Journal of Physics B: Atomic, Molecular and Optical Physics
J. Phys. B: At. Mol. Opt. Phys. 48 (2015) 125101 (6pp)

doi:10.1088/0953-4075/48/12/125101

Site-speciﬁc formation of metastable OCS2+
studied by Auger-electron-ion coincidence
methodT Kaneyasu1,3, M Ito2, K Soejima2, Y Hikosaka1,4 and E Shigemasa1
1
2

UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan
Department of Environmental Science, Niigata University, Niigata 950-2181, Japan

E-mail: kaneyasu@saga-ls.jp
Received 30 March 2015, revised 21 April 2015
Accepted for publication 28 April 2015
Published 19 May 2015
Abstract

We have studied the site-speciﬁc formation of metastable OCS2+ in the C1s and the O1s Auger
decays of OCS molecules by using an Auger-electron–ion coincidence technique. The
coincidence measurement reveals the metastable character of the Auger ﬁnal states with the(3π)−2 conﬁguration. The formation of metastable OCS2+ is observed negligibly in the C1sAuger decay and only weakly in the O1s Auger decay, in contrast to the favorable metastableformation previously reported for S2p Auger decay. It is found that the origin of the sitespeciﬁcity in the formation of metastable OCS2+ arises from the localization effect of the valenceorbital involved in the Auger transition. The dissociative dication states related to individual
fragmentation pathways are also derived from the coincidence measurement.
Keywords: metastable dication, Auger decay, electron–ion-coincidence technique(Some ﬁgures may appear in colour only in the online journal)
1. Introduction

photoelectron coincidence (TOF-PEPECO) [4], both of which
can provide dication spectra with vibrational resolution. In
particular, the TOF-PEPECO method has been successfully
applied to studies of the valence double photoionization ofvarious molecules, and the basic properties of the dication
states have been clariﬁed [5].
Besides valence double ionization, molecular dications
with two holes in valence orbitals are efﬁciently produced
through Auger decay following core-hole creation in molecules composed of light atoms. The molecular dications thus
formed either show metastable character or rapidly dissociate
into fragments, depending on the properties of their potential
surfaces. Auger electron spectroscopy can provide information on the stability and dissociation properties of the molecular dications [6, 7]. However it is usually difﬁcult to derive
detailed dication dynamics from molecular Auger spectra, due
to the many overlapping Auger transitions, most of which
produce dissociative dication states leading to broad spectral
shapes. One promising method for disentangling these complex molecular Auger spectra, and determining the stability
and dissociation pathways of the dications, is the coincidence

While doubly-charged molecular ions (dications) are inherently unstable due to the Coulomb repulsion between the
nuclei, the existence of metastable molecular dications having
lifetimes of the order of at least microseconds has been known
from mass spectroscopic studies since the 1930s [1]. The
stability and the fragmentation of molecular dications havelong been subjects of study in the research ﬁeld of atomic andmolecular physics, and a number of experimental and theoretical works have been carried out [2]. However, details of
the electronic structures of molecular dications have been
largely unknown until recently because of experimental difﬁculties in studying the inherently unstable species. Experimental breakthroughs for precise spectroscopic studies onmolecular dications were made through efﬁcient electron–
electron coincidence techniques such as threshold photoelectron coincidence [3] and time-of-ﬂight photoelectron–
3

Present address: SAGA Light Source, Tosu 841-0005, Japan
Present address: Graduate School of Medicine and Pharmaceutical
Sciences, University of Toyama, Toyama 930-0194, Japan

