BL3U

Exchange Interaction of the Rydberg Electron Bound by Doubly Charged
Ions in Small Krypton Clusters Studied by Resonant Auger Spectroscopy
M. Nagasaka1, T. Hatsui2, H. Setoyama1, N. Kosugi1 and E. Rühl3
Institute for Molecular Science, Myodaiji, Okazaki 444-8585 Japan
2
XFEL Project Head Office, RIKEN, Sayo-cho, Hyogo 679-5148, Japan3
Physikalische Chemie, Freie Universität Berlin, Takustr. 3, D-14195 Berlin, Germany
1interaction of the Rydberg electron with thesurrounding atoms is short-range interaction and isalmost proportional to the number of the nearest
neighbor atoms.

In the core-to-Rydberg excitation of small krypton
clusters, we have evaluated the surface-site dependentexchange (EX) interaction of the Rydberg electronwith the nearest neighbor atoms by using X-ray
absorption spectroscopy (XAS) [1 – 3]. After the
resonant Auger decay following the core-to-Rydberg
excitation, the spectator-type Auger final state can be
regarded as a Rydberg electron bound by a doubly
charged ion. The Rydberg electron bound by a singly
charged core-hole state could be decreased by adoubly charged ion, increasing the short-range EXinteraction. In the present work, we have evaluatedthe site-dependent EX energies of the Rydberg
electron with the neighbor atoms in a doubly ion byusing resonant Auger electron spectroscopy (RAS).
The experiments were performed at BL3U. Kr
clusters with <N>~15 were formed by the supersonicgas expansion method. RAS spectra were measured
with a hemispherical electron energy analyzer(SCIENTA SES-200 combined with MBS A-1).
Figure 1 shows RAS spectra after the Kr 3d5/2-15p
Rydberg excitation (91.37 eV) at the edge-site atomof small Kr clusters [1 – 3]. Because the ratio of edge
and face sites is 3:1 in this excitation energy, the ratioin RAS is set to be 3:1 for the fitting procedure. As
shown in Table 1 and Fig. 1, the observed final-stateenergies of both the 5p and 6p states are redshiftedfrom the atomic one, indicating the induced
polarization (PL) effect is dominant. As in the case of
XAS, the energy shift of the Rydberg states (i = 5, 6)
is given by ǻEi (RAS) = PL (+2) + EXi (+2), where
PL (+2) can be approximated as 4 PL (+1). Table 1
shows EX in the doubly charged ion at different
surface sites. EX (+2) is almost proportional to thenumber of nearest neighbor atoms within the firstcoordination shell. EX (+2) of the 5p state is 2.3 times
larger than EX (+1) of the 5p state, indicating
contraction of the Rydberg orbital as demonstrated inFig. 2. Table 1 shows the 6p electron bound by the
doubly charged ion has a similar EX energy to the 5p
electron bound by the singly charged ion, also
indicating contraction of the Rydberg orbital.
In conclusion, RAS spectra of small krypton
clusters show redshifts from the atomic peaks,
indicating the PL effect is dominant even in thelowest Rydberg state. Furthermore, the EX effect isalso enhanced in the Auger final state, indicating thecontraction of the Rydberg orbital under the stronger
potential by the doubly charged ion. The EX

Fig. 1. RAS spectra of different surface sites in small
krypton clusters for the 4s-25p and 4s-26p final states
after the edge-site excitation in the 3d5/2-15p state(91.37 eV).

Fig. 2. Schematic Rydberg orbitals bound by singly
and doubly charged ions at a surface site of small
krypton clusters.
Table 1. Surface-site dependent EX energies (in eV)of the Rydberg electron bound by doubly charged
ions of small krypton clusters. The bulk site
corresponds to the large krypton clusters [4].
eVǻEPL(+2) EX (+2)
EX (+1)4s-25pEdge-0.51 -2.28
1.77
0.74
Face
-0.84 -3.16
2.32
1.01
Bulk
-1.0
-4.3
3.3
1.47
4s-26pEdge-1.66 -2.28
0.62
0.22
Face
-2.22 -3.16
0.94
0.28
Bulk
-2.8
-4.3
1.5
0.42
[1] M. Nagasaka, T. Hatsui and N. Kosugi, in thisvolume.
[2] M. Nagasaka, T. Hatsui and N. Kosugi, J. Electron
Spectrosc. Relat. Phenom. 166-167 (2008) 16.
[3] A. Knop, B. Wassermann and E. Rühl, Phys. Rev.
Lett. 80 (1998) 2302.
[4] S. Peredkov et al., Phys. Rev. A 72 (2005)
021201.

45

BL2B

Electron Ion Coincidence Spectroscopy of C60
Using Synchrotron Radiation Photoionization
C. Huang1, H. Katayanagi1, 2, B. P. Kafle1, Md. S. I. Prodhan2,
H. Yagi1, K. Nakajima1, 3 and K. Mitsuke1, 21Institute for Molecular Science, Okazaki 444-8585, Japan
2
Graduate University for Advanced Studies, Okazaki 444-8585, Japan
3
Science Research Center, Hosei University, Chiyoda-ku, Tokyo 102-8160, Japan
have similar tendencies. The different behaviors ofthe ı(C60-2n2+)/ı(C60+) curves for different detection
methods can be understood [2] from expectation thatthe magnitude of the average internal energy
remaining in nascent C60z+ ions produced in PEPICO
is larger than that in pulsed ion sampling.
The appearance energies and peak positions of theyield curves for the fragment ions were found to be
shifted in the higher hν direction than thosesummarized in [2].

