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Direct observation of the electronic states ofphotoexcited hematite with ultrafast 2p3d X-ray
absorption spectroscopy and resonant inelastic
X-ray scattering†
Ahmed S. M. Ismail, ‡a Yohei Uemura, ‡ab Sang Han Park,‡c Soonnam Kwon,cMinseok Kim,c Hebatalla Elnaggar,a Federica Frati,a Yasuhiro Niwa,d Hiroki Wadati,ef
Yasuyuki Hirata,e Yujun Zhang,e Kohei Yamagami,e Susumu Yamamoto, e
Iwao Matsuda,e Ufuk Halisdemir,g Gertjan Koster,g Bert M. Weckhuysen a andFrank M. F. de Groot*a
Hematite, a-Fe2O3, is an important semiconductor for photoelectrochemical water splitting. Its low
charge carrier mobility and the presence of midgap states provide favourable conditions for electron–
hole recombination, hence affecting the semiconductor’s photoelectrochemical efficiency. The natureof the excited state and charge carrier transport in hematite is strongly debated. In order to further

Received 14th June 2019,
Accepted 15th October 2019

understand the fundamental properties of the hematite photoexcited state, we conducted femtosecond

DOI: 10.1039/c9cp03374b

thin-films at the Pohang Accelerator Laboratory X-ray Free Electron Laser (PAL-XFEL). The observed

2p (L3) X-ray absorption (XAS) and 2p3d resonant inelastic scattering (RIXS) measurements on hematite
spectral changes and kinetic processes are in agreement with previous 3p XAS reports. The potential

rsc.li/pccp

additional information that could be acquired from 2p3d RIXS experiments is also discussed.

Introduction
X-ray absorption spectroscopy (XAS) combined with the pump–probe method has become a powerful technique to investigate
electronic states of photocatalytic materials.1–4 XAS provides
information on the element specific electronic structure of thematerial under investigation. In addition, XAS can be measured
under operando conditions.5,6 In combination with calculations,
XAS enables the determination of the detailed local structurea

Inorganic Chemistry and Catalysis, Debye Institute for Nanomaterials Science,
Utrecht University, Universiteitslaan 99, 3584 CG Utrecht, The Netherlands.
E-mail: f.m.f.degroot@uu.nlbInstitute for Catalysis, Hokkaido University, Kita 21 Nishi 10, Kita-ku, Sapporo,
Hokkaido 001-0021, Japanc
PAL-XFEL, Pohang Accelerator Laboratory, 77 Cheongam-Ro, Nam-Gu, Pohang,
Gyeongbuk 37673, South KoreadPhoton Factory, Institute for Materials Structure Science, KEK, Tsukuba 305-0801,
Japane
Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581,
Japan
f
Graduate School of Material Science, University of Hyogo, Kamigori,
Hyogo 678-1297, Japang
Faculty of Science and Technology and MESA + Institute for Nanotechnology,
University of Twente, P.O. Box 2171, 7500 AE Enschede, The Netherlands† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9cp03374b‡ These authors contributed equally to this work.

This journal is © the Owner Societies 2020and electronic states of materials.7,8 There have been pioneeringstudies to investigate photoexcited states of metal oxides withthe pump–probe XAS methodology.7,9–16 While synchrotron
pump probe XAS experiments are limited by the pulse durationof X-rays (B100 ps) or by the reachable flux for slicing experiments, the birth of X-ray free electrons lasers (XFEL) has
expanded the horizon of the time resolved studies based on
the pump–probe XAS methodology.1,9,11,17–20 XFELs can provide
ultrashort and intense X-ray pulses down to 10 fs. The numberof photons in an X-ray pulse from XFELs are at least millions oftimes larger than that of an X-ray pulse in a synchrotron. Suchshort pulse duration has enabled the study of femtosecond
dynamics, which have also been achieved with femtosecond
optical laser spectroscopies. In addition to the capability of
tracking ultrafast dynamics, such high intense X-ray pulses enablesophisticated photon-in/photon-out spectroscopic measurements
such as X-ray emission spectroscopy (XES)21–23 and resonant
inelastic X-ray scattering (RIXS)24–30 with a very short temporal
resolution. RIXS spectra provide additional opportunities toobserve electronic excitations in materials, which would be
impossible only with optical spectroscopies. At PAL-XFEL,31,32
one can measure the L3 XAS of 3d transition metal elementswith a time resolution of less than 300 fs.33 In addition, 1 eV
resolved 2p3d RIXS34–36 can also be measured, which extends
the information provided on the electronic states of 3d metals.

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Here, we report the first time resolved L3 XAS and 2p3d RIXS
experiments in PAL-XFEL to observe the photoexcited state ofhematite (a-Fe2O3).
Hematite is intensely studied as a semiconductor for thesolar-assisted water splitting processes. This is mainly due to its
rich abundance in the earth crust and to its optical bandgap ofB2 eV that enables it to absorb light in the visible wavelength
range of the solar spectrum.37,38 However, the intrinsic electronic properties of hematite make it diﬃcult for the semiconductor to carry out the water splitting reaction directly, since its
hole diﬀusion length is so small, 2–4 nm, that the electrons andholes can easily recombine in short time scales.39,40 Therefore,
ultrafast spectroscopic investigations of the nature of thehematite photoexcited states have gained increasing interest
due to its relevance to its water splitting efficiency.12,38,41–43 For
example, Vura-Weiss et al. conducted the first femtosecond
extreme-ultra violet (XUV) spectroscopic studies on hematite
thin films and reported that the observed photoinduced Fe(II)states originate mainly from ligand to metal charge transfer(LMCT) i.e. the charge transfer from oxygen to iron, and not fromthe optical d–d excitation.44 Further theoretical and experimental
XUV spectroscopic reports suggested that an additional reason
for the low hematite performance is the formation of small
polarons that localize the minority carriers at recombination
centres.45,46
Our time resolved studies of 2p X-ray absorption spectroscopy (XAS) and 2p3d resonant inelastic X-ray scattering (RIXS)
of iron elucidate the electronic structure of the photoexcitedhematite. The results also demonstrate the potential of 2p3d
RIXS experiments to understand the electronic nature of excitedstates in photocatalytic materials.

