Molecular Physics
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Extreme ultraviolet time-resolved photoelectron
spectroscopy of aqueous aniline solution:enhanced surface concentration and pumpinduced space charge effect
Christopher W. West, Junichi Nishitani, Chika Higashimura & Toshinori
SuzukiTo cite this article: Christopher W. West, Junichi Nishitani, Chika Higashimura & Toshinori Suzuki(2020): Extreme ultraviolet time-resolved photoelectron spectroscopy of aqueous aniline solution:enhanced surface concentration and pump-induced space charge effect, Molecular Physics
To link to this article: https://doi.org/10.1080/00268976.2020.1748240

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MOLECULAR PHYSICS e1748240
https://doi.org/10.1080/00268976.2020.1748240

RESEARCH ARTICLE

Extreme ultraviolet time-resolved photoelectron spectroscopy of aqueous aniline
solution: enhanced surface concentration and pump-induced space charge effect
Christopher W. West, Junichi Nishitani, Chika Higashimura and Toshinori Suzuki

Department of Chemistry, Graduate School of Science, Kyoto University, Kyoto, Japan
ABSTRACTARTICLE HISTORY

We present extreme ultraviolet (XUV) time-resolved photoelectron spectroscopy (TRPES) of an aqueous aniline solution. One-colour XUV-induced photoemission signal of aniline was observed withmuch greater intensity than expected from its molar fraction in the bulk solution, indicating thataniline is hydrophobically segregated on the liquid surface. The concentration dependence of thephotoelectron intensity is found to be well correlated with the surface concentration of aniline
estimated by surface tension measurements. Similar segregation was observed also for phenol inaqueous solution. The enhanced surface concentration of aniline makes its XUV-TRPES to be highly
vulnerable to the pump-induced space charge effects (PISC). The PISC caused by a moderate pumpintensity was not completely corrected using a simple mean field model, and reduction of the pumppulse intensity was necessary. The spectra measured at lower pump intensity were corrected for
PISC, which enabled us to extract the information on the excited state dynamics of aniline in aqueous solution under 240 nm photoexcitation. Two components with lifