Journal of Electron Spectroscopy and Related Phenomena 213 (2016) 17–21

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Journal of Electron Spectroscopy and
Related Phenomena
journal homepage: www.elsevier.com/locate/elspec

Electron emission relevant to inner-shell photoionization ofcondensed water studied by multi-electron coincidence spectroscopy
Y. Hikosaka a,∗ , R. Mashiko b , Y. Konosu b , K. Soejima b , E. Shigemasa c,da

Graduate School of Medicine and Pharmaceutical Sciences, University of Toyama, Toyama 930-0194, Japan
Department of Environmental Science, Niigata University, Niigata 950-2181, JapanUVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japand
SOKENDAI, Okazaki 444-8585, Japanbca r t i c l e

i n f oArticle history:
Received 4 September 2016
Received in revised form 17 October 2016
Accepted 18 October 2016
Available online 5 November 2016
Keywords:
Condensed water
Auger decay
Multi-electron coincidence spectroscopy
Magnetic-bottle electron spectrometera b s t r a c t
Multi-electron coincidence spectroscopy using a magnetic-bottle electron spectrometer has been applied
to the study of the Auger decay following O1s photoionization of condensed H2 O molecules. Coincidence
Auger spectra are obtained for three different photoelectron energy ranges. In addition, the energy distribution of the slow electrons ejected in the Auger decay of the O1s core hole is deduced from three-foldcoincidences.
© 2016 Elsevier B.V. All rights reserved.

1. Introduction
Absorption of a single high-energy photon by a condensed
matter often results in emissions of two or more electrons fromthe surface. Auger decay following inner-shell photoionization of
the constituents of the condensed matter is one of the important origins of the multi-electron emissions, while the emissionsof multiple valence-band electrons also occur sizably as a result ofmulti-electron photoemission and inelastic scattering of initiallyejected electron [1]. Electron coincidence spectroscopy, whichenables one to clarify energy correlations between electrons
emitted in pair, is especially useful to identify the individual pathways of the multi-electron emission processes [2,3]. In order toobtain reasonable coincidence efﬁciencies, electron spectrometers with large acceptance solid angles have been adapted to theelectron coincidence measurements. For example, Thurgate andco-workers have improved photoelectron-Auger electron coincidence apparatuses based on two hemispherical analyzers [4],and Mase and co-workers have developed a coincidence apparatus consisting of a symmetric mirror analyzer and a cylindricalmirror analyzer [5]. Kirschner and co-workers have utilized twohemispherical energy analyzers installed with two-dimensional∗ Corresponding author.
E-mail address: hikosaka@las.u-toyama.ac.jp (Y. Hikosaka).
http://dx.doi.org/10.1016/j.elspec.2016.10.006
0368-2048/© 2016 Elsevier B.V. All rights reserved.

detectors [6,7]. Meanwhile, Gotter et al. have demonstrated angleresolved photoelectron-Auger electron coincidence measurements
using an array of seven hemispherical spectrometers [8]. However,statistics and energy resolution of coincidence spectra have beenoccasionally compromised even in such two-electron coincidence
measurements.
In this work, we have introduced for the ﬁrst time the magnetic bottle technique [9] into electron coincidence investigationof multi-electron emissions from solid surfaces. This technique,enabling one to collect almost all the electrons emitted into thewhole solid angle, has been successfully used in electron coincidence studies for gas-phase atoms and molecules. Thanks to theextremely-high collection-efﬁciency, not only two-electron coincidence but also multi-electron coincidences (three-fold or more)
have been effectively achieved, and varieties of multi-electron
emission pathways including such topical processes as triple photoionization [10,11] and double core photoionization [12] havebeen disclosed. In the present study, the feasibility of a magneticbottle electron spectrometer for solid samples is demonstrated by
its application to multi-electron emissions from water molecules
condensed thickly on solid surface.
Electron emission processes from water molecules are of particular interest, because of their connection with radiation damageof bio-cells. The DNA strand breaks in bio-cells exposed to ionizing
radiations are caused partly by the impacts of electrons emitted
from the ionization of cell-solvent water, where the impact of