Experiments and calculations
Sample preparation
A 50 nm hematite (a-Fe2O3) polycrystalline thin-film was prepared on indium tin oxide (ITO)/fused silicon dioxide substrateby pulsed laser deposition (PLD). First, a 5 nm ITO thin film
was deposited on a fused silicon dioxide substrate by translatinga KrF excimer laser (l = 248 nm) on a rotating ITO target with anincident angle of 451 with a fluency of 2 J cm2 and a repetition
rate of 1 Hz. The ITO target was then replaced with hematitetarget and hematite thin-film was deposited at the same deposition conditions. The deposition experiment was conducted at
700 1C and under 1.3 kPa oxygen pressure. The average surface
roughness of the hematite thin-film was determined to be
4.7 nm using Atomic Force Microscopy. X-ray Photoelectron
Spectroscopy (XPS) was employed to confirm iron and oxygen
electronic properties in hematite. Further information on thesample characterization can be found in the ESI.†The X-ray setup at PAL-XFEL
All XAS and RIXS spectra were measured in the soft X-ray
spectroscopy and scattering (SSS) beamline of PAL-XFEL. An
illustration of the SSS beamline of PAL-XFEL is shown in Fig. 1.

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Fig. 1 The experimental setup of the SSS beamline of PAL-XFEL. A
microchannel plate (MCP) detector was employed to measure fluorescent
X-ray and a CCD detector was employed to measure RIXS signals.

In the SSS beamline, soft X-rays between 250 eV and 1200 eV are
available. In order to monochromatise the X-ray, a grating
optics with 200 lines per mm was employed and the estimatedenergy resolution of the monochromatised X-ray was about
0.5 eV at 710 eV for the XAS experiments and about 1.0 eV for
the RIXS experiments at 710 eV. The monochromatised X-ray
was focused by a pair of Kirkpatrik–Baez mirrors to reduce the
X-ray size to less than 50 mm (H)  50 mm (V) on the sample
position. Since the X-ray intensities are different in each X-ray
pulse, it was necessary to measure the monochromatised X-ray
intensity (I0) and the fluorescent X-ray intensity (If) shot by shotin order to obtain XAS correctly. Each I0 intensity was observedby the electrons emitted from a Si3N4 thin-film. The photoelectrons were measured with a microchannel plate (MCP) detector.
The If intensity was measured with another MPC detector byobtaining fluorescent X-ray photons from the sample. Twoboxcar integrators (SR250, Stanford Research Systems) were
employed to collect both the signals from the MCP detectors.
The boxcar integrators were synchronised with the trigger
signals from the XFEL in order to gather only the signals fromthe X-ray pulses. The output signals from the boxcars weretransferred to the digitizers (SR245, Stanford Research Systems).
A Ti:sapphire laser was employed to excite the sample. Its
fundamental wavelength was converted to 400 nm with a BBO
crystal. The laser pulses were transferred into the measurement
chamber and focused on the sample position. The spot size ofthe laser was approximately 210 mm (H)  210 mm (V) for its 1/e2
value. The angle between the X-ray and the laser was set to about
11. The excitation laser fluence was set to B8 mJ cm1, which
was found to be the optimum laser fluence for exciting B20% of
the iron atoms and in the same time causing minimal to nosample damage or nonlinear effects.
The X-ray setup at Spring-8
Transient XAS spectra of the Fe L3 edge in the picoseconds
regime were measured at the BL07LSU beamline in SPring-8.47
During the beamtime, the operation mode of SPring-8 was‘‘H-mode’’,48 where one electron bunch between bunch trainsis detected. The X-rays from the isolated bunch arrives every
B4.76 ms (B210 kHz). In order to synchronise with the excitation
laser (B1 kHz), an X-ray chopper was employed to select X-ray
pulses. XAS spectra were measured in a Partial Electron Yield

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PCCP(PEY) geometry. An MCP detector was employed to measure theintensity of electrons coming out from the sample surface. The
fluence of the excitation laser was the same as in PAL-XFEL andthe temporal resolution of the set up was estimated to be 80 ps.
Further details on the setup were explained in ref. 47.
Theoretical calculations
The X-ray absorption and X-ray emission processes have beenmodelled with crystal field multiplet theory. The use of thecharge transfer multiplet (CTM) model is based on the argumentthat the 2p core hole state is localised on one site allowing a local
model to describe the 2p3d excitation and the 3d2p X-ray emission
decay. The 2p XAS spectra are simulated from transitions from the
3d5 ground state to the 2p53d6 final state, which are based on themodel described by de Groot et al. and Vura-Weiss et al.44,50
The parameters we used for the calculations are identical to theparameters used in these studies, i.e. atomic values are used forthe electron–electron interactions and spin–orbit couplings
and a value of 0.5 eV was used for the octahedral crystal field.
The multiplet-averaged Fe(II) calculation is shifted by 0.7 eVwith respect to Fe(III), following the experimentally determined
shift between Fe(II) and Fe(III) sites in CoFe2O4.51

Results and discussion
2p X-ray absorption spectroscopy of photoexcited hematite
The Fe L3 edge XAS spectrum and transient (diﬀerence) spectraof hematite are shown in Fig. 2. Each diﬀerence spectrum wascalculated by subtracting the reference spectrum (the spectrum
at a delay time of 10 ps) from the photoexcited XAS spectrumat the corresponding delay time. A distinctive change wasobserved at 709 eV in the diﬀerence XAS spectra at positiveFig. 2 Fe L3 edge XAS spectrum of the hematite thin film (a), the transient
Fe L3 edge XAS spectra of photoexcited hematite at several delay times (b)and the calculated spectra of Fe L3 edge XAS by the multiplet theory. The
solid black line in panel (c) is the smoothed representation of the experimental data points that helps to clarify the spectral shape of the transient
spectra.

