Chemical Science

Research Frontiers 2020

Formation of Xe 4d double-core-hole states in intense EUV-FEL fields
studied by multielectron-ion coincidence spectroscopy

Linear signals
Nonlinearsignals

tag withionsSignals

Electron Energy

Electron Energy

Electron spectrum
Nonlinearsignals

Signals

Fig.1. Schematics of electron spectra obtainedby multielectron-ion coincidence spectroscopy.

78Xe+

Xe
150

22.7 Mb

100

500P2

σDCH

22.1 Mb

P1

EUV-FEL

Binding Energy (eV)The nonlinear optical response of materials inthe extreme ultraviolet (EUV) and X-ray regions has
attracted a great deal of attention for its importance invarious applications of intense ultrashort free-electron
lasers (FELs). Atoms and molecules in intense FEL
fields have been studied as a benchmark for clarifying
the underlying processes, which revealed that they
typically undergo multiple ionization by multiphoton
absorption [1].
In the multiple ionization process involving innershell electrons, Auger electron emission processes
associated with the decay of the inner-shell holecompete with the subsequent multiphoton absorption
process, resulting in the emission of many electrons
as well as charged ions depending on the ionization
pathway. To understand the nonlinear ionization
mechanism involving such multiple pathways in detail,it is necessary to clarify the electronic states involvedin each ionization process.
Electron spectroscopy is a powerful technique forthis purpose since it allows the measurement of theenergy of electrons emitted during ionization, fromwhich intermediate and/or final electronic states can beidentified. It has been applied to nonlinear ionization
involving doubly excited states [2] and double-corehole (DCH) states [3], but nonlinear signals are usually
weak such that they are often smeared out by signals
originating from strong one-photon (linear) processes.
In EUV and X-ray intense laser fields, highly
charged ions tend to be generated in high-ordernonlinear processes where a large number of photons
are absorbed. This means that the ions can be used as“tags” for separating electron signals associated with atarget nonlinear process from other dominant signals
mainly produced by one-photon processes (Fig. 1). In

4d –1

Xe2+

5p–1

Xe4+

28.5 Mb
A2'4d –15s–15p–2
A2A''3 5s–15p–3
17.8 Mb–1–2
P3 4d 5p A3'A3
3.4 Mb–44d –2

4d –15p–1
2.3 Mb
AdA1'A1

5s–15p–2

5p–3

5p5s–25s–15p–1
5p–2

1.8 MbXe3+

Photoelectronemission
Auger decay

Fig.2. Energy diagram of relevant electronic
states of neutral and ionic Xe atoms [4]. P and A
stand for electron emissions by photoionization
and Auger decays, respectively.

this work, we introduced multielectron-ion coincidence
spectroscopy to tag electrons with the counterpart
ion produced by the ionization of a single atom. We
incorporated this idea in the multiphoton ionization ofXe by EUV-FEL irradiation as a benchmark system.
By relating electron signals to Xe4+ ions generated
by the two-photon process, we successfully captured
electron signals that are associated with the twophoton DCH formation of Xe 4d inner-shell states [4].
Experiments were conducted at the soft X-ray
beamline, SACLA BL1. Intense EUV laser pulses
were focused by a Kirkpatrick-Baez mirror onto thediffusive Xe gas introduced into the interaction regionof a magnetic bottle-type electron spectrometerequipped with an ion detection unit. Electrons
produced in the interaction region were guided toa microchannel plate (MCP) detector placed in one
end of a time-of-flight (TOF) tube by magnetic fields
formed by a permanent magnet and a solenoid coil,while the counterpart ions were introduced into theTOF tube by pulsed electric fields and detectedwith the same MCP. The chamber was kept underan ultrahigh-vacuum condition (<5 × 10–10 Torr), andcoincidence measurements were carried out at a
typical event rate of 0.25 events/shot to suppress
false coincidence events to less than 20% of the totalevents.
Irradiation with EUV-FEL pulses (91 eV, 30 fs,60 Hz, 1.6 × 1012 W/cm2) produced XeZ+ ions (Z ≤ 4).