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Y. Hikosaka et al. / Journal of Electron Spectroscopy and Related Phenomena 213 (2016) 17–21

Fig. 1. Schematic of the magnetic bottle electron spectrometer.

slow electrons can efﬁciently induce DNA strand breaks throughresonant capture processes [13,14]. An important fraction of theslow electrons can be produced in the Auger decay of the corehole formed by the initial inner-shell ionization. For isolated watermolecules, the slow electron formation relevant to the inner-shell
photoionization has been investigated by multi-electron coincidence spectroscopy [15]. It was observed that the autoionizing O
atoms produced by the dissociation of Auger-ﬁnal H2 O2+ states
remarkably yield discrete structures in the energy distribution of
the slow electrons [15]. It is interesting to investigate whether theenergy distribution of the slow electrons from solvent water condensed by hydrogen bonding also shows such discrete structures.
In this study, we have investigated multi-electron emissions
from condensed water molecules and obtained coincidence Auger
spectra with satisfactory statistics. Moreover, the energy distribution of the slow electrons ejected in the Auger decay of the O1score hole is deduced from three-fold coincidences, where any tangible structure due to O autoionization cannot be identiﬁed. To ourknowledge, such a three-fold coincidence has never been achieved
for solid samples, which veriﬁes the powerfulness of the magnetic
bottle technique in electron coincidence spectroscopy for multielectrons ejected from surfaces.

2. Experimental
The experiment was performed at the beamline BL4B of the synchrotron radiation facility UVSOR. The single bunch operation of thestorage ring provided light pulses with a 178-ns repetition period
for the 110-ps-width light-pulses. The synchrotron radiation froma bending magnet of the storage ring was monochromatized bya grazing incidence monochromator using a varied-line-spacing
plane grating. The monochromatized light was focused to 0.5 mm(horizontal) × 0.3 mm (vertical) in size.
Electron coincidence measurements were carried out with themagnetic bottle electron spectrometer [16] whose schematic isshown in Fig. 1. A strong permanent magnet whose top is of a conical shape (∼1 T at the tip) is placed a few mm away from the focal
point of the monochromatized light. A 1.5-m long solenoid tube,
forming a homogeneous magnetic ﬁeld (∼1 mT) in its inside, faces
to the permanent magnet across the focal point of the monochromatized light. Electrons formed around the focal point are capturedby the magnetic ﬁeld lines and are guided towards the microchannel plate detector located around the opposite end of the solenoid
tube. Signals from the detector are fed into a multi-stop time-todigital converter (Roentdek TDC8). Conversion from the electron
time-of-ﬂight to kinetic energy was calibrated by measuring Ar
2p photoelectron spectra at many different photon energies. Theenergy resolving power of the apparatus was estimated, from thephotoelectron lines, to be nearly constant at E/E = 35 [16]. It wasalso estimated from the coincidence yields between Ar 2p photoelectrons and the corresponding Auger electrons that the electron

Fig. 2. Total electron yield spectra around oxygen K-edge, measured before andunder the cooldown (cooling time of 1–5 h) of the copper wire. The photon energy
resolution is around E/E = 5000. Intensities of the spectra were normalized at
530 eV.

