Chemical Physics Letters 603 (2014) 46–50

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Site-speciﬁc formation of metastable dications following inner-shellionization of CO2
Y. Hikosaka a,⇑, Y. Shibata a, K. Soejima a, H. Iwayama b, E. Shigemasa bab

Department of Environmental Science, Niigata University, Niigata 950-2181, JapanUVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japana r t i c l e

i n f oArticle history:
Received 20 March 2014
In ﬁnal form 17 April 2014
Available online 24 April 2014a b s t r a c t
An Auger-electron–ion coincidence method is applied to study the site-speciﬁc formation of metastable
CO2+
2 in the C1s and O1s Auger decays of CO2. It is found that only dication states with the electronic conﬁguration of 1pg 2 are relevant to the metastable formation. These dication states are favorably producedin the O1s Auger decay, because of the charge distribution of the 1pg orbital localizing at the O site.
Fragmentation channels of dissociative CO2+
2 states are also investigated.
Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction
Inner-shell excitation/ionization in a molecule composed oflight atoms is usually followed by Auger decay, which dominantly
produces the molecular ion with holes in the valence shell. The
core hole generated by the initial inner-shell excitation/ionization
is localized at an atomic site, and then the localization should be tosome extent preserved in the Auger ﬁnal states. This view has led
to the idea of site-speciﬁc fragmentation [1], or selective bondbreaking around the speciﬁc atomic site where the initial core–hole is created. Site-speciﬁc fragmentation has been widely
observed not only in gas-phase molecules [2,3] but also inadsorbed or condensed molecules [4].
The ion yield spectra in the C1s and O1s inner-shell ranges of
CO2 [5] show that the increase of the metastable CO2+
2 yield is more
remarkable above the O1s threshold than above the C1s threshold.
This observation indicates a unique site-speciﬁc phenomenon thatmetastable dication formation, not fragmentation, presents sitespeciﬁcity. The site-speciﬁcity in the metastable dication formation is conﬁrmed by the present investigation. Figure 1 shows
time-of-ﬂight spectra of ions observed in coincidence with C1s or
O1s photoelectrons, which were obtained with the electron–ion
coincidence spectrometer described in Section 2. While the difference in the intensities of each fragment ion is at most 40% between
the two spectra, the metastable CO2+
2 yields associated with theO1s ionization are about three times larger than those with the
C1s ionization.
In this Letter, we have studied the mechanism of the site-speciﬁc formation of metastable CO2+
2 following inner-shell ionization⇑ Corresponding author. Fax: +81 25 262 6147.
E-mail address: hikosaka@env.sc.niigata-u.ac.jp (Y. Hikosaka).
http://dx.doi.org/10.1016/j.cplett.2014.04.030
0009-2614/Ó 2014 Elsevier B.V. All rights reserved.

of CO2, by using an Auger-electron–ion coincidence method. We
have identiﬁed Auger ﬁnal states associated with the formationof metastable CO2+
2 , as well as those with individual fragmentation
channels. Only CO2+
2 states with two holes at the 1pg orbital, whichis the highest-occupied valence orbital in the neutral ground CO2,
are observed as metastable dications. These dication states arefavorably formed by the O1s Auger decay, because of the large
charge distribution of the 1pg orbital at the O site.
2. Experimental
The experiment was performed at the undulator beamline BL6U
in UVSOR. The synchrotron radiation was monochromatized by agrazing incidence monochromator using a varied-line-spacing
plane grating. The detailed description of the electron–ion coincidence spectrometer used in the present work was already given
elsewhere [6,7]. Brieﬂy, the coincidence spectrometer consists ofa double toroidal electron analyzer and a three-dimensional ion
momentum spectrometer, each of which is equipped with a timeand position-sensitive delay line detector (Roentdek DLD40). It is
worth mentioning that dissociation of singly-charged CO2 ionstates produced by resonant Auger decay of C1s excited states in
CO2 has been already investigated by the same kind of method
as in the present study [8].
Electrons ejected at 54.7° with respect to the electric vector of
the incident light were sampled (the acceptance solid angle is 5% of
4p sr) and then were dispersed in energy by toroidal deﬂectors.
Through the present measurements, the pass energy of the analyzer was set to 200 eV, and electrons within an energy range corresponding to 15% of the pass energy were observed
simultaneously. The electron transmission efﬁciency as a functionof electron kinetic energy was determined by observing Ar 3p