Photoelectron-photoion coincidence (PEPICO)
spectroscopy is one of the most powerful methods forstudying the kinetics and dynamics of chemical
reactions involving excited molecules and ions. We
therefore consider that unimolecular decomposition
processes of fullerenes can be closely studied using a
PEPICO method. This report presents a recent results
obtained with an electron detector newly incorporated
into a photoionization time-of-flight mass
spectrometer using synchrotron radiation of hȞ =25-120 eV.
All experiments were carried out at the bending
magnet beam line BL2B. Parent C60z+ and fragment
C60-2nz+ ions (n  1, z = 1-3) were measured at hȞ = 50
– 120 eV. Since no small ionic carbon cluster Cmz+ (m= 1-5) was observed, the emission of neutral C2 rather
than Cmz+ is the main fragmentation channel of C60z+.
As for the singly charged fragments, C58+ and C56+
were detected only at a very narrow hȞ range. Above
60eV, their intensities become negligibly weak. This
probably means that C60+ ions with their internalenergy higher than ∼60 eV are stabilized rapidly byemitting one or two electron(s), before being
converted to high-vibrationally excited states throughnonadiabatic transitions (i.e. internal conversion)
followed by statistical redistribution of excess energy
and sequential ejection of C2 units [2].
Taking mass spectra with scanning themonochromator allowed us to measure the ion yield
curves for C60z+ and C60-2nz+ as a function of hȞ. The
relative total photoionization cross section was
calculated from the total ion yield curve of C60 andagrees well with our former results [1]. Next, the ratioı(C60-2nz+)/ı(C60+)
betweentherelative
photofragmentation cross section for C60-2nz+ (n  1, z
= 1-3) and that for C60+ was calculated from the yield
curves of the respective ions. This ratio was measured
also in pulsed ion sampling by applying a positive
pulsed voltage to the repeller electrode. Figure 1
shows our results of ı(C582+)/ı(C60+) determined by
PEPICO and pulsed ion sampling, together with theprevious data of Reinköster et al. and Juranic et al. [3,4]. The two data from the literature have beenmeasured by pulsed ion sampling.
All the results of pulsed ion sampling show that theratio almost levels off after passing its maximum
value. In contrast, our PEPICO curve exhibits apronounced decrease after the maximum at 75 eV.
The ratios of ı(C562+)/ı(C60+) and ı(C542+)/ı(C60+)

PEPICO
Pulsed Ion Sam pling
Juranic et al.
R einkoster et al.

㪇㪅㪈㪍

2+

+

σ(C58 ) 㩷/ σ (C60 )

㪇㪅㪈㪉

㪇㪅㪇㪏

㪇㪅㪇㪋

㪇㪅㪇㪇
㪋㪇

㪋㪌

㪌㪇

㪌㪌

㪍㪇

㪍㪌

㪎㪇

㪎㪌

㪏㪇

㪏㪌

㪐㪇

㪐㪌 㪈㪇㪇 㪈㪇㪌 㪈㪈㪇 㪈㪈㪌 㪈㪉㪇

Photon energy / eV

Fig. 1. Ratio between the photofragmentation
cross section for C582+ and that for C60+.
[1] K. Mitsuke et al., J. Phys. Chem. A 111 (2007)
8336.
[2] J. Kou et al., Phys. Chem. Chem. Phys. 7 (2005)
119.
[3] B. Reinköster et al., J. Phys. B 37 (2004) 2135.
[4] P. N. Juranic et al., Phys. Rev. A 73 (2006) 042701.

46

BL2B

Performance of the Mass Gate Incorporated in the Photofragment
Imaging Apparatus for Measuring Momentum Distributions
in Dissociative Photoionization of Fullerenes

B. P. Kafle1, H. Katayanagi1. 2, C. Huang1, S. I. Prodhan2, H. Yagi1 and K. Mitsuke1, 21Institute for Molecular Science, Okazaki 444-8585, Japan
2
Graduate University for Advanced Studies, Okazaki 444-8585, Japan