detection efﬁciencies decrease slowly with electron kinetic energy
from ∼50% (E ≈ 0 eV) to ∼40% (E = 600 eV) [16].
In the present measurement, a copper wire of a 0.3-mm diameter was placed at the focus point of the monochromatized light,
at right angles to the photon beam and the solenoid tube axis. No
particular cleaning was applied to the wire surface, and hence itshould be severely contaminated with carbon and oxygen. Water
molecules were adsorbed on the surface by continuously coolingthe wire down to the liquid nitrogen temperature, under exposure
to the background pressure inside the chamber (∼1.8 × 10−8 Torr)in which water vapor should be one of the dominant residual gases.
In practice, the background pressure decreased to ∼1.0 × 10−8 Torrduring cooling, according to the condensation of water vapor.
Fig. 2 shows total electron yield spectra around oxygen K-edge,measured before and under cooling of the copper wire. Through thepresent work, both the entrance and exit slits of the monochromator were set to be extremely narrow in order to reduce the electron
count rate to less than 10 kHz, and the photon energy resolution
reached the best value (E/E = 5000) of the monochromator. As
seen in Fig. 2, before the cooldown of the copper wire, the total
electron yield spectrum shows a single peak around 533 eV. As cooling time goes on, while the peak around 533 eV becomes weakerand shifts lower photon energy side, an enhancement appearedin 535–545 eV remarkably grows up. The spectrum after 2-h cooling, which exhibits a broad band with maxima around 537 V and
542 eV and a weak shoulder around 535 eV, resembles closely the
NEXAFS spectra of ice [17–19], certifying thick adsorption of watermolecules on the wire surface. The spectral intensity increases withincreasing cooling time of more than 2 h, implying that the thickness of the condensed water layer is still comparable to the escapedepths of the ejected electrons (∼1 nm for 500 eV electrons [20]).
This is because, considering that electrons generated at a distantdepth much beyond their escape depth can hardly get out into thevacuum, further growth of the layer whose thickness already got
much beyond the escape depth does not results in such an increaseof the electron yields.
3. Results and discussion
Coincidence datasets were accumulated for electrons emitted
from the copper wire on irradiation of 738.2 eV photons, before andunder the cooldown. The measurement under the cooldown wasstarted after an advance cooling of 2 h, to ensure sizable adsorption of water molecules on the wire surface. The photon energy

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Fig. 4. Photoelectron spectrum obtained by integrating the coincidence counts on
the map in Fig. 3(b) over the range of x = 450–530 eV. The three kinetic energy ranges
for the extractions of the curves in Figs. 5(a)-(c) are indicated.

Fig. 3. Two-dimensional maps showing energy correlations of the coincident electrons emitted from the copper wire on irradiation of 738.2 eV photons, (a) beforeand (b) under the cooldown to the liquid nitrogen temperature. The measurement
under the cooldown was started after an advance cooling of 2 h, to ensure a thickadsorption of water molecules on the wire surface. The accumulation times were
3 h and 20 h, respectively, with count rates around 5 kHz.

is ∼200 eV above the O1s threshold of condensed water (∼533 eV[17,21]). The two-dimensional maps in Fig. 3 display the energy correlations of the coincident electrons included in the accumulated
coincidence datasets, where the coincidence counts are plotted asa function of faster (x-axis) and slower electron energies (y-axis).
Several diagonal lines seen commonly in these two maps are artifacts due to rebounds in the electron signals. On the map from theaccumulation before the cooldown [Fig. 3(a)], a weak structure running vertically is discernible around (x, y) = (450 eV, 230–260 eV).
This structure is ascribable to the coincidences between the C1 s
photoelectrons (450 eV) and the C1 s Auger electrons (230–260 eV)ejected from the carbon contamination on the wire. The corresponding structure becomes less remarkable in the map from theaccumulation under the cooldown [Fig. 3(b)], but otherwise a clear
structure running horizontally appears around (x, y) = (400–530 eV,
200 eV). The emergent structure is assignable to the coincidences
between the O1s photoelectrons (200 eV) and the O1s Auger electrons (400–530 eV) from condensed water molecules.
To display the O1s photoelectron structure closely, the coincidence counts on the map in Fig. 3(b) are integrated in the rangeof x = 450–530 eV and projected onto y-axis. The spectrum thus
obtained is presented in Fig. 4. The O1s photoelectron peak isobserved around a kinetic energy of 205 eV, and a continuous
structure is remarkably exhibited on the lower kinetic energy
side. The continuous structure is attributed mainly to the photoelectrons with reduced energies due to inelastic scatterings on

Fig. 5. Auger electron spectra obtained in coincidence with (a) O1s photoelectrons,(b) energy-loss photoelectrons due to inelastic scatterings, and (c) electrons in the
broad maximum. The photoelectron energy ranges set for the extractions of these
Auger spectra are indicated in Fig. 4. In (c), the raw Auger spectrum (solid black)
includes a large contribution from the inelastic scattering process, and net spectrum(red ﬁlled area) is obtained by assuming that the contribution from the inelastic
scattering is described by the distribution in (b). (For interpretation of the references
to colour in this ﬁgure legend, the reader is referred to the web version of this article.)