47

Y. Hikosaka et al. / Chemical Physics Letters 603 (2014) 46–50

0.0020C+

O+

0.0015Intensity (arb. units)

Binding energy of dication state (eV)O1s ionization
C1s ionization

80
1.0

CO22+

CO+

0.0010

70

60

50

(a)
C5

0.5

B2
B3

C2 B8B6

0.0005CO
0.00001.0-0.0005
1600

30

B1

0.0

2+

1400

40

B5 C1s Auger

1800

2000

2200

(b)

CO++O+(c)

C++O++O(d)

O++O++C(e)

CO2++O

0.5

2400

Time-of-flight of ion (ns)photoelectron peaks at different photon energies. It was estimatedthat the electron energy resolution with the pass energy of 200 eV
was around 3 eV (FWHM).
According to each electron detection, a pulsed electric ﬁeld of
270 V/cm was applied to the interaction region, in order to extract
the associated ions to the ion momentum spectrometer. The pulsed
electric ﬁeld ensured a 4p collection solid angle for ions withkinetic energies of less than 7 eV. In Figure 1, the O+ peaks exhibit
hollow shapes, which is probably due to escapes of higher kinetic
energy ions whose emission directions are perpendicular to theﬂight tube. The ion observation allows multi-hit detection, whichenables us to identify fragmentation channels from Auger ﬁnal
states. The detection efﬁciency of the ion detector was estimatedto be 43% by comparing coincidence yields for ion-pair CO+ + O+
and that for single-hit CO+, both of which are produced from Auger
ﬁnal CO2+2 . Contributions from false coincidences were evaluated tobe negligible in the present measurements, by a separate coincidence measurement where the pulsed electric ﬁeld for observing
ions was triggered independent of electron detection.
Auger-electron–ion coincidence datasets were accumulated forCO2 at photon energies of 403 eV (for C1s Auger decay) and 643 eV(for O1s Auger decay). At each photon energy, coincidence datasets
were obtained with three different settings to kinetic energy window: one for observing inner-shell photoelectrons and the othertwo to cover the whole Auger spectral ranges.
3. Results and discussion
Figures 2a and 3a show C1s and O1s Auger spectra of CO2,
respectively, which were obtained from the total events in the
coincidence datasets. Each spectrum was made by combining thespectra with two different kinetic-energy windows. These Auger
spectra exhibit several band structures corresponding to the formation of different CO2+
2 states. The spectral features essentiallyagree with the previous measurements [9–11], though the vibrational structures of low-lying CO2+
2 states [11] cannot be resolved
with the present resolution. In these ﬁgures, the same labels asin [9,12] are adopted to the band structures, and the assignmentsof the electronic conﬁgurations of these bands are presented inTable I of [12]. Some of the band structures are common in thetwo spectra, but the relative intensities are largely different
between them. The lowest band (B1) around a binding energy of
38 eV is assigned to the formation of CO2+
2 states with two holes
at the 1pg orbital [12]. Different combinations of two valence holes

Intensity (arb. units)

0.0
Figure 1. Time-of-ﬂight spectra of ions observed in coincidence with C1s and O1s
photoelectrons, which were observed at the photon energies of 403 eV and 643 eV,
respectively. Intensities are normalized to the photoelectron counts. Signals in thetime-of-ﬂight range below 1400 ns were masked, to veto large noises resulting fromthe pulsed electric ﬁeld for the ion extraction; the C2+ and O2+ peaks could not be
observed accordingly.