We have developed a velocity imaging
spectrometer, to obtain a reliable velocity distributionof the fullerene fragments using synchrotron radiation[1-3]. Uniqueness of this apparatus is incorporation ofa cylindrical mass gate and retarding grid near theend of the drift tube which are used to eliminate theoverlapping of images due to other ionic fragments.
Here, we will present preliminary experimental
results using Kr to demonstrate their capability ofseparating ions with a particular charge from thosewith other charges. The experimental chamber was
uniformly filled by Kr sample which was irradiatedwith a synchrotron radiation in the photoionization
region lying between the repeller and extractor
electrodes. A pulsed voltage of +75V was applied to
the repeller electrode for pushing out photoions,while continuous voltages of 68 and 0 V were applied
to the extractor electrode and drift tube , respectively.
Figure 1(a) shows the TOF mass spectra of Krz+
measured in the vicinity of the 3d-15/25p1 resonancepeak at hν = 91.2 eV, with the mass gate andretarding electrode being grounded. In order to realize
isolation of one of the charged state z from the others,e.g. selecting Kr2+ from Krz+ (z = 1-3), the following
method was adopted: First, setting the voltage of themass gate (VM) to zero, the critical voltage on theretarding grid (VG) was optimized in such a way thatno signal was observed. Now, all the ionic species
were reflected back as demonstrated in the Fig. 3 of [1].
Then, a pulsed voltage was applied as VM, at thetiming when a bunch of Kr2+ ions arrives at the massgate, so that they gain additional kinetic energies toovercome the potential barrier at retarding grid andcan reach the detector. Figure 1(b) shows the spectrum
observed at the same voltage combination as that usedin Fig. 1(a) except that VM and VG were set to 5 and 75
V, respectively. The width of the VM was 5 µs and its
rising edge was preceded by 22 µs by that of thepulsed voltage applied to the repeller. Only a single
peak assigned to Kr2+ appears in Fig. 1(b). Thisconfirms that the ion selection using the mass gateworks well. We also tested our spectrometer for its
capability of separating a specific fragment from theother ions produced from SF6 (not shown here). From
the above observation we consider that this setup canbe applied for exclusive imaging detection of C58+
after excluding C60+ and C56+ with the same kinetic
energies.

㩷
㪈㪇㪇

a)

㪢㫉㩿㪉㪂㪀

㪍㪇㩷㪠㫅㫋㪼㫅㫊㫀㫋㫐㩷㪏㪇㪋㪇㪢㫉㩿㪊㪂㪀

㪉㪇

㪢㫉㩿㪂㪀

㪇
㪈㪇

㪈㪌

㪉㪇

㪉㪌

㪊㪇

㪊㪌

㪋㪇

㪫㪦㪝㩷㩿㫄㫀㪺㫉㫆㫊㪼㪺㪀㩷㪏㪇㪻㪼㫃㪸㫐㩷㪔㩷㪉㪉㩷µ㫊㪭㪤㪞㪔㩷㪌㪭

㪢㫉㩿㪉㪂㪀㪍㪇㪋㪇㩷㪢㫉㪉㪂㩷㪠㫆㫅㩷㪰㫀㪼㫃㪻㩷㩿㪸㫉㪹㪅㫌㫅㫀㫋㫊㪀

b)

㪉㪇

㪇
㪈㪇

㪈㪌

㪉㪇

㪉㪌

㪊㪇

㪊㪌

㪋㪇

㪫㪦㪝㩷㩿µ㫊㪀Fig. 1. (a) TOF spectrum of Kr at hν = 91.2 eV. The
mass gate and retarding grid are kept grounded. (b)TOF spectrum of Kr measured with VM = 5 V and VG= 75 V.
[1] B. P. Kafle, H. Katayanagi and K. Mitsuke, AIP
Conf. Proc. 879 (2007) 1809.
[2] Md. S. I. Prodhan et al., Chem. Phys. Lett. 469(2009) 19.
[3] H. Katayanagi et al., in preparation.

47

BL2B

Ion Imaging Study of Dissociative Photoionization of C60
H. Katayanagi1. 2, C. Huang1, S. I. Prodhan2, H. Yagi1, B. P. Kafle2,
K. Nakajima3 and K. Mitsuke1, 2
1
Department of Photo-Molecular Science, Institute for Molecular Science, Okazaki 444-8585,
Japan
2
Graduate University for Advanced Studies, Okazaki 444-8585, Japan
3
Science Research Center, Hosei University, Tokyo 102-8160, Japandissociation dynamics of fullerenes [4, 5].
In the present study, we found that the fragments
could be observed even without mass selection. The
mass selection will allow us more quantitative
analyses on translational energy release distributions.
We developed a potential switch mass gate [2], whichenables us to select a specific fragment from a seriesof fragments. We are now testing the mass gate
experimentally.

The photoion images of fullerene molecular beams,
which consist of parent and fragment ions, wereobtained by the velocity map imaging (VMI)
technique [1]. The fragments were observed as weakshoulders in transverse profiles of the beam.
The design of our imaging setup was described
elsewhere [2, 3]; we explain the experimental
procedure briefly. The fullerene (C60) sample was
loaded in a cylindrical quartz cell and heated up by aheater around 700-800 K in vacuum. The C60 vapor
passed through two apertures and reached theionization region, where the C60 molecular beamintersected the monochromatized synchrotron
radiation (SR). Ions produced at the ionization region
were extracted by a VMI electrode assembly andprojected on to a position sensitive detector (PSD) of
40 mm in diameter and 375 mm away from theionization region. No mass selection was made. We
thus obtained two-dimensional projections of
three-dimensional scattering distributions of the ionson the PSD.
The photoion images were recorded in the photon
energy range from 60 to 110 eV. Fig. 1 shows thephotoion images at 60 (a) and 110 (b) eV. Vertical
stripes along x axis correspond to the beam. Imagesof other photon energies resemble the images shown
here. Background images obtained without the C60
beam was subtracted from all the images.
Figure 2 shows one-dimensional projections of theimages along y axis with integration of pixel
intensities with respect to x. These projections can,therefore, be regarded as the transverse intensity
profiles of the beam. An intense central component of
the profiles originates from parent ions of C60. In
addition to the intense component, we can recognize
shoulders on both sides. Intensities and widths of theshoulders increased with increasing photon energy.
The shoulders can be assigned to the photofragments
produced by the dissociative photoionization of C60
with SR, since the fragments obtained translationalenergy by the dissociative photoionization andescaped from the central component. This assignment
is supported by computer simulations of C60
fragmentation. The shapes of the shoulders agreed
qualitatively with results of the simulations.
Furthermore, the energy dependence can beinterpreted as that the smaller and more highly
translationally accelerated fragments can be producedwith increasing photon energy. This interpretationagrees with our previous conclusions on theFig. 1. Photoion images of C60 molecular beams(parent and fragment ions) at 60 (a) and 110 (b) eV. In
panel (a), x axis corresponds to the direction of themolecular beam and y axis to the synchrotron
radiation. Images are 22.5×22.5 mm in size. Pixel
size is 0.176×0.176 mm.
㩷