Research Frontiers 2020

40(a)
Xe2+

30

2020×10–55P13010A1100–50

110Intensity(arb. units)

Electron Energy (eV)40
A1'

1A1'0102030

40

P1A12030

Electron Energy (eV)403040(b)Xe4+30
P12020A3,A3"

10A2'×10–61

100–1

P3P2A3'02Intensity(arb. units)

Electron Energy (eV)

40A2XeZ+ ions with lower charge (Z=1–3) can be generated
by single-photon ionization, while for the formation ofXe4+ ions, the absorption of more than two photons
is required because of the FEL photon energy used(see Fig. 2).
Figure 3(a) shows a two-dimensional correlation
map of electrons that are simultaneously detectedwith Xe 2+ ions. The observed features are formedby electrons generated by the photoionization of Xe
4d inner-shell levels (4d photoelectron; P1) and theultrafast Auger decay to Xe2+ (Auger electron; A1, A1’)[5]. These features are also identified in the electron
map for Xe4+ ions (Fig. 3(b)), indicating that Xe4+ canbe produced via the Xe 2+ states by the absorptionof another EUV photon. On the other hand, broad
electron distributions that are not explained by thesequential photoionization process mentioned above
are observed in the energy region of 26–38 eV. Theseelectron energies agree well with the kinetic energiesof Auger electrons (A2) associated with the decay of
Xe2+ (4d −2 ) into Xe3+ (4d −1 5p−2 ), suggesting that thetwo-photon formation of Xe 4d DCH states is involvedin the generation of Xe4+ ions.
A quantitative analysis of the Auger electron
intensities associated with these two pathways, (A1,A 1’) and (A 2, A 2’), revealed that the contribution ofthe Xe 4d DCH pathway is comparable to that of thesequential one [4]. This result indicates that the Xe 4dDCH states are efficiently formed from the transient
Xe+ (4d −1 ) states (lifetime of ~6 fs) even though theEUV-FEL pulses employed here have a rather longpulse duration of 30 fs. A numerical simulation basedon coupled rate equations for two-photon absorption
processes showed that the efficient two-photon
formation of the Xe 4d DCH states becomes possible
when the ionization cross section σDCH of Xe+(4d −1 )states, Xe+(4d −1) + hν → Xe2+(4d −2 ) + e−, falls in therange between 27 and 64 Mb. The obtained results
suggest that σ DCH is considerably larger than thenormal 4d cross sections (~20 Mb), which could be
attributed to the resonance excitation from the 4d −1 to4p−1 states in Xe [4].
The present work clearly demonstrates thepower of multielectron-ion coincidence spectroscopy
for gaining a deeper understanding of nonlinear
phenomena induced by intense EUV and X-ray laser
fields. This coincidence technique can be extended tomolecular systems in which core holes can be createdin different atomic sites. Since such DCH states are
known to sensitively reflect the local environmentof core-hole sites within a molecule [3], the present
approach is expected to become a key technique foruncovering nonlinear reaction pathways for a novel
X-ray chemical analysis.

10P3P22 A3'0A2'20P130

A3,A"31040
A22030Electron Energy (eV)40Fig.3. Two-dimensional map of electrons detected incoincidence with (a) Xe2+ and (b) Xe4+ ions, obtained bytriple covariance analysis [4]. Electron energy regions
observed in each process are indicated by color bars.

Mizuho Fushitani a,*, Yasumasa Hikosaka band Akiyoshi Hishikawaa,cGraduate School of Science, Nagoya University
Institute of Liberal Arts and Sciences,
University of ToyamacResearch Center for Materials Science, Nagoya Universityab*Email: fusitani@chem.nagoya-u.ac.jp
References[1] M. Yabashi et al.: J. Phys. B 46 (2013) 164001.
[2] A. Hishikawa et al.: Phys. Rev. Lett. 107 (2011) 243003.
[3] P. Lablanquie et al.: J. Phys. B 49 (2016) 182002.
[4] M. Fushitani, Y. Sasaki, A. Matsuda , H. Fujise , Y.
Kawabe, K. Hashigaya, S. Owada, T. Togashi, K. Nakajima,
M. Yabashi, Y. Hikosaka, and A. Hishikawa: Phys. Rev.
Lett. 124 (2020) 193201.
[5] M. Fushitani et al.: J. Electron Spectrosc. Relat. Phenom.
184 (2012) 561.

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