the escape from the inside of the water condensation. On thecontinuum structure, a broad maximum is discernible around a
kinetic energy of 180 eV. One may consider that this structure isdue to an enhancement in the inelastic scattering at the speciﬁc
energy loss corresponding to the valence excitation in the counterpart water molecules. In the meanwhile, for monolayer H2 O
adsorbed on Si surface, the formation of 1 s satellite states resulting from the promotion of a valence electron accompanying O1s
ionization is observed with a similar energy separation [22]; thissingle-molecule process possibly contributes to the enhancement
observed for the present multi-layer sample.
Fig. 5 presents Auger electron spectra observed in coincidence with O1s photoelectrons (region (a) in Fig. 4), energy-loss

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Y. Hikosaka et al. / Journal of Electron Spectroscopy and Related Phenomena 213 (2016) 17–21

photoelectrons due to inelastic scatterings (region (b) in Fig. 4),
and electrons in the broad maximum (region (c) in Fig. 4). The
Auger spectrum in coincidence with O1s photoelectrons, shown in
Fig. 5(a), resembles the conventional Auger spectrum [23]. Considerable intensity is observed even below 420 eV, which is ascribableto an energy loss of the Auger electrons due to the inelastic scattering. In the Auger spectrum derived in coincidence with energy-loss
photoelectrons due to inelastic scatterings [Fig. 5(b)], the O1s Augerfeature is less pronounced compared to that in Fig. 5(a), implyingthat effect from inelastic scattering is enhanced in the selection to
the low energy tail of the O1s photoelectron peak. This observation
can be understood as follows: the selection of photoelectrons withenergy loss picks out the photoemission events for H2 O molecules
distant from the outermost surface and thus the Auger electronsejected from these molecules are also subject to remarkable inelastic scattering.
The raw coincidence Auger spectrum deduced for the broad
maximum [solid black curve in Fig. 5(c)] includes a large contribution from the ordinary inelastic scattering process, corresponding
to the background intensity underling the enhancement structure
seen in Fig. 4. Assuming that the spectral shape of the contribution
from the ordinary inelastic scattering process is rarely dependent on the kinetic energy of the energy-loss photoelectron and is
described by the spectrum in Fig. 5(b), the contribution can be reasonably subtracted from the raw spectrum. The residual spectrum,
shown as red ﬁlled area in Fig. 5(c), exhibits an Auger structurein which low kinetic energy tail due to the inelastic scattering isless remarkable. Thus, it is likely that the main contributor to theenhancement is the formation of the 1 s satellite states. The Auger
structure in the red ﬁlled area in Fig. 5(c) lies in a similar energy
range to the Auger decay of the O1s state [Fig. 5(a)], though the 1 ssatellite states gain an excess energy of the valence excitation compared to the main O1s state. A similar behavior is observed in theAuger decay of the O1s satellite states in isolated H2 O molecules[15]. These observations are understood in terms of the spectator
behavior of the excited valence electron: the excited valence electron is kept in the initially-excited orbital through the Auger decay
and the Auger ﬁnal states are correspondingly more excited.
Since O1s photoionization of a single H2 O molecule is predominantly followed by the Auger decay, the O1s photoelectron andAuger electron are basically emitted in pair from the molecule.
However, a considerable fraction of these electrons should be lost in
the way to the ejections into the vacuum. In practice, in the coincidence dataset accumulated under the cooldown, coincidence countbetween O1s photoelectron and O1s Auger electron is 4.5% of thetotal count of the O1s photoelectron and 1.4% of that of the Auger
electron, where the Auger electrons in the kinetic energy range of
450–530 eV are chosen. Here, the total count of the Auger electron
was observed to be three times as much as that of the photoelectron. The higher ejection probability for the Auger electrons is agood correspondence to the fact that the electron escape depth for
the Auger electrons (kinetic energy ∼500 eV) is a few times larger
than that for the O 1 s photoelectron (kinetic energy ∼200 eV) [20].
Next, we inspect the slow electron emission accompanying theAuger decay of the O1s core hole. Such slow electrons should beobserved in three-fold coincidence with the O1s photoelectron
and the fast Auger electron. Fig. 6 displays the energy correlation
between the photoelectrons (x-axis) and additional slow electrons(y-axis) in the kinetic energy range of 0–6 eV, obtained from thetriple coincidence events including a fast Auger electron in the
kinetic energy range of 450–530 eV. A vertical stripe is seen at
the O1s photoelectron energy, delineating the distribution of theslow electrons accompanying the Auger decay of the O1s core hole.
The slow electron spectrum extracted by the yields along the vertical stripe is shown in the right panel. The slow electron spectrum
shows a continuous distribution decreasing gradually as increas-