40953-4075/15/125101+06$33.00

1© 2015 IOP Publishing Ltd Printed in the UK

J. Phys. B: At. Mol. Opt. Phys. 48 (2015) 125101

T Kaneyasu et alobservation of energy-selected Auger electrons and productions [8].
Another important property of molecular Auger decay is
the site-speciﬁcity in the electronic relaxation process relevantto the localization character of the initial core-hole [9, 10].
With the localized positive charges being preserved in theAuger decay, the subsequent molecular fragmentation often
takes place accompanying bond-breaking near the atomic siteat which the initial core-hole was created. The Auger-electron–ion coincidence technique has also been applied forstudying such site-speciﬁc phenomena in detail [11–13]. It isnoteworthy that the site-speciﬁc formation of the metastabledication following the C1s and O1s Auger decays of CO2 hasbeen revealed recently by an Auger-electron–ion coincidence
method [14].
In this paper, we report a study of the stability andfragmentation of OCS2+ formed via Auger decay of OCS
molecules using the Auger-electron–ion coincidence technique. The dication spectra of low-lying OCS2+ states havepreviously been measured by TOF-PEPECO with vibrational
resolution [15]. The overall features of the Auger decays,
S2p, C1s and O1s Auger spectra, and assignments of thespectral features based on a semi-empirical calculation havealso been reported [16]. The effect of the nuclear motion on
the Auger transitions has been theoretically studied [17].
Recently, a high-resolution S2p Auger spectrum, andassignments to metastable states on the basis of a theoretical
calculation have been reported [7]. Meanwhile, Auger-electron–ion coincidence experiments have been performed only
for S2p core-hole creation, and reported in our previous work[18], and later in [19].
In the present work we focus on the Auger decays of
OCS following C1s and O1s ionization, in order to elucidate
the inﬂuence of localized core-holes on the Auger transitions.
It is found that the metastable ﬁnal states are less favored in
the Auger transitions subsequent to C1s and O1s ionization,in sharp contrast to the intense production of metastabledications for S2p Auger decay [18–20]. The site-speciﬁcformation of metastable OCS2+ is discussed by combining ourprevious result for the S2p Auger decay. The present result
also clariﬁes the dissociative dication states which are related
to the individual fragmentation processes.

light were analyzed in energy by the DTA. The pass energy of
the DTA was set to Epass = 240 eV. The resolving power ofthe DTA was estimated to be better than Epass/ΔE = 100.
Following the detection of an electron, fragment ions wereextracted from the ionization region into the momentum
spectrometer by a pulsed electric ﬁeld. In this study, thepulsed electric ﬁeld was set to ensure a 4π-collection solid
angle for fragment ions having kinetic energies less than 5 eV.
The electron arrival positions and ion arrival positions andtimes-of-ﬂight (TOFs) were recorded in list format. The PSD
for ions was operated in a multi-hit mode to facilitate theidentiﬁcation of dissociation pathways.
Coincidence data sets were recorded at photon energies
of 306 eV and 550 eV for C1s and O1s ionization, respectively. To cover a binding energy range of 25–50 eV in theAuger electron observations, we performed two sets ofmeasurements with different settings of the kinetic energy
window of the DTA for each core ionization. Auger electron
spectra related to the formation of metastable dications andindividual fragmentation pathways were extracted from thecoincidence data sets in an off-line analysis in which false
coincidence events were carefully subtracted from the raw
data. False coincidence rates were estimated from separate
measurements in which the pulsed extraction ﬁeld was triggered by signals from a pulse generator, independent ofelectron detection.

3. Results and discussion
3.1. Conventional Auger spectra

Figures 1(a) and (b) show conventional Auger spectra of OCS
following C1s and O1s ionization, respectively. These spectra
were derived without discrimination by detected ion species
from the coincidence data sets. The Auger spectra are plottedon binding energy scales calculated by subtracting the Auger
electron energies from the C1s−1 and O1s−1 ionization energies of 295.43 and 540.28 eV [16], respectively. The kinetic
energy scales are also shown. The spectra exhibit several
broad structures resulting from the overlaps of many transitions. We have attached labels to the band structures
according to [17]. Although some of the bands, C1, C2, O3,
O4, O5 and O6, are not fully resolved in the present study, themeasured spectral proﬁle essentially agrees with the previous
measurements [16]. The spectral features differ quite signiﬁcantly from those in the S2p Auger spectrum, which
exhibits intense narrow peaks in the binding energy region
below 40 eV [16].
The band structures in the Auger spectra can be assigned
in detail from the general agreement with the theoretical
Auger energies [17]. The calculated energies of the Auger
ﬁnal states with large transition intensities in the C1s and O1s
Auger decays are indicated by the vertical bars in ﬁgure 1. For
the energies indicated, the energy shifts due to nuclear
dynamics are taken into account. Note that the valence electronic conﬁguration of the OCS molecule is described as(6σ)2(7σ)2(8σ)2(9σ)2(2π)4(3π)4. A weak band structure

2. Experiment
The experiment was performed at the bending magnet
beamline BL4B of the UVSOR facility. Synchrotron radiationfrom the bending magnet was monochromatized to a bandwidth of 0.5 eV by a grazing incidence monochromator. The
photon energy scale was calibrated through total ion yieldmeasurements of the inner-shell resonances in OCS. Detailsof the electron–ion coincidence spectrometer are given in[21, 22]. Brieﬂy, it consists of a double toroidal electronenergy analyzer (DTA) [23] and an ion momentum spectrometer, both of which are equipped with time- and positionsensitive detectors (PSDs). The electrons emitted at an angleof 54.7° with respect to the electric ﬁeld vector of the ionizing2

J. Phys. B: At. Mol. Opt. Phys. 48 (2015) 125101

T Kaneyasu et al

Figure 1. (a) C1s and (b) O1s conventional Auger spectra derived

Figure 2. (a) One-dimensional TOF spectrum of the product ionsfrom the total events of the coincidence data. Vertical bars indicate
the calculated energy positions of the Auger ﬁnal states with largetransition intensities in the C1s and O1s Auger decays [17]. The
labels attached in the Auger spectra are adopted from [17].

measured in coincidence with C1s Auger electrons. (b) Twodimensional TOF map representing the formations of ion-pairs
following C1s Auger decay. Projections of the two-dimensional map
are plotted in the bottom and right panels, representing the summed
TOF spectra for the ﬁrst-hit and second-hit ions. Note that the weak
TOF peaks observed between 3600 and 4000 ns are due toexperimental artifacts.

ranging from 30 to 35 eV is observed only in the O1s spectrum. This lowest band O1 is assigned to the formation of(3π)−2 states. In the binding energy region of 35–38 eV,
intense band structures are discernible in both spectra. These
structures are attributed to the dication states characterized by
two-hole conﬁgurations; one hole in the 3π orbital and theother hole in the 2π, 9σ, or 8σ orbital. A large number of
dication states exist above 38 eV, where both spectra exhibit
the most intense band structure; these spectral features are dueto the complex overlaps of transitions involving the 7σ, 8σ,
9σ, 2π and 3π orbitals.

metastable OCS2+, while the two-dimensional TOF map canbe used to determine the TOF ranges necessary to selectindividual ion-pair fragmentation routes. Note that false
coincidence events are included in the data plotted in ﬁgure 2.
Prior to discussion of the coincidence Auger spectra, we
ﬁrst describe the dissociation mechanisms of OCS2+ which
can be identiﬁed from the two-dimensional map. While false
coincidence events appear as horizontal and vertical stripes on
the two-dimensional map, true coincidences between ion pairs
appear as diagonal island structures. The two-body fragmentations into CO+ + S+, O+ + CS+ and CO+ + S2+ are
observed as diagonal structures with slopes of −1, since thetwo fragments are emitted in opposite directions with thesame momenta. The detection of CO+ + S2+ events resulting
from the dissociation of triply charged OCS3+ can be assumed
to be due to false coincidences, since the detection window of
the Auger electron energy used here is much lower than the
triple ionization threshold of OCS at 60 eV [24]. Three-bodyfragmentations into O + C+ + S+, O+ + C + S+ and O+ + C+ + S
can be identiﬁed as diagonal structures with slopes steeper
than −1, indicating that sequential processes play a role in thethree-body fragmentation of OCS2+ [25, 26].

3.2. Coincidence Auger spectra

To extract Auger spectra ﬁltered by the coincident detectionof metastable dications and other individual fragmentation
signatures, which we refer to as coincidence Auger spectra,
we discriminate the coincidence events using the ion TOF
information. As an example, a one-dimensional TOF spectrum of product ions and a two-dimensional map representing
the TOFs of the ﬁrst-hit and second-hit ions on the detector
for C1s Auger decays are shown in ﬁgures 2(a) and (b),respectively. The one-dimensional TOF spectrum can be used
to determine the ion TOF range necessary to extract thecoincidence Auger spectrum related to the formation of3

J. Phys. B: At. Mol. Opt. Phys. 48 (2015) 125101

T Kaneyasu et al

Figure 3. Conventional and coincidence Auger spectra of OCS subsequent to (a) C1s and (b) O1s ionization. The coincidence spectra are
extracted from the coincidence data sets by ﬁltering the Auger electron signals related to the formation of metastable OCS2+ and individual
fragmentation pathways. The intensities of the coincidence spectra are normalized to the corresponding conventional spectrum.

The coincidence Auger spectra for metastable OCS2+ andfor individual fragmentation pairs are shown in ﬁgures 3(a)and (b) for C1s and O1s ionization, respectively. The coincidence Auger spectra for three-body fragmentation toO+ + C+ + S are not presented, since the count rates were too
low to reveal any reliable structure in the derived spectra. The
coincidence Auger spectra are normalized to the conventional
Auger spectra shown in the top panel of each ﬁgure, and thus
the spectral intensities directly reﬂect the decay branchingratios.
For both C1s and O1s Auger decay, the coincidence
observation reveals the metastable character of the Auger ﬁnal
states lying in the range 30–35 eV, which are attributed to
dication states with two holes in the non-bonding 3π orbital.
In contrast to the intense production of metastable OCS2+ in
S2p Auger decay [18], the metastable ﬁnal states are lessfavored in the C1s and O1s Auger decays. This site-speciﬁcformation of metastable OCS2+ is discussed in the next
section.
The present observation conﬁrms the dissociative character of the dication states identiﬁed in the Auger-electron–

ion coincidence experiment for S2p Auger decay [18]. In
general, two-body fragmentation to CO+ + S+ dominates thedissociation of the dication states lying in the binding energy
range 35–45 eV, while the high-lying dication states above
45 eV mainly lead to three-body fragmentation. We here note
only the characteristics of the dissociative dication statesfound in the C1s and O1s Auger decays. For two-bodyfragmentation to CO+ + S+, a small component is observed
around 33 eV only in the O1s Auger decay. This weak band
structure is peaked at slightly higher binding energy than that
related to the metastable dication. Thus this structure canpossibly be attributed to Auger decays into vibrationallyexcited (3π)−2 states which pre-dissociate into CO+ + S+ witha lifetime shorter than the order of microseconds. It is clearthat the O+ + CS+ channel is correlated with the band structures lying above 38 eV for both C1s and O1s Auger decays.
In contrast O+ + CS+ fragmentation is hardly discernible in
S2p Auger decay [18] because of the weak population of the
dication states above 38 eV. The structures correspond to theoverlap of many Auger ﬁnal states having two holes in the 7σ,
8σ, 9σ, 2π and 3π valence orbitals. The centers of the band
4