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delay times. The maximum of the peaks around 709 eV in eachdiﬀerence spectrum at a positive delay time are comparable toeach other, while the peak width at a delay time of 10 ps islarger. It is expected that the laser excites electrons in the Fe 3d
orbitals, eﬀectively reducing Fe(III) to Fe(II). Vura-Weis et al.
have observed the excited state of hematite with transient
3p XAS experiments and they calculated the XAS spectra of
the excited states with multiplet calculations.44 Similar calculations of the 2p XAS spectra of Fe(II) were performed by de Grootet al. and Haverkort et al.49,50 The diﬀerence spectra between
the calculated Fe L3 XAS of Fe(II) and the calculated the XAS of
Fe(III) are shown in Fig. 2(c). The XAS spectra of Fe(II) werescaled in order to reproduce the spectral shapes of the diﬀerence XAS (Dmtcalc = mtFe(II)(E + DE)  amtFe(III)(E)). We estimatedthe values of a and DE value based on the knowledge of XAS of
the mixed-valence iron compounds such as Fe3O4, where adetailed fitting procedure determined a shift between octahedral
Fe(II) and octahedral Fe(III) to be 0.7 eV.51 Diﬀerent a values were
explored in order to reproduce the spectral features of thediﬀerence XAS. The combination of a = 0.5 and DE = 0.7 eV
were found to be the best values to reproduce the experimental
spectra. The main peak position of the diﬀerence spectra in
Fig. 2(b) was reproduced by the calculated spectra. The photoexcited state at a delay time of 0.2 ps is the initial excited stateand is equal to the time resolution of our experiment as shown
below in Fig. 3(b). We expect that no structural change occurs at
this time scale, implying that the crystal field value 10Dq can beapproximated as being equal to the ground state value of Fe(III).
Kinetic traces of 2p3d X-ray absorption spectroscopy ofphotoexcited hematite
The kinetic traces of the XAS at 709 eV are shown in Fig. 3(a)and (b). To avoid possible artificial eﬀects resulting from thelong-term fluctuations at the XFEL, we sequentially recorded
the photoexcited signal and the reference signal (at a delay
time of 10 ps) for each delay time. For shorter delay times,

Fig. 3 The kinetic traces of the transient XAS at 709 eV. (a) From a delay
time of 0.4 ps to that of 1.0 ps, (b) from a delay time of 3 ps to that of
10 ps, (c) Fe L3 XAS and the transient XAS spectrum at 0.2 ps.

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the kinetic trace was observed between 0.4 ps and 1.0 ps. The
transient signal increased upon irradiation by the laser pulses.
In order to estimate the time resolution of the experimental
setup, the rise of the transient signal was fitted with a convolutionof a step function and a Gaussian function. The time resolution for
the experiments was estimated to be 180 fs by the full width of half
maximum (FWHM) of the Gaussian function. The estimated value
in the experiment seems to be reasonable considering the laser
pulse duration (B50 fs), the pulse duration of X-ray (o100 fs) andthe jitter between the optical laser and the XFEL, although thisvalue is slightly shorter from the time resolution estimated by thechange of the reflectance of the laser pulse from a YAG single
crystal.33 It should be noted that an arrival timing tool or a timing
compensation tool is yet to be installed in the beamline.52,53 As aresult, a precise measurement of the delay time between each X-ray
pulse and optical laser pulse or the compensation of the timing
drift between them could not be performed. Therefore, wemeasured the position of the delay stage for a delay time of
0 ps at the beginning and the end of each 12 h shift. Based onthese measurements, we have established that the position for adelay time 0 did not shift by more than 100 fs. Hence, we
concluded that there is no significant timing drift on a timescale of 12 h.
The decay process shown in Fig. 3(a) is a fast decay process,whose kinetic constant (1/k) is B0.2  0.1 ps based on the fit on
the decrease of the intensity of the transient signal. A fast
process within 1 ps after photoexcitation was observed byseveral research groups43,44 and similar values for the process
were proposed (0.24 ps,44 0.25 ps43). The kinetic trace in Fig. 3(a)in the longer delay times (40.2 ps) was fitted well with aconvolution of a Gaussian function (FWHM: 0.18 ps) and an
exponential function (f (t) = A  exp(t/k) + B, where A and B are
arbitrary constants). Considering our experimental conditions,the fast process we observed should correspond to the fast
process observed in the previous studies.42–44 This process is
ascribed to be a relaxation process of the excited electrons inhigher energy states to the bottom of the conduction band.42,44
For delay times over 1 ps, the peak intensity of the diﬀerence
signal gradually decreased as in Fig. 3(b). The kinetic trace in
Fig. 3(b) was fitted as a single exponential function and its kinetic
constant of the decay process (1) was estimated to be B6 ps. A
similar time constant was observed in optical spectroscopy,43
which was claimed to be a recombination process between therelaxed electrons and the holes, while such process was notfound in the XUV spectroscopies.41,44
The electronic structure of the iron sites in transition metaloxides such as a-Fe2O3 is described by charge transfer multiplet
theory, where the ground state of Fe in a-Fe2O3 is described as|3d5 + 3d6Li. The 3d5 configuration represents the ground stateof Fe(III) and ‘‘3d6
L’’ represents the state where one electron is
excited from O2p to Fe3d with a charge transfer energy (D) thatis diﬀerent from the band gap due to strong correlation eﬀects.
The ground state is the bonding combination of both configurations.
Upon optical laser excitation, a valence electron is excited into the
conduction band, which in the CTM model refers to an anti-bonding
state |3d6
L + 3d5i. For simplicity we approximate the ground state2688 | Phys. Chem. Chem. Phys., 2020, 22, 2685--2692