㪇㪅㪌

㪇㪅㪇
㪈㪇㪋

㩷

Int. (arb.)

㪈㪅㪇

㪈㪈㪉

㪈㪉㪇

50

110 eV㪈㪉㪏㪈㪊㪍
Y / pixel

㪈㪋㪋

㪈㪌㪉

Fig. 2. Transverse intensity profiles of the beam.
Curves are plotted every 10 eV in the photon energy
range from 50 to 110 eV. Peak intensities of thecurves are normalized to unity.
[1] A. T. J. B. Eppink and D. H. Parker, Rev. Sci
Instrum. 68 (1997) 3477.
[2] B. P. Kafle et al., AIP Conf. Proc. 879 (2007)
1809.
[3] S. I. Prodhan et al., Chem. Phys. Lett. 469 (2009)19.
[4] J. Kou et al., Phys. Chem. Chem. Phys. 7 (2005)
119.
[5] K. Mitsuke et al., AIP Conf. Proc. 811 (2006) 161.

48

BL2B

Dissociative Photoionization of Perfluorocyclobutane
K. Okada1, 2, M. Sakai1, C. Huang2, H. Yagi2, H. Katayanagi2, 3,
K. Mitsuke2, 3 and K. Tabayashi1
1Department of Chemistry, Hiroshima University, Higashi-Hiroshima 739-8526, Japan
2
Institute for Molecular Science, Okazaki 444-8585, Japan
3
Graduate University for Advanced Studies, Okazaki 444-8585, Japan
Molecular photoionization and ionic fragmentation
processes are of fundamental importance in theupper-atmospheric chemistry and plasma physics.
Perfluorocyclobutane is extensively used as a reagent
for dry etching of semiconductors. Ravishankara et al.
[1] report that the lifetime of this molecule in theatmosphere is about 1000 years. The use of the gas inindustry has atmospheric implications for global
warming. Because of the absence of data above theexcitation energy of 27 eV, we measured in this study
yield spectra of the fragment ions produced by thephotoionization of perfluorocyclobutane.
The experiments have been performed on thebeamline BL2B at the UVSOR facility. The
experimental setup has been described in a previous
paper [2], except a minor change for the purpose inthis study: A thickness monitor was removed and aneedle for gas inlet was installed. Synchrotron
radiation was irradiated at right angles to the effusive
beam of the sample gas. Fragment ions produced in
the ionization region were extracted by a pulsed
electric field toward a time-of-flight (TOF)
spectrometer. The pulse and ion signals were fed into
the start and stop pulse inputs of a multi-stoptime-to-digital converter (FAST ComTec, model
P7888), respectively, to record TOF signals. Partial
ion yield spectra were obtained by measuring a seriesof TOF spectra while scanning the photon energy.
The pressure in the chamber during the measurements
was kept at 1.5 × 10-6 Torr.
A variety of fragment ions such as CFk+ (k = 1–3),
C2Fm+ (m = 1–4), C3Fn+ (n = 1–5), and C4F7+ weredetected in the TOF spectra. This is in contrast to theprevious report [3], in which C4F7+ could not be
detected by the measurement of thresholdphotoelectron–photoion coincidence spectra acquired
at photon energies of 10–27 eV. However, theproduction of the C4F7+ ion is reasonable as observedin this study, if considering the fragmentation of otherperfluorocarbon compounds.
Figure 1 shows partial ion yield curves of thefragment ions. The branching fractions at the photon
energy of 30 eV are 46.2 % (C3F5+), 37.5 % (C2F4+),
7.2 % (CF3+), 3.0 % (CF+), 1.7 % (CF2+), 1.7 %(C3F3+), 1.1 % (C4F7+) and others. While the C3F5+
and C2F4+ fragments are most abundant at all theenergies studied here, the yield of these ionsdecreased greatly with increase in the photon energy.
The yield at ~ 170 eV becomes about 1/5 of that at 25
eV. The yield of CF3+ gives an approximately

constant value up to 55 eV, and decreases
monotonically at the higher energies. The yield of
CF+ increases with the photon energy up to about 80
eV, and then gradually decreases. As a result, theyields of small fragment ions are relatively enhancedwith the excitation energy.

