientational distribution of molecules is determinedby the orientation of the transition dipole moment
relative to the electric vector of the light. Thisanisotropy is reflected in the angular distribution of
the products created either from the initial core hole
state or from states following the rapid decay.
Processes like photoelectron emission, Auger electron
ejection, fluorescence, and photodissociation, all bear
the vestige of the symmetry of the initial excitedstate.
An electron analyzer with toroidal shapes is one of
the most suitable analyzers to observe the anisotropic
electron emissions, owing to the simultaneousmulti-energy and multi-angle detection with areasonably-high energy-resolution. Recently, a“double toroidal” electron analyzer, which has beenoriginally developed by a French group [1], has been
constructed. The analyzer consists of a four element
electrostatic conical lens and two toroidal deflectors
connected, as illustrated in Fig. 1. The conicalsymmetry of the lens system limits the detection
angle of electrons to 54.7q with respect to thecylindrical symmetry axis of the analyzer. The
incident electrons are focused by the lens, and thendispersed in energy between the double toroidal
deflectors, and may reach a delay-line-type imaging
detector (Siegmund Scientific) mounted on the output
plane. The imaging detection without any exit slit
enables us to observe electrons within an energy
range more than 10 % of the pass energy. Figure 2
shows an observed image for Kr M4,5N2,3N2,3 Augerlines with kinetic energies in 50-57eV. The Augerlines are partially resolved, and accordingly four ormore concentric circles are seen on the image. Since
we set the cylindrical symmetry axis of the analyzer
to be parallel to the electric vector of the incident
radiation, no anisotropy should essentially beobserved along the concentric circles. However, theconcentric circles in Fig. 2 are, in practice, far fromisotropy; particularly, a lack of intensity is detected at
the top-right part of the image, which is due to apotential distortion by a gas nozzle. The other minor
intensity modulations on the concentric circles areprobably due to a transmission-efficiency fluctuation
depending on angle. Actually the analyzer has three
supporting struts in the lens system to keep its
geometrical structure appropriate, which may induce
insensitive areas on the detector.
The acceptance solid-angle of the analyzer isestimated to be about 6×10-2 sr. Such large acceptanceFig. 1 A side view of the chamber with a schematic
cutting image of the toroidal electron analyzer.

Fig. 2 An image observed by a delay-line-type
imaging detector. The Kr M4,5N2,3N2,3 Auger lines
with kinetic energies in 50-57eV are partiallyresolved. A lack of intensity at the top-right part ofthe image is due to a potential distortion by the gasnozzle.
[1] C. Miron et al, Rev. Sci. Instrum. 68, 3728 (1997).

46

BL4B

Observation of Vacuum Ultraviolet Fluorescence Following the N 1s
Excitations of N2O
Y. Hikosaka and E. Shigemasa
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585
producing the same fluorescing species. In contrast,
the Auger final states following these twocore-excited states should be quite different fromeach other, according to the electronic properties of
the core-excited states. The present observationsuggests that most of the fluorescing species are not
the molecular Auger final states, but the excited
fragments produced by the dissociations after thecore-hole relaxations. One interesting aspect seen inFig. 2 is that the intensity ratios of several peaks
between the two spectra are apparently different: thepeaks at 460 and 510 Å in the spectrum of 424 eV isstronger than those of 401 eV, while the peaks at 610
Å exhibit the opposite behavior. This reflects different
Auger final-states accessed by the resonant Augerprocesses.

Resonant Auger is the dominant decay process ofmolecular core-excited states. The dynamics of theresonant Auger transition, which is closely related tothe electronic structure of the core-excited state, hasreceived considerable attention, often in connection
with state-specific fragmentation induced bycore-excitation. Electron spectroscopy is the standard
means to investigate Auger final-states accessed fromeach core-excited state, and many interesting findingssuch as lifetime/vibrational interference, detuning
effects, fragmentation dynamics etc., have beenstudied in great detail.
When the Auger final-states have sufficient internalenergies, fluorescence may be emitted from either themolecular ions or fragments formed throughdissociations of the molecular ions. The ultraviolet
fluorescence may be a unique probe for identifying
the multielectron processes such as double excitationsin the core ionization continua, because it is expectedthat such processes may lead to highly-excitedmolecular ions, fragment ions, and neutral fragments
produced after the Auger decays.
We have constructed a 0.3-m Seya-Namioka-type
fluorescence spectrometer, for observing the vacuum
ultraviolet fluorescence following the molecular
inner-shell excitation. A photo of the spectrometer isshown in Fig. 1. The dispersed fluorescence isdetected by a position sensitive detector composed ofthree microchannel plates and a resistive anodeencoder, where the front microchannel plate is coated
by CsI. The position sensitive detector enables us toobserve the fluorescence spectrum with its spectral
range about 300 Å simultaneously. The wavelength
resolution is estimated to be about 5 Å.
As our first attempt, we have measured the vacuum
ultraviolet fluorescence from N2O at the N 1s
excitation region. Figure 2 indicates dispersed
fluorescence spectra at the incident photon energies
of 401 and 424 eV, where the S*(Nt) and V*
resonances lie, respectively. The wavelength region
simultaneously-observed was set to 450-750 Å,though we did not calibrate the wavelength precisely;
an error within a few tens of Å is anticipated in thewavelength scale of Fig. 2. Many fluorescence lines
can be seen in the spectra. Most of the lines in the
short wavelength region may be regarded ascorresponding to the transitions between ionic states,
probably the emissions from atomic fragment ions;however, at the present stage we do not attempt toassign the fluorescence lines, due to the anticipatederror in the wavelength scale. The same fluorescencelines are observable in the two spectra, implying that
the S*(Nt) and V* core-excited states result in

Fig. 1

A photo of the vacuum ultraviolet
fluorescence spectrometer.

hQ = 401 eV
hQ = 424 eV450500550600650700750

Wavelength (A)Fig. 2 Dispersed fluorescence spectra from N2O
measured at two different photon energies.

47