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and laser excited state to be |3d5i and |3d6
Li respectively,
keeping in mind that both states are mixed. In addition, the|3d6
LiR and |3d6
LiR 0 states represent the relaxed states from theinitial excitation. The overall process can then be described as:

where t1 = 0.2 ps and t2 = 6 ps. In the 2p XAS process each state
3dN is excited to a final state 2p53dN+1, where the spectral shapeof the anti-bonding |3d6
Li state will have a |2p53d7
Li final state
similar to a Fe(II) ground state. This justifies the first-orderapproximation of its spectral shape that is related to a Fe(II)species in the local electronic structure of a-Fe2O3.
2p3d resonant inelastic scattering of photoexcited hematite
2p3d RIXS measures the 2p XAS excitation of the 3d5 groundstate to 2p53d6, followed by the decay back to the 3d5 manifold.
A recent static 2p3d RIXS study of a-Fe2O3 has been publishedby Miyawaki et al.54 We note that the lifetime of the 2p core hole
state is B10 fs based on its lifetime broadening. We assume thatthe 2p core hole decay is faster than any other reorganisation
process and that 2p3d RIXS is eﬀectively an instantaneous probe.
The 2p3d RIXS spectra were measured at 710 eV, i.e. at the
maximum of the static a-Fe2O3 2p XAS spectrum. The delay
times of each measurement were 1 ps and 10 ps as shown in
Fig. 4. The RIXS spectra were normalised to the intensity ofelastic X-ray peak (energy loss (Eloss) B 0 eV). Each RIXS
spectrum was recorded in two hours. Due to the increased
possibility for time drift in the delay times around 0.2 ps, we
could not record the RIXS spectrum for such short delay times.
In addition, in order to guarantee that RIXS from photoexcitedstates are observed, the kinetic traces at 709 eV were observed
before each RIXS measurement (Fig. 4).
When we compared the transient RIXS at a delay time of 1 ps
to that of 10 ps, we noticed that the overall diﬀerence signal
seemed to increase in the later delay time. We are not certain
about the interpretation of this eﬀect due to the low energy
resolution of the spectra and the interference between Fe(II)and Fe(III) features. One possible explanation could be the longlife time of the Fe(II) excited states that are generated athematite defect sites. We observed in another related timeresolved XUV study that these states could have a life time of
up to B350 ps. It is possible that the concentration of thesestates increases over time after subsequent optical excitations,which could be reflected in the increase of the diﬀerence signal
in RIXS at 10 ps.
Examining the time evolution of the RIXS diﬀerence signals
showed spectral changes in the coloured areas at between
3 eV and 8 eV. The theoretical 2p3d RIXS spectrum of
Fe(III) contains an elastic peak and a range of d–d excitations
between 2 and 7 eV (Fig. 4). The ratio between both the RIXS
spectra is given by the excitation energy, essentially as given by
the XAS calculations. This implies that Fe(III) has B10 times
more intensity than Fe(II). The spectrum of Fe(II) has a strong

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Fig. 4 2p3d RIXS spectra for 10 ps, 1 ps, 10 ps (top) and the diﬀerence
RIXS spectrum for each delay time (bottom). The incident X-ray energy
was 710 eV. The calculated spectra were normalised so that their elastic
scattering intensities were half of the experimental spectra for clarity.

peak at B1.8 eV related to the 5T2 - 5T1 d–d transition, which
will compensate the intensity loss of Fe(III) at B1.8 eV, implyingthat only between 3 and 8 eV intensity is lost. We note thatan excitation energy of 710 eV, more than B90% of the signal isrelated to Fe(III) based on the theoretical 2p XAS spectral
shapes, which implies that we mainly see Fe(III) in the 2p3d
RIXS. One could also look specifically at the Fe(II) species, but
this would require comparing the RIXS spectra recorded at theexcitation energy of 710 eV as well as that of 709 eV, which was
not measured due to the time constraints. We essentiallyobserve a decrease of the region between 2 and 7 eV
indicating that the amount of Fe(III) species is reduced. The
data quality unfortunately does not allow us to make more
specific statements regarding the 2p3d RIXS spectral shapes.
We note that by measuring the 2p3d RIXS at 709 eV wewould specifically target the transient Fe(II) species. Its 2p3d
RIXS spectrum would reveal its d–d excitations and as such thenature of its state in much more details compared to 2p XAS. In
addition there would be much less signal related to the Fe(III)species.
Photoexcited states of hematite
It has been proposed that several diﬀerent excited states exist for thephoto-absorption process in hematite by optical spectroscopy.43