Fig. 1. Partial ion yield curves of the fragment ionsproduced by the photoionization of perfluorocyclobutane.

[1] A. R. Ravishankara, S. Solomon, A. A. Turnipseed
and R. F. Warren, Science 259 (1993) 194.
[2] T. Mori, J. Kou, M. Ono, Y. Haruyama, Y.
Kubozono and K. Mitsuke, Rev. Sci. Instrum. 74(2003) 3769.
[3] G. K. Jarvis, K. J. Boyle, C. A. Mayhew and R. P.
Tuckett, J. Phys. Chem. A 102 (1998) 3230.

49

BL2B

Construction of a Velocity Map Imaging Spectrometerand Its Performance Test Using Rare Gases
Md. S. I. Prodhan1, H. Katayanagi1, 2, C. Huang2, H. Yagi2,
B. P. Kafle2, K. Nakajima2, 3, K. Mitsuke1, 2
1
The Graduate University for Advanced Studies (SOKENDAI), Okazaki 444-8585, Japan
2
The Institute for Molecular Science, Okazaki 444-8585, Japan;
3
Science Research Center, Hosei University, Tokyo 102-8160, Japan.
Report is made on velocity map imaging (VMI)
spectrometer based on a time-of-flight (TOF) technique.
Thus we observed the momentum distributions of thescattered cations produced by dissociative photoionization of gaseous fullerenes by irradiation ofsynchrotron radiation [1, 2, 3]. The basic performance ofthe spectrometer was tested with rare gases for attaining
high kinetic energy resolution of the photofragment
images at hν = 35 eV and 300 K. The 3D velocity
distributions were reconstructed using the inverse Abeltransformation (IAT) from the measured 2D images
projected on a position-sensitive detector (PSD) to thecross-sectional images in the perpendicular plane of thespectrometer. Using the speed distributions extracted
from these cross-sectional images, we evaluated thetemperatures by the least-squares fit of the experimental
data points to the Maxwell-Boltzmann distributions, as
demonstrated in Fig. 1.
In addition, we have reproduced the images of raregases at 300 K by computer simulations to comparethem with those obtained experimentally. The number of
He atoms in this simulation was set to 105. It was
assumed that each He+ ion flies from the origin withkeeping its original thermal velocity of a parent He. The
speed was calculated by generating random numbers
utilizing Metropolis method from the MaxwellBoltzmann distribution. The 2D image projected on the
PSD was estimated by the spatial density function.
Figure 1 (a) shows the best focused 2D raw image of
He+ ions. The applied voltages at the MCP andelectrodes were VMCP = -2200 V, VR = 640 V, VT = - 350
V, and VE = 353 V. Panel (b) represents the crosssectional image obtained using IAT from Panel (a) [4].
The solid curve in Panel (c) denotes the MaxwellBoltzmann distribution fitted to the data points (filled
circles, extracted from (b)). The temperature obtained by
the fittings was 282 ± 2 K which is explained in terms ofsystematic underestimate inherent in the IAT analysis.
Similarly, the temperature was evaluated to be 287 ± 3 Kfrom the simulated image of He+ in Panel (d). A few
scattered data points result from the application of IAT
in the image processing.
Similar experimental works were executed for otherrare gases, Ne and Ar. All the temperatures were foundto be in good agreements with those from thecorresponding simulated images.

He

282 ± 2 K

Expt.

(a)

(c)

(b)

He

287 ± 3 K

(f)

Simu.

(d)

x
y
(e)

Fig. 1. Panels (a) and (d) show experimental andsimulated 2D projections of He+ ions at 300 K,
respectively. Panels (b) and (e) are cross-sectional
images obtained from Panels (a) and (d), respectively.
Panels (c) and (f) are the speed distributions extracted
from Panels (b) and (e), respectively. The solid curves
denote the best fitted Maxwell-Boltzmann distributions.
[1] A. T. J. B. Eppink and D.H. Parker, Rev. Sci. Instrum.
68 (1997) 3477.
[2] B. P. Kafle, H. Katayanagi and K. Mitsuke, in“Synchrotron Radiation Instrumentation”, Am. Inst.
Phys., CP879 (2007) 1809.
[3] H. Katayanagi et al., Rev. Sci. Instrum. (2008)
submitted.
[4] S. M. Candel, Comput. Phys. Commun. 23 (1981)
343.

50

BL3U

Exchange Interaction of the Rydberg Electron Bound by Singly Charged
Ions in Small Krypton Clusters Studied by X-Ray Absorption Spectroscopy
M. Nagasaka1, T. Hatsui2 and N. Kosugi1
Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan
2
XFEL Project Head Office, RIKEN, Sayo-cho, Hyogo 679-5148, Japan1

different surface sites of krypton clusters, and found
EX is derived from the repulsive and short-range
interaction between the Rydberg electron and thenearest neighbor atoms of the ionized atom. EX is a
key to understand the surrounding structure of surface
sites in clusters.