0.2
0.1
0.0
0.04

0.02

0.00
0.010

0.005

0.000
220

230

240

250

260

270

Auger electron energy (eV)Figure 2. (a) C1s Auger spectrum of CO2 derived from total events, and thoseﬁltered by coincidences with (b) CO+ + O+, (c) C+ + O+, (d) O+ + O+ and (e) CO2+. The
intensities of the coincidence Auger spectra are corrected by assuming the 43%
detection efﬁciency for each single ion. The intensity of the O+ + O+ spectrum isunderestimated, because of the detection dead time. The labels of the band
structures, indicated in (a), are adopted from [9,12], and the assignments of theirelectronic conﬁgurations are presented in Table I of [12].

in the four outer-valence orbitals lead the band structures in thebinding energy of 40–65 eV [12]. Peaks above 65 eV are attributed
to states with a hole in the inner-valence orbitals [12]. Above thetriple ionization threshold at 74 eV [13], a small contribution fromdouble Auger decay forming triply-charged CO2 ion should be
included in the Auger yields. Note that the valence electronic
conﬁguration of the neutral ground state in CO2 is (3rg)2(2ru)2(4rg)2(3ru)2(1pu)4(1pg)4, where the 3rg and 2ru orbitals are categorized to inner-valence orbital [14].
Prior to the investigation of metastable CO2+
2 formation, we pay
attention to the dissociation channels of the CO2+
2 states. When thetwo positive charges of Auger ﬁnal CO2+
2 states are localized in oneof the fragments, the dissociation channel is observed as the coincidence between an Auger electron and a single dication fragment.
Figure 4a shows a time-of-ﬂight spectrum of single ions detectedin coincidence with Auger electrons, which were extracted fromthe coincidence dataset accumulated for the C1s Auger decay withthe kinetic energy window of 245–275 eV. Although possible dication fragments are C2+, O2+, CO2+ and O2+
2 , only a faint structure
attributable to CO2+ is discernible in the time-of-ﬂight spectrum.
The O2+
2 fragments cannot be identiﬁed due to the overlap with

48

Y. Hikosaka et al. / Chemical Physics Letters 603 (2014) 46–50

Binding energy of dication state (eV)

1.0

70

60

50

(a)

B6

C4

O+

30

B5 O1s Auger
B3

(a)

0.5

40

Counts

80

B1

400

C+

CO+
CO22+

200

CO2+
0

0.0

(b)

CO +O

+

0.5

(b)

0.2

C++O++O

(c)

0.1
0.0
0.04

+

(d)

+

O +O +C

0.02

0.00
0.010

1800

2000

2200

2400

400

O+

C+

200

CO+

0

CO2++O(e)

2400

Time-of-flight of second ion (ns)Intensity (arb. units)

0.0

1600

Time-of-flight of ion (ns)Counts

1.0

1400
+

CO+
2200

2000

256

1800

64
16

O+

1600

4
1400

1

0.005
1400

0.000
470

480

490

500

510

Auger electron energy (eV)Figure 3. Same as Figure 2, for the O1s Auger decay of CO2.

the O+ peak. The atomic dication fragments of C2+ and O2+ could not
be observed in the present measurement, because the signals in
the time-of-ﬂight range below 1400 ns were masked in order toveto large noises resulting from the pulsed electric ﬁeld for theion extraction. Note that the C+, O+ and CO+ peaks in the time-ofﬂight spectrum result from ion pair events in which one of the ions
are missed in the detection.
Meanwhile, the dissociation channels forming fragment ion
pairs should be identiﬁed as coincidences between an Auger electron and two fragment ions. Figure 4b shows a two-dimensional
map for the correlations between two ions detected in coincidencewith an Auger electron, extracted from the same coincidence dataset as for Figure 4a. On the two-dimensional map, clear structures
for coincidences between fragment ion pairs of CO+ + O+, C+ + O+
and O+ + O+ can be identiﬁed, while any reasonable structure forthe remaining pair of O+2 + C+, whose formation needs a large conformation change, is indiscernible. The CO+ + O+ structure on thismap shows a clear ridge with gradient of 1. This feature is characteristic of the coincidences between the two fragment ions fromtwo-body dissociation in which the fragments are emitted into the
opposite direction with an equal momentum. A weak diagonal tailcan be found at the lower right-hand side of the CO+ + O+ structure,
which continues to (x, y)  (2000, 2000). The coincidences in the
tail part are due to the dissociation of metastable CO2+
2 and the formation of the CO+ + O+ pair in the drift region [15–20]. Thus, therelevant CO2+
states have lifetimes in the same order as the2