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From our XAS and RIXS results we expect three diﬀerent excitedstates in the first 10 ps, in line with the observed states proposedby optical spectroscopic measurements. The initial photoexcitedstate was observed at a delay time of 0.2 ps. After photoabsorption, the 3d5 ground state of Fe(III) is excited to the
3d6
L charge transfer state. The charge transfer state symmetry
is similar to that of a Fe(II) ground state and results in transient
peak at 709 eV. In a single particle picture, the 3d6L state can beinterpreted as an electron in the conduction band coupled to ahole in the valence band (Fig. 5). Since the wavelength of the
optical laser is larger than the band gap of hematite, the excitedelectrons and holes have excess energy (often known as hot
carriers). The hot electron–hole excitations relate to hot electrons
and holes which are distributed in the conduction and valencebands, respectively. The transient signal intensity at 709 eV
decreases after 1 ps, which corresponds to the relaxation processof the hot carriers. During the relaxation of the hot carriers, thehot electrons relax to the bottom of the conduction band while
the holes relax to the top of the valence band releasing extra
energy. The fast decay in Fig. 2 and the simulation of thedifference spectra in Fig. 3 agree with this hypothesis. During
the relaxation the local structure of the Fe atoms is affected,
which modifies the Fe–O and Fe–Fe bond distances. These
structural changes affect the electronic structure (10Dq, etc.) of
the excited Fe atoms, thereby also changing the Fe L3 XAS
spectral shape.
The transient signal intensity decreased further after 10 ps,
which agrees with the charge recombination process observedin optical spectroscopy. The population of the excited electrons
is decreased by the recombination events, i.e. by the decay ofanti-bonding 3d6L to bonding 3d5 states. The intensity of theFig. 5 Schematic diagram of the possible pathways of the excited statesof hematite upon visible light excitation. Before photoexcitation (|3d5i), thevalence of Fe is 3+, which has 5 electrons in the 3d orbitals. The 3d orbitalsof Fe are split into 3 tg orbitals and 2 eg orbitals because of its octahedral
symmetry. After photoexcitation (|3d6
Li, B200 fs), an excited electron
occupies tg or eg orbitals. Within 1 ps (|3d6
LiR 0 B 1 ps), the excited electron
relaxes to occupy the tg orbital. In the later delay time (B10 ps), some ofthe excited Fe species change to another excited state (|3d6
LiR 0 ) whileothers relax to the ground state. It is noted that the excited electron isspin-down because the spin-up states are all occupied. At a neighboring
site, the spin-up and spin-down signature is reversed as a-Fe2O3 is an
antiferromagnet.

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diﬀerence spectra at 709 eV is attributed to the number of Fe(II)atoms and the decrease of Fe(II) atoms leads to the reduction of thediﬀerence signals around 10 ps. Interestingly, we observe that the
Fe(II) species remains present for several hundreds of picoseconds.

Conclusions
In this study, we have reported the first femtosecond Fe L3 XAS
and femtosecond 2p3d RIXS of hematite using the PAL-XFEL.
Upon photoexcitation, electrons are transferred from the hematite valence band (mainly O 2p) to the conduction band (mainly
Fe 3d). We detected three relaxation events after photoexcitation:
the first process is the relaxation of hot electrons to a lower
energy state. This relaxation process of the hot carriers with akinetic constant of 0.2 ps was observed by Fe L3 XAS with a time
resolution of 180 fs. The second process is a fast carrier recombination, which was observed as the decrease of the transient XAS
signal at 709 eV with a kinetic constant of 6 ps. In this process,part of the Fe(II)-like species relaxed back to 3d5 Fe(III). However,
our observations also show that some Fe(II) species remain at alonger time scale (in Fig. S2 in ESI†). This long-living photoexcited state could be a trapping state of the excited electrons
with a local structural change around photoexcited iron atoms inhematite to form small polarons.
To the best of our knowledge, this is the first report on
femtosecond transient RIXS of a solid-state material. Our result
demonstrates exciting possibilities for studying the photoexcited states of solid-state materials using femtosecond RIXS.
In future experiments, where RIXS spectra will be measured at
diﬀerent excitation energies and better energy resolution, moredetailed information will be revealed about specific electronic
states of the photoexcited materials. It could also be possible todescribe charge carrier dynamics in ultrafast time scales withelectronic configurations at each distinctive delay time.
Although there have been several pioneering works of timeresolved 2p3d RIXS,24–30 the opportunity to perform time-resolved
2p3d RIXS experiments was limited since only the Linac Coherent
Light Source (LCLS, Menlo Park, CA, USA) was able to provide thesoft X-rays needed.55,56 In addition to the Pohang XFEL, new soft
X-ray beamlines are being implemented in novel XFEL facilities
such as in LCLS-II,57 European XFEL33 and Swiss FEL.36 Suchpowerful FEL facilities, providing ultrashort soft X-ray pulses, will
make it possible to address structural and electronic configurations of excited states of semiconductors consisting of 3d transition metals thereby opening possibilities to understand themechanism of e.g. a solar-assisting water splitting reaction.

Conflicts of interest
There are no conflicts to declare.

Acknowledgements
This work was financially supported by the European ResearchCouncil (ERC) under the European Union’s Horizon 2020

2690 | Phys. Chem. Chem. Phys., 2020, 22, 2685--2692

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research and innovation programme (grant agreement No 340279),
Netherlands Center for Multiscale Catalytic Energy Conversion(MCEC), a Gravitation Program from the Netherlands Organisationfor Scientific Research (NWO), Grant for collaborative research in
Institute for Catalysis, Hokkaido University (18A1005) and a basic
science research program funded by the Ministry of Education,
Government of the Netherlands (No. NRF-2017R1D1A1A02018484).
This work was carried out by the joint research in the Synchrotron
Radiation Research Organization and the Institute for Solid State
Physics, the University of Tokyo (Proposal No. 2019A7592). The
experiments were performed at SSS end-station of PAL-XFEL(proposal no. 2018-2nd-SSS-001 or 2019-1st-SSS-020) funded by
the Ministry of Science and ICT of Korea.

References
1 M. Chergui and E. Collet, Chem. Rev., 2017, 117, 11025–11065.
2 C. J. Milne, T. J. Penfold and M. Chergui, Coord. Chem. Rev.,
2014, 277–278, 44–68.
3 C. Bressler and M. Chergui, Annu. Rev. Phys. Chem., 2010, 61,
263–282.
4 C. Bressler and M. Chergui, Chem. Rev., 2004, 104, 1781–1812.
5 Y. Iwasawa, K. Asakura and M. Tada, XAFS Techniques for
Catalysts, Nanomaterials, and Surfaces, Springer International
Publishing, 2017.
6 J. A. Van Bokhoven and C. Lamberti, X-Ray Absorption and
X-Ray Emission Spectroscopy: Theory and Applications, John
Wiley & Sons, UK, 2016.
7 T. J. Penfold, J. Szlachetko, F. G. Santomauro, A. Britz, W. Gawelda,
G. Doumy, A. M. March, S. H. Southworth, J. Rittmann, R. Abela,
M. Chergui and C. J. Milne, Nat. Commun., 2018, 9, 478.
8 T. J. Penfold, S. Karlsson, G. Capano, F. A. Lima, J. Rittmann,
M. Reinhard, M. H. Rittmann-Frank, O. Braem, E. Baranoﬀ,
R. Abela, I. Tavernelli, U. Rothlisberger, C. J. Milne and
M. Chergui, J. Phys. Chem. A, 2013, 117, 4591–4601.
9 Y. Uemura, D. Kido, A. Koide, Y. Wakisaka, Y. Niwa, S. Nozawa,K. Ichiyanagi, R. Fukaya, S. Adachi, T. Katayama, T. Togashi,
S. Owada, M. Yabashi, K. Hatada, A. Iwase, A. Kudo,
S. Takakusagi, T. Yokoyamaa and K. Asakura, Chem. Commun.,2017, 53, 7314–7317.
10 Y. Obara, H. Ito, T. Ito, N. Kurahashi, S. Thürmer, H. Tanaka,
T. Katayama, T. Togashi, S. Owada, Y.-I. Yamamoto,
S. Karashima, J. Nishitani, M. Yabashi, T. Suzuki and
K. Misawa, Struct. Dyn., 2017, 4, 044033.
11 Y. Uemura, D. Kido, Y. Wakisaka, H. Uehara, T. Ohba, Y. Niwa,
S. Nozawa, T. Sato, K. Ichiyanagi, R. Fukaya, S.-I. Adachi,
T. Katayama, T. Togashi, S. Owada, K. Ogawa, M. Yabashi,
K. Hatada, S. Takakusagi, T. Yokoyama, B. Ohtani and
K. Asakura, Angew. Chem., Int. Ed., 2016, 55, 1364–1367.
12 D. Hayes, R. G. Hadt, J. D. Emery, A. A. Cordones, A. B. F.
Martinson, M. L. Shelby, K. A. Fransted, P. D. Dahlberg,
J. Hong, X. Zhang, Q. Kong, R. W. Schoenlein and L. X. Chen,
Energy Environ. Sci., 2016, 9, 3754–3769.
13 H. Wen, M. Sassi, Z. Luo, C. Adamo, D. G. Schlom,
K. M. Rosso and X. Zhang, Sci. Rep., 2015, 5, 15098.

This journal is © the Owner Societies 2020

View Article Online

Open Access Article. Published on 16 October 2019. Downloaded on 10/7/2024 4:08:40 AM.
This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.

PCCP

14 F. G. Santomauro, A. Lübcke, J. Rittmann, E. Baldini,
A. Ferrer, M. Silatani, P. Zimmermann, S. Grübel, J. A.
Johnson, S. O. Mariager, P. Beaud, D. Grolimund, C. Borca,
G. Ingold, S. L. Johnson and M. Chergui, Sci. Rep., 2015,
5, 14834.
15 Y. Uemura, H. Uehara, Y. Niwa, S. Nozawa, T. Sato,
S. Adachi, B. Ohtani, S. Takakusagi and K. Asakura, Chem.
Lett., 2014, 43, 977–979.
16 M. H. Rittmann-Frank, C. J. Milne, J. Rittmann,
M. Reinhard, T. J. Penfold and M. Chergui, Angew. Chem.,
Int. Ed., 2014, 53, 5858–5862.
17 N. A. Miller, A. Deb, R. Alonso-Mori, B. D. Garabato,J. M. Glownia, L. M. Kiefer, J. Koralek, M. Sikorski, K. G.
Spears, T. E. Wiley, D. Zhu, P. M. Kozlowski, K. J. Kubarych,
J. E. Penner-Hahn and R. J. Sension, J. Am. Chem. Soc., 2017,
139, 1894–1899.
18 H. T. Lemke, K. S. Kjaer, R. Hartsock, T. B. van Driel,
M. Chollet, J. M. Glownia, S. Song, D. Zhu, E. Pace,
S. F. Matar, M. M. Nielsen, M. Benfatto, K. J. Gaﬀney,
E. Collet and M. Cammarata, Nat. Commun., 2017, 8, 15342.
19 M. L. Shelby, P. J. Lestrange, N. E. Jackson, K. Haldrup,
M. W. Mara, A. B. Stickrath, D. Zhu, H. T. Lemke,
M. Chollet, B. M. Hoﬀman, X. Li and L. X. Chen, J. Am.
Chem. Soc., 2016, 138, 8752–8764.
20 H. T. Lemke, C. Bressler, L. X. Chen, D. M. Fritz, K. J.
Gaﬀney, A. Galler, W. Gawelda, K. Haldrup, R. W. Hartsock,
H. Ihee, J. Kim, K. H. Kim, J. H. Lee, M. M. Nielsen,
A. B. Stickrath, W. Zhang, D. Zhu and M. Cammarata,
J. Phys. Chem. A, 2013, 117, 735–740.
21 S. E. Canton, K. S. Kjær, G. Vankó, T. B. van Driel, S.-I. Adachi,
A. E. Bordage, C. Bressler, P. Chabera, M. Christensen,
A. O. Dohn, A. Galler, W. Gawelda, D. Gosztola, K. Haldrup,
T. Harlang, Y. Liu, K. B. Móller, Z. Németh, S. Nozawa, M. Pápai,
T. Sato, T. Sato, K. Suarez-Alcantara, T. Togashi, K. Tono,
J. Uhlig, D. A. Vithanage, K. Wärnmark, M. Yabashi, J. Zhang,
V. Sundström and M. M. Nielsen, Nat. Commun., 2015, 6, 6359.
22 K. Haldrup, W. Gawelda, R. Abela, R. Alonso-Mori,
U. Bergmann, A. Bordage, M. Cammarata, S. E. Canton,
A. O. Dohn, T. B. van Driel, D. M. Fritz, A. Galler, P. Glatzel,
T. Harlang, K. S. Kjær, H. T. Lemke, K. B. Móller, Z. Németh,
M. Pápai, N. Sas, J. Uhlig, D. Zhu, G. Vankó, V. Sundström,
M. M. Nielsen and C. Bressler, J. Phys. Chem. B, 2016, 120,
1158–1168.
23 J. Kern, R. Tran, R. Alonso-Mori, S. Koroidov, N. Echols,
J. Hattne, M. Ibrahim, S. Gul, H. Laksmono, R. G. Sierra,
R. J. Gildea, G. Han, J. Hellmich, B. Lassalle-Kaiser, R. Chatterjee,
A. S. Brewster, C. A. Stan, C. Glöckner, A. Lampe, D. DiFiore,
D. Milathianaki, A. R. Fry, M. M. Seibert, J. E. Koglin, E. Gallo,
J. Uhlig, D. Sokaras, T.-C. Weng, P. H. Zwart, D. E. Skinner,
M. J. Bogan, M. Messerschmidt, P. Glatzel, G. J. Williams,
S. Boutet, P. D. Adams, A. Zouni, J. Messinger, N. K. Sauter,U. Bergmann, J. Yano and V. K. Yachandra, Nat. Commun., 2014,
5, 4371.
24 M. Beye, F. Sorgenfrei, W. F. Schlotter, W. Wurth andA. Föhlisch, Proc. Natl. Acad. Sci. U. S. A., 2010, 107,
16772–16776.

This journal is © the Owner Societies 2020

Paper

25 R. M. Jay, S. Eckert, M. Fondell, P. S. Miedema, J. Norell,
A. Pietzsch, W. Quevedo, J. Niskanen, K. Kunnus andA. Föhlisch, Phys. Chem. Chem. Phys., 2018, 20, 27745–27751.
26 R. M. Jay, J. Norell, S. Eckert, M. Hantschmann, M. Beye,
B. Kennedy, W. Quevedo, W. F. Schlotter, G. L. Dakovski,
M. P. Minitti, M. C. Hoﬀmann, A. Mitra, S. P. Moeller,
D. Nordlund, W. Zhang, H. W. Liang, K. Kunnus, K. Kubicek,
S. A. Techert, M. Lundberg, P. Wernet, K. Gaﬀney,
M. Odelius and A. Föhlisch, J. Phys. Chem. Lett., 2018, 9,3538–3543.
27 K. Kunnus, I. Josefsson, I. Rajkovic, S. Schreck, W. Quevedo,
M. Beye, C. Weniger, S. Grübel, M. Scholz, D. Nordlund,
W. Zhang, R. W. Hartsock, K. J. Gaﬀney, W. F. Schlotter,
J. J. Turner, B. Kennedy, F. Hennies, F. M. F. de Groot,
S. Techert, M. Odelius, P. Wernet and A. Föhlisch, Struct.
Dyn., 2016, 3, 043204.
28 J. Norell, R. M. Jay, M. Hantschmann, S. Eckert, M. Guo,
K. J. Gaﬀney, P. Wernet, M. Lundberg, A. Föhlisch and
M. Odelius, Phys. Chem. Chem. Phys., 2018, 20, 7243–7253.
29 V. Vaz da Cruz, F. Gel’mukhanov, S. Eckert, M. Iannuzzi,
E. Ertan, A. Pietzsch, R. C. Couto, J. Niskanen, M. Fondell,M. Dantz, T. Schmitt, X. Lu, D. McNally, R. M. Jay, V. Kimberg,
A. Föhlisch and M. Odelius, Nat. Commun., 2019, 10, 1013.
30 P. Wernet, K. Kunnus, I. Josefsson, I. Rajkovic, W. Quevedo,
M. Beye, S. Schreck, S. Grubel, M. Scholz, D. Nordlund,
W. Zhang, R. W. Hartsock, W. F. Schlotter, J. J. Turner,
B. Kennedy, F. Hennies, F. M. de Groot, K. J. Gaﬀney, S. Techert,
M. Odelius and A. Fohlisch, Nature, 2015, 520, 78–81.
31 H.-S. Kang, C.-K. Min, H. Heo, C. Kim, H. Yang, G. Kim,
I. Nam, S. Y. Baek, H.-J. Choi, G. Mun, B. R. Park, Y. J. Suh,
D. C. Shin, J. Hu, J. Hong, S. Jung, S.-H. Kim, K. Kim, D. Na,
S. S. Park, Y. J. Park, J.-H. Han, Y. G. Jung, S. H. Jeong,
H. G. Lee, S. Lee, S. Lee, W.-W. Lee, B. Oh, H. S. Suh,Y. W. Parc, S.-J. Park, M. H. Kim, N.-S. Jung, Y.-C. Kim,
M.-S. Lee, B.-H. Lee, C.-W. Sung, I.-S. Mok, J.-M. Yang,
C.-S. Lee, H. Shin, J. H. Kim, Y. Kim, J. H. Lee, S.-Y. Park,
J. Kim, J. Park, I. Eom, S. Rah, S. Kim, K. H. Nam, J. Park,J. Park, S. Kim, S. Kwon, S. H. Park, K. S. Kim, H. Hyun,
S. N. Kim, S. Kim, S.-M. Hwang, M. J. Kim, C.-y. Lim,
C.-J. Yu, B.-S. Kim, T.-H. Kang, K.-W. Kim, S.-H. Kim,
H.-S. Lee, H.-S. Lee, K.-H. Park, T.-Y. Koo, D.-E. Kim and
I. S. Ko, Nat. Photonics, 2017, 11, 708–713.
32 PAL-XFEL, http://pal.postech.ac.kr/paleng/.
33 S. H. Park, M. Kim, C.-K. Min, I. Eom, I. Nam, H.-S. Lee,
H.-S. Kang, H.-D. Kim, H. Y. Jang, S. Kim, S.-M. Hwang,
G.-S. Park, J. Park, T.-Y. Koo and S. Kwon, Rev. Sci. Instrum.,
2018, 89, 055105.
34 L. J. P. Ament, M. van Veenendaal, T. P. Devereaux, J. P. Hill
and J. van den Brink, Rev. Mod. Phys., 2011, 83, 705–767.
35 F. de Groot and A. Kotani, Core Level Spectroscopy of Solids,
CRC Press, 2008.
36 A. Kotani and S. Shin, Rev. Mod. Phys., 2001, 73, 203–246.
37 K. Sivula, F. L. Formal and M. Grätzel, ChemSusChem, 2011,
4, 432–449.
38 A. G. Tamirat, J. Rick, A. A. Dubale, W.-N. Su and
B.-J. Hwang, Nanoscale Horiz., 2016, 1, 243–267.