Small krypton clusters show different surface sites(corner, edge, and face), and show different redshifts
in the core level as a function of the number ofnearest neighbor atoms in X-ray photoelectron
spectroscopy (XPS) [1]. The single core hole state
formed as a result of core ionization is stabilized byinduced polarization (PL) of surrounding atoms,
which leads to redshift of the cluster peaks. On theother hand, the excitation to low lying Rydberg statesshow blueshift of the cluster peaks in X-ray
absorption spectroscopy (XAS) [2]. The blueshift
behavior indicates destabilization of the excited states,
which consist of the Rydberg electron bound by thesingly ionized core stabilized by PL. The
destabilization would be introduced by exchangeinteraction (EX) based on Pauli’s exclusion principle
for the Rydberg electron overlapping with neighboratoms. In the present work, we have measured XASof krypton clusters with <N> ~15, and discuss EX of
the Rydberg electron in singly charged ion at different
surface sites of krypton clusters [3].
The experiments were performed at BL3U.
Krypton clusters were formed by the supersonic gas
expansion method. XAS spectra were measured witha time-of-flight mass spectrometer to select only Kr2+
dimer ions.
Figure 1 shows XAS spectra of krypton clusters.
The 3d5/2-15p Rydberg states of some surface sites areblueshifted from the atomic one, whereas the 3d5/2-16p
states are redshifted. The initial state effect in van der
Waals clusters is negligible; therefore, the energy
shift (ǻEi) of the Rydberg state (i = 5, 6) in XAS issimply given by ǻEi (XAS) = PL (+1) + EXi (+1),
where PL (+1) is obtained by XPS and is notdependent on each Rydberg state i. Table 1 shows thesurface site dependent EX energies of different
Rydberg states. EX is found to be almost proportional
to the coordination number of nearest neighbor atoms.
It means EX is mainly caused by the overlap of the
Rydberg electron with the nearest neighbor atoms. EX
in the low lying 5p Rydberg electron is larger than the
PL effect, whereas EX in the 6p Rydberg electron issmaller than PL. Therefore, the energy shift of the 5p
Rydberg state in XAS is blueshifted compared to theatomic one. This is reasonable considering the radiusof the 5p Rydberg orbital (3.64 Å) is close to the vander Waals distance of krypton (4.03 Å); on the other
hand, the 6p orbital (8.13 Å) is distributed beyond thefirst coordination shell.
In conclusion, we have measured EX of the
Rydberg electron bound by singly charged ion at

Fig. 1. XAS spectra of different surface sites inkrypton clusters with the 3d5/2-15p and 3d5/2-16p states.
Table 1. Exchange (EX) energies (in eV) of Rydberg
states at different surface sites of krypton clusters,
together with the energy shift (ǻE) in XAS and theinduced polarization (PL) energy determined by XPS.
The bulk site corresponds to the large clusters [4].
eV
3d5/2-15p
Corner
Edge
Face
Bulk
3d5/2-16p
Corner
Edge
Face
Bulk

ǻE

PL(+1)

EX (+1)

0.05
0.17
0.22
0.40

-0.39
-0.57
-0.79
-1.07

0.44
0.74
1.01
1.47

3.6
6.0
8.2
12

3
5
8
12

-0.22
-0.35
-0.51
-0.65

-0.39
-0.57
-0.79
-1.07

0.17
0.22
0.28
0.42

4.9
6.3
8.0
12

3
5
8
12

CN

[1] T. Hatsui et al., J. Chem. Phys. 123 (2005)
154304.
[2] A. Knop, B. Wassermann and E. Rühl, Phys. Rev.
Lett. 80 (1998) 2302.
[3] M. Nagasaka, T. Hatsui and N. Kosugi, J. Electron
Spectrosc. Relat. Phenom. 166-167 (2008) 16.
[4] M. Tchaplyguine et al., J. Chem. Phys. 127 (2007)
124314.

51

BL4B

Formation of Metastable Fragments around the Cl 2p Ionization
Thresholds of HCl
Y. Hikosaka, T. Kaneyasu and E. Shigemasa
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan
spectrum is the broad peaks around the Cl 2p
thresholds, which are completely absent from the ionyield spectrum. A similar enhancement of the neutral
particle yield is observed around the N 1s ionizationthreshold of N2 [1]. In analogy to the N2 case, we
attribute the enhancements in HCl to the observationof metastable fragments formed via either spectator
Auger decay from high-Rydberg states or byphotoelectron recapture due to the post-collisioninteraction. Coincidence detections of neutral particle
and ions reveal pair formation of H*+Cln+ [3]. It isfound that efficient H* formation around theionization thresholds is related to the high-Rydberg
HCl+ states, which are populated via either spectator
resonant Auger decay or photoelectron recapture. In
addition, H* is revealed to be produced by the
dissociation of the HCl2+ states formed by normal
Auger decay of the Cl 2p core-ionized states [3].