2200

2400 0 200 400

Time-of-flight of first ion (ns)

1600

1800

2000

CountsFigure 4. (a) Time-of-ﬂight spectrum of single ions detected in coincidence with
Auger electrons, and (b) two-dimensional map for times-of-ﬂight of ion-pairsmeasured in coincidence with Auger electrons. They were derived from thecoincidence dataset obtained for the C1s Auger decay with the kinetic energy
window of 245–275 eV which corresponds roughly to the right-half range of theAuger spectrum in Figure 2a. The C+, O+ and CO+ peaks in the time-of-ﬂight
spectrum in (a) result from ion pair events in which one of the ions are missed in
the detection. On the map in (b), coincidence counts are plotted as a function ofboth time-of-ﬂight of ﬁrst ion (horizontal axis) and that of second ion (vertical axis),
and are presented on a logarithmic scale. The time-of-ﬂight spectra of the ﬁrst andsecond ions, which are the projections of the coincidence counts on the map onto
the horizontal and the vertical axes, are shown in the top and right panels,
respectively.

++
time-of-ﬂight of CO2+2 (2 ls in the present case). The C + O structure also exhibits a diagonal feature, but the slope is steeper than
1. This feature in coincidences between fragments from themolecular center and one end is peculiar of two-step dissociation
+
+
+
+
+
+
[21,22]: CO2+
2 ? CO + O ? C + O + O . The feature of the O + O
structure cannot be discussed, because a part of the structure is
missing due to the detection dead time of 30 ns.
Auger electron spectra ﬁltered by coincidences with CO+ + O+,
C+ + O+, O+ + O+ and CO2+ are shown in Figures 2 and 3 for theC1s and O1s Auger decays, respectively. The intensities of the coincidence Auger spectra are corrected by assuming the 43% detection
efﬁciency for each single ion, and thus the relative intensities ofeach band in the spectra directly reﬂect the branching ratio forthe different fragmentation channels. Note that, because of thedetection dead time, the intensity of the O+ + O+ spectrum should
be underestimated.
One sees in Figures 2 and 3 that, while the total intensities ofindividual fragment channels are similar in the C1s and O1s Auger
decays, the contributions from the individual dication states are

Y. Hikosaka et al. / Chemical Physics Letters 603 (2014) 46–50

Binding energy of dication state (eV)
70
60
50
40
30

Intensity (arb. units)

80

B5

(a)

1.0

C5

0.5

C1s Auger

B2
B3

C2 B8B6

B1

0.0
220

Intensity (arb. units)

270

Binding energy of dication state (eV)
70
60
50
40
30

80
1.0

230
240
250
260
Auger electron energy (eV)

B5

(b)