Phys. Chem. Chem. Phys., 2020, 22, 2685--2692 | 2691

View Article Online

Open Access Article. Published on 16 October 2019. Downloaded on 10/7/2024 4:08:40 AM.
This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.

Paper

39 M. Barroso, S. R. Pendlebury, A. J. Cowan and J. R. Durrant,
Chem. Sci., 2013, 4, 2724–2734.
40 A. Kay, I. Cesar and M. Grätzel, J. Am. Chem. Soc., 2006, 128,
15714–15721.
41 J. Husek, A. Cirri, S. Biswas and L. R. Baker, Chem. Sci.,
2017, 8, 8170–8178.
42 S. R. Pendlebury, X. Wang, F. L. Formal, M. Cornuz,
A. Kafizas, S. D. Tilley, M. Grätzel and J. R. Durrant, J. Am.
Chem. Soc., 2014, 136, 9854–9857.
43 S. Sorenson, E. Driscoll, S. Haghighat and J. M. Dawlaty,
J. Phys. Chem. C, 2014, 118, 23621–23626.
44 J. Vura-Weis, C.-M. Jiang, C. Liu, H. Gao, J. M. Lucas,
F. M. F. de Groot, P. Yang, A. P. Alivisatos and S. R. Leone,
J. Phys. Chem. Lett., 2013, 4, 3667–3671.
45 L. M. Carneiro, S. K. Cushing, C. Liu, Y. Su, P. Yang, A. P.
Alivisatos and S. R. Leone, Nat. Mater., 2017, 16, 819–825.
46 A. J. E. Rettie, W. D. Chemelewski, D. Emin and C. B.
Mullins, J. Phys. Chem. Lett., 2016, 7, 471–479.
47 K. Takubo, K. Yamamoto, Y. Hirata, Y. Yokoyama, Y. Kubota,
S. Yamamoto, S. Yamamoto, I. Matsuda, S. Shin, T. Seki,
K. Takanashi and H. Wadati, Appl. Phys. Lett., 2017, 110, 162401.
48 SPring-8 several bunch modes, http://www.spring8.or.jp/en/
users/operation_status/schedule/bunch_mode_b19/.

2692 | Phys. Chem. Chem. Phys., 2020, 22, 2685--2692

PCCP

49 M. U. Delgado-Jaime, K. Zhang, J. Vura-Weis and F. M. F. deGroot, J. Synchrotron Radiat., 2016, 23, 1264–1271.
50 M. W. Haverkort, M. Zwierzycki and O. K. Andersen, Phys.
Rev. B: Condens. Matter Mater. Phys., 2012, 85, 165113.
51 B. Liu, C. Piamonteze, M. U. Delgado-Jaime, R.-P. Wang,
J. Heidler, J. Dreiser, R. Chopdekar, F. Nolting and F. M. F.
de Groot, Phys. Rev. B, 2017, 96, 054446.
52 M. R. Bionta, N. Hartmann, M. Weaver, D. French, D. J.
Nicholson, J. P. Cryan, J. M. Glownia, K. Baker, C. Bostedt,
M. Chollet, Y. Ding, D. M. Fritz, A. R. Fry, D. J. Kane,
J. Krzywinski, H. T. Lemke, M. Messerschmidt, S. Schorb,
D. Zhu, W. E. White and R. N. Coﬀee, Rev. Sci. Instrum.,
2014, 85, 083116.
53 T. Katayama, S. Owada, T. Togashi, K. Ogawa, P. Karvinen,
I. Vartiainen, A. Eronen, C. David, T. Sato, K. Nakajima,
Y. Joti, H. Yumoto, H. Ohashi and M. Yabashi, Struct. Dyn.,
2016, 3, 034301.
54 J. Miyawaki, S. Suga, H. Fujiwara, M. Urasaki, H. Ikeno,
H. Niwa, H. Kiuchi and Y. Harada, Phys. Rev. B, 2017, 96, 214420.
55 T. Funk, A. Deb, S. J. George, H. Wang and S. P. Cramer,
Coord. Chem. Rev., 2005, 249, 3–30.
56 X. Liu, W. Yang and Z. Liu, Adv. Mater., 2014, 26, 7710–7729.
57 LCLS-II, https://lcls.slac.stanford.edu/lcls-ii.

This journal is © the Owner Societies 2020