J. Phys. B: At. Mol. Opt. Phys. 48 (2015) 125101

T Kaneyasu et al−1
Auger decays from the S2p−1
3/2 and 2p1/2 core-hole states(1.2 eV splitting) are overlapped in the Auger spectra. Thus,
the metastable (3π)−2 structures in the S2p spectra areobserved at slightly higher energy than those for the C1s andO1s Auger decays. While the metastable state is barely
observed above 36 eV in the C1s and O1S coincidence Auger
spectra, the S2p coincidence Auger spectrum shows a sizable
intensity in this range. These structures are possibly due to theformation of the bound (8σ)−1(3π)−1 and (2π)−1(3π)−1 states
identiﬁed in the high-resolution Auger spectrum [7]. The
weak structure around 35 eV, and part of the peak structure at
36 eV are attributable to an imperfection in the false coincidence subtraction, since there are no metastable states in thisenergy region.
Here we concentrate on the formation of metastabledication states with two holes at the non-bonding 3π orbital.
The intensity of the metastable dication state in the S2p Auger
decay is much higher than those in the C1s and O1s Auger
decays (see ﬁgure 4). This site-speciﬁc formation of themetastable dications can be qualitatively explained by thespatial distribution of the valence orbital involved in theAuger decay processes. The localization character of the 3π
orbital can be quantitatively described in terms of the Mulliken population analysis, and the orbital populations havebeen calculated to be 0.64, 0.32 and 3.04 for around O, C andS atoms, respectively [16]. Due to the localization of the 3π
valence orbital around the S atom, the valence orbital should
show a favorable overlap with the S2p core-hole and thus theS2p core-hole can be preferably ﬁlled by a 3π electron. Next
to the exclusive population around the S atom, the 3π orbitalis weakly localized around the O atom. Thus a small amountof the metastable dications is observed in the O1s Auger
decay while a nearly negligible yield of the metastable dications is produced in the C1s Auger decay. The present
observation conﬁrms the interpretations for the localization
effect of valence orbitals on the Auger transitions in OCS[16, 17]. We expect that such site-speciﬁc phenomena emerge
as a general property of the Auger decays involving localized
and non-bonding valence orbitals in molecules. Indeed asimilar site-speciﬁc production of metastable dications has
been observed also in the Auger decays of CO2 molecules [14].

Figure 4. Conventional (dashed red lines) and coincidence (solid

blue lines) Auger spectra of OCS following C1s and O1s ionization,compared with the corresponding spectra for the S2p ionization [18].
The coincidence spectra are obtained by ﬁltering with the detectionof metastable OCS2+. The energy resolution for the C1s and O1s
Auger spectra is estimated to be ΔE ∼ 2.4 eV, and that for the S2p
Auger spectrum ΔE ∼ 1 eV. Note that the binding energy scale for
the S2p Auger spectrum is calculated with respect to the ionizationenergy of the S2p−1
1/2 core-hole state. The scale is correct for theAuger transitions from the S2p−1
1/2 state but is shifted by 1.2 eV for
those from the S2p−1
3/2 state.

structures are close to the appearance potential of 42.5 eV in
the valence double ionization [27]. The OCS2+ states related
to three-body fragmentations to O + C+ + S+ and O+ + C + S+
show broad structures above 35 eV, and most three-bodyfragmentations originate from dication states lying
above 40 eV.

4. Summary
We have studied the site-speciﬁc formation of metastable
OCS2+ in the C1s and O1s Auger decays of OCS moleculesby using the Auger-electron–ion coincidence technique. The
coincidence measurement reveals the metastable character ofthe Auger ﬁnal states with a (3π)−2 conﬁguration, and thecorrelations of the dissociative dication states with individual
fragmentation pathways. The metastable ﬁnal states produced
following C1s and O1s ionization are populated with much
lower probabilities compared to the observation for the S2p
ionization. The origin of the site-speciﬁcity in the metastableproduction can be understood in terms of the localization
effect of the valence orbitals involved in the Auger transition.

3.3. Site-specific formation of metastable OCS2+

Next we discuss the mechanism of the site-speciﬁcity in the
production of the metastable dications. Figure 4 shows themetastable OCS2+ coincidence Auger spectra and conventional Auger spectra following C1s and O1s ionization, alongwith the corresponding S2p Auger spectra from our previous
study [18]. As mentioned above, the metastable character ofthe dication states is allocated to the (3π)−2 states lying in thebinding energy region of 30–35 eV. Note that the binding
energy scale for the S2p spectra is calculated with respect tothe ionization energy of the 2p−1
1/2 core-hole state, while the5

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We are grateful to the UVSOR staff for the stable operation of
the storage ring. The authors would like to thank Dr J R
Harries for critical reading of the manuscript. This work waspartially supported by the Japan Society for Promotion of
Science, Grant-in-Aid for Scientiﬁc Research.

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