Formation of metastable fragments has recently
been found in the inner-shell region of N2 [1, 2]. The
metastable fragments observed trace the formationsof high-Rydberg N2+ states which dissociate into
N*+N+ pairs. The N2+ states are produced throughresonant Auger decay of inner-shell excited stateslying below the inner-shell threshold [1] and of
multiply excited states lying above the threshold [2].
The N2+ states are populated also in thenear-threshold region through photoelectron recapture
due to the post-collision interaction [1]. Theseprevious works demonstrate that the observation ofmetastable fragments enables us to probe the decay
dynamics of molecular inner-shell excited states [1]and also to perform detailed spectroscopic studies on
the multiply excited states embedded in the
inner-shell ionization continuum [2].
In this work, we have investigated metastableformation in the vicinity of the Cl 2p ionizationthresholds of HCl [3]. The monochromatized SR lightand the target gas beam effusing from a hypodermic
needle crossed each other at a right angle. Two
microchannel plate (MCP) detectors with an active
diameter of 14.5 mm, facing each other across theinteraction region, were placed at pseudo-magic
angles with respect to the electric vector of the lightand at right angles with respect to the gas beam [1].
The acceptance solid angle of each MCP detector was
estimated to be 0.18 sr. Two grids (80% transmissioneach) were mounted in front of each detector, and thegrids facing the interaction region were grounded.
One of the detectors was dedicated to neutral particle
observation and the other to fragment ions: thesecond grid of the neutral particle detector was held at+50 V while that of the fragment-ion detector washeld at +5 V. Both the front plates of the two MCP
stacks were held around -2 kV in order to prevent
detection of electrons. The neutral particle detector
was sensitive to neutral metastable particles with
sufficient internal energy (> ~9 eV), as well as both
VUV and SX photons.
Figures 1(a) and (b) show a fragment ion and aneutral particle yield spectrum measured in thephoton energy range of 203.5 - 211.0 eV. The neutral
particle yields are due to the detection of neutral
metastable fragments, as well as VUV/SX photons.
Here, the metastables of the parent HCl cannot be
detected with our experimental geometry in which theneutral particle detector is placed perpendicular to thegas flow. Blow the Cl 2p ionization thresholds,
Rydberg structures are seen in both the spectra. An
intriguing characteristic of the neutral particle yield(a) IonIntensity (arb. units)

20000

4s

4p

4s

4p

3d 5s 4d 5d

3d 5s 4d 5d

10000
0

(b) Neutral Particle
10000
5000
0
204

205

206

207

208

209

210

211

Photon energy (eV)Fig. 1. (a) Ion yields and, (b) neutral particle yields
in the vicinity of the Cl 2p ionization thresholds of
HCl. The photon energy resolution is set to about 40
meV. The ion yield spectrum was measured with aretardation of +5 V.
[1] Y. Hikosaka, P. Lablanquie and E. Shigemasa, J.
Phys. B: At. Mol. Opt. Phys. 38 (2005) 3597.
[2] Y. Hikosaka, T. Gejo, T. Tamura, K. Honma, Y.
Tamenori and E. Shigemasa, J. Phys. B: At. Mol. Opt.
Phys. 40 (2007) 2091.
[3] Y. Hikosaka, T. Kaneyasu and E. Shigemasa, J.
Korean Phys. Soc. 53 (2008) 3798.

52

BL4B

High-Resolution Multi-Electron Coincidence Spectrometer for Studies ofAtomic and Molecular Auger Processes
Y. Hikosaka and E. Shigemasa
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan
resolution. Figure 2 shows Auger spectra obtained incoincidence with 2p3/2 photoelectrons; thus, Augerlines associated with the 2p1/2 core are removed fromthe ordinary Auger spectrum. While the componentsof the band structures are not resolved on the
spectrum observed without retardation, a 180-eV
retardation clearly separates the individual Augerlines.

When an inner-shell electron in an atom ormolecule is removed, the core-hole state decays via
the Auger transition. The kinetic energy of the Auger
electron is element-specific, and thus Auger electron
spectroscopy is widely used as a powerful analytical
tool in many different fields of research. However,the detailed interpretation of Auger spectra is difficult,even for atoms and small molecules. This is because
inner-shell ionization is concomitant with theexcitation and ionization of valence electrons, and allof these core-hole states contribute to theconventional Auger electron spectra. In theory, it ispossible to filter out structures due to individual
contributions by using the photoelectron-Auger
electron coincidence method, which allows one tocorrelate initial core-hole states with the relevant
Auger decays. In practice, the acceptance solid angleof a conventional electron spectrometer is too small
to allow the recording of such a coincidence spectrumwith sufficient statistics. The experimental limitation
was fully solved by the introduction of the magnetic
bottle electron spectroscopic technique [1], andhighly-efficient coincidence studies using thistechnique have been successfully performed [2-4].
However, since this technique is based ontime-of-flight analysis, the energy resolutions for fastelectrons are often insufficient to observe individual
Auger lines.
While the retardation of electron kinetic energy
improves the energy resolution of time-of-flightanalysis in general, the adequateness is notestablished for the magnetic bottle electron
spectroscopic
techniqueusingastronginhomogeneous magnetic field. In this work, we havetested an electric retardation for a magnetic bottle
electron analyzer. Our analyzer is essentially the sameas that developed by Eland et al. [1], except for the
1.5-m flight path in the present analyzer instead of theoriginal 5-m path. A strong permanent magnet is
located close to the interaction region, and themagnetic field guides electrons through a longsolenoid toward a position sensitive detector.
Electrodes which retard electrons are located aroundthe entrance of the flight tube. The energy resolution
was evaluated by measuring He 1s photoelectrons at
different photon energies.
Figure 1 shows the evolutions of energy resolution(FWHM) with electron energy, which were observedwith a 100-eV retardation or no retardation. It isshown that the retardation improves the energy
resolutions. The coincidence measurement of Ar 2p
Auger lines demonstrates more clearly the improved

no retardation
100 V retardation

FWHM (eV)

10

1

0.1

1

10

100

Electron energy (eV)Fig. 1. Evolutions of energy resolution (FWHM)
with electron energy, observed with a 100-eV
retardation (red dots) and no retardation (black dots).