Fig. 6. Two-dimensional map showing energy correlation between the O1s photoelectrons and slow electron below a kinetic energy range of 6 eV, which is extracted
from the triple coincidence events including a fast Auger electron in the kinetic
energy range of 450–530 eV. The top panel shows an enlargement of the spectrum in
Fig. 4, and the right panel plots the slow electron spectrum extracted by integrating
the horizontal range for the O1s photoelectrons.

ing the electron energy. It is reported for isolated H2 O moleculesthat a similar continuous distribution of slow electrons is produced
through the double Auger decay of the O1s core-hole state [15];it is likely, however, that the contribution from the double Auger
decay is minor in the slow electrons ejected from condensed water.
This is because, while the sum of the energies of the two Auger
electrons emitted in the double Auger process should show theH2 O3+ states populated ﬁnally, the corresponding H2 O3+ structures
cannot be identiﬁed in the sum of the energies of the fast Auger
electron and slow electron (not shown). Therefore, majority of theslow electrons observed in the O1s decay of the condensed water
is probably the secondary electrons produced by scatterings of theejecting photoelectrons and Auger electrons.
The total counts of the spectrum in the right panel in Fig. 6, corresponding to the yields of the three-fold coincidences among theO1s photoelectron, the fast Auger electron and the slow electron,
are about 17% of the number of the coincidences between the O1s
photoelectron and the fast Auger electron. By taking the detection
efﬁciency (∼50%) of the slow electron into account, ∼34% of thepairs of the O1s photoelectron and fast Auger electron are accompanied by the ejection of a slow electron in the range of 0–6 eV.
This observation implies that slow electrons are efﬁciently formedby the inelastic scattering of photoelectrons and Auger electrons.
Note that, considering that the escape depth [20] for the slow electrons less than 6 eV is an order of magnitude larger than that of thefast Auger electron, the ejection probability for the slow electrons
should be signiﬁcantly large.
The distribution of the slow electrons emitted after O1s photoionization in isolated H2 O molecules remarkably exhibits below
2 eV the autoionization lines of superexcited O atoms [15]. These
superexcited O atoms are formed by double OH-bond breaking at
highly-excited H2 O2+ states populated after the Auger decay of theO1s core-hole state. The corresponding autoionization lines are
hardly discernible in the slow electron distribution in the rightpanel of Fig. 6. Since the superexcited O formation constitutes only
0.8% in the O1s decay of the isolated H2 O molecules [15], even if
this process occurs similarly in condensed phase, the autoionization lines are masked by the slow electrons formed efﬁciently by
the inelastic scattering. On the other hand, it is anticipated thatthe formation of superexcited O atoms is suppressed in condensed

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phase, because of fast electron transfers from neighbor molecules
to the Auger ﬁnal states before their dissociation.
In conclusion, we have applied multi-electron coincidence
spectroscopy using a magnetic bottle electron spectrometer tomulti-electron emissions from condensed water molecules. Coincidence Auger spectra were obtained not only for the O1s decay
but also for the satellite decay. Moreover, three-fold electron coincidence was achieved. The present work demonstrates that themagnetic bottle technique is promising in electron coincidence
measurements of solid samples.
Acknowledgments
The authors thank Kazuhiko Mase (KEK-PF) for fruitful discussions. We are grateful to the UVSOR staff for the stable operationof the storage ring. This work was supported by the Cooperative
Research Program of the Institute for Molecular Science. YH thanks
ﬁnancial supports from Shimadzu Science Foundation and Daiko
Foundation.
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