O1s Auger
B3

B6
0.5

C4

B1

0.0
470

480
490
500
510
Auger electron energy (eV)Figure 5. (a) C1s and (b) O1s Auger spectra (red) of CO2 ﬁltered by coincidenceswith metastable CO2+
2 , compared with total Auger spectra (black) which are the replots of the spectra in Figure 2a and b. (For interpretation of the references to color
in this ﬁgure legend, the reader is referred to the web version of this article.)largely different between the two Auger decays. The dication statesin the binding energy range of 40–50 eV, which are the states with
two outer-valence holes, are predominantly dissociate into theCO+ + O+ pair. This observation conﬁrms the interpretations of
the ion coincidence yields resulting from valence double photoionization [23] and from electron-impact double ionization [17]. A
theoretical calculation proposed predissociation pathways of the
dication states lying in this binding energy range into CO+ + O+
[24]. Figures 2 and 3 show that the dication states above 55 eV dissociate rarely into CO+ + O+ and preferably into the three-bodyfragmentation channels. The two-body fragmentation to CO2+ + O
is observed in a similar binding energy range to the three-bodyfragmentations, but the intensity is much weaker.
Next, we inspect the Auger ﬁnal states relevant to metastable
CO2+2 and the mechanism of the site-speciﬁc formation. Auger electron spectra associated with the formation of metastable CO2+2 canbe extracted by ﬁltering the coincidence events including a single
ion signal in the time-of-ﬂight range of 2000–2060 ns (see Figure 4a). The coincidence Auger electron spectra thus obtained areplotted in Figure 5, in comparisons with the total Auger spectra.
One ﬁnds in both coincidence spectra that the formation of metastable CO2+
2 is associated with only the lowest band (B1) attributed
to the 1pg 2 conﬁguration. The main contributors to this band are
calculated to be the a1Dg and b1R+g states of the 1pg 2 conﬁguration[12,14], while a high-resolution C1s Auger spectrum shows a certain contribution from the c1Ru state in this band [11]. The potential energy surfaces of these CO2+
2 states are calculated to have deep
minima [24,25], and in practice these CO2+
2 states populated byvalence double photoionization are observed to have lifetimes in
the order of ls or more [15].
The intensities of the B1 structures in the coincidence spectra
do not reproduce the whole intensities in the corresponding ranges

49of the total Auger spectra, and other components underlie the B1structures in the total spectra. Apart from background signals,
the Auger decay of the photoionization satellite states, which preferably produces excited CO2+
2 states by spectator behavior of initially-excited electrons, should contributes to the yields in thisenergy range. The Auger electron spectra ﬁltered by coincidenceswith C+ + O+, shown in Figures 2c and 3c, exhibit some intensities
in this energy range, which is probably due to the dissociation of
the excited CO2+
2 states produced by the Auger decay of the photoionization satellite states.
In Figure 5, while the CO2+
2 states in the same binding energy
range are relevant to the metastable CO2+
2 formation following
the different core–hole creations, the intensity of the metastable
CO2+
2 formation in the O1s core–hole decay is much higher thanthat in the C1s one, as already observed in the time-of-ﬂight spectra in Figure 1. The favorable production of the relevant 1pg 2 states
following the O1s core–hole creation is qualitatively explained byconsidering the charge distribution of the 1pg orbital involved in
the Auger decay processes. Since the 1pg orbital with the dominant
oxygen lone-pair character is largely localized around the O atomin the CO2 molecule, the O1s core–hole is ﬁlled preferentially by
the 1pg electron [11]. Thus the Auger transition involving the 1pg
electrons is favored in the decay of the O1s core–hole states, resulting in the favorable formation of metastable CO2+
2 states with the
conﬁguration of 1pg 2.
In conclusion, the mechanism of the site-speciﬁc formation ofmetastable CO2+
2 following inner-shell ionization of CO2 has beenstudied by using an Auger-electron–ion coincidence method. The
site-speciﬁcity arises from the two conditions: (1) only dication
states of 1pg 2 are relevant to the metastable formation, and (2)the formation of these dication states are favorable in the O1s
Auger decay, because of the charge distribution of the 1pg orbital
localized at the O site. Such conditions may be often fulﬁlled for
the Auger decay in which valence electrons with localized andnon-bonding characters are involved, and thus site-speciﬁc formation of metastable dication may occur in many other molecules.
Acknowledgements
The authors are grateful to the staff of the UVSOR facility for
their support and stable operation of the storage ring during thecourse of the present experiment. This work was supported by
the Cooperative Research Program of the Institute for Molecular
Science.
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