Coincidence yields (arb. units)

Coincident Ar 2p3/2 Auger spectrum

no retardation
with 180-V retardation

170

180

190

200

210

Auger energy (eV)Fig. 2. Ar 2p Auger spectra obtained in coincidencewith 2p3/2 photoelectrons: with a 180-eV retardation(red) and no retardation (black).
[1] J. H. D. Eland et al., Phys. Rev. Lett. 90 (2003)
053003.
[2] Y. Hikosaka et al., Phys. Rev. Lett. 97 (2006)
053003.
[3] Y. Hikosaka et al., Phys. Rev. Lett. 98 (2007)
183002.
[4] Y. Hikosaka et al., Phys. Rev. Lett. 102 (2009)013002.

53

BL4B

Dissociation Pathways of C2D22+ Studied by an Auger-Electron−Ion
Coincidence Method

E. Shigemasa1, T. Kaneyasu1, 2, Y. Hikosaka1, M. Fushitani3 and A. Hishikawa3
1
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan
2
Kyushu Synchrotron Light Research Center, Tosu 841-0005, Japan
3
Dept. of Photo-Molecular Science, Institute for Molecular Science, Okazaki 444-8585, Japan
states with the 1πu-2 configurations. The second
lowest peak in the 1πu-2 band lies at 34.7 eV, where
C2D+ and C2+ fragments are produced. The CD2+
fragments related to the V-channel, as well as theatomic fragments C+ and D+ yield the highest energy
peak in the 1πu-2 band at 35.0 eV. The second bandaround 40 eV in the Auger spectrum seems to becomposed of at least three components: 38.5 eV for
C2D+ and D+, 40.6 eV for CD+ and D+, and 42.4 eV for
C2+. The highest binding energy band centered at 50
eV enhances violent fragmentation leading to the
productions of C+, D+, CD+, and C2+.

The acetylene dication is one of the smallest
metastable polyatomic dications, whose dissociationand isomarization from the acetylene (HCCH) into thevinylidene (H2CC) configurations have been
extensively investigated. By using the PEPIPICO
technique, it was found that the acetylene dications
dissociate from excited states above 34 eV, where five
three-body reactions and three two-body reactions
including the dissociation channel via the vinylidene
form (CH2+ + C+: V-channel) have been identified.
The isomarization time of the acetylene decation
produced following the Auger decay was estimated toproceed within 60 fs [1]. Very lately, the visualizationof ultrafast hydrogen migration in deuterated
acetylene dication, which occurs in a recurrent manner,
was nicely demonstrated by using intense ultrashortlaser pulses [2].
An attempt to identify the fragmentation patterns for
specific electronic states of the acetylene dication wasmade more recently, where the momenta of two
positively charged ions were measured in coincidencewith the Auger electrons [3]. In addition to the
V-channel, the other two channels in the two-bodyreactions, such as the acetylene channel (CH+ + CH+:A-channel) and deprotonation channels (C2H+ + H+:
P-channel), were correlated with some Auger final
states. In order to gain a further insight into thedissociation mechanism of the Auger final states, wehave performed an Auger-electron−ion coincidence
study on fragmentations of deuterated acetylene
dication, C2D22+, formed via Auger decay. With theuse of our Auger-electron−ion coincidence
spectrometer [4], we have identified Auger final states
relevant to the individual fragmentations including thethree-body reaction, as well as to the formation ofmetastable C2D22+.
Figure 1 shows the coincidence Auger spectra
extracted from the coincidence data sets, with the
maximum intensity in each spectrum normalized tounity. It is found that the atomic fragment ions tend
to be produced in the entire region of Auger electrons,
while the molecular ions except for CD+ exhibit
specific productions for certain Auger final states.
The parent dications are mainly observed at the lowest
band, which shows a clear maximum around 33.3 eV.
The shaded area in the C2D22+ spectrum displays theCD+ spectrum multiplied by an appropriate factor.
We come to the conclusion that the parent dications
are essentially produced only from the lowest energy

Fig. 1. KVV Auger spectrum of C2D2 (pink), incomparison with coincidence Auger spectra related tomolecular ions (blue), and atomic ion formation (red).
[1] T. Osipov et al., Phys. Rev. Lett. 90 (2003)
233002.
[2] A. Hishikawa, A. Matsuda, M. Fushitani and E.
Takahashi, Phys. Rev. Lett. 99 (2007) 258302.
[3] T. Osipov et al., J. Phys. B 41 (2008) 091001.
[4] T. Kaneyasu, Y. Hikosaka and E. Shigemasa, J.
Electron Spectrosc. Relat. Phenom. 156-158 (2007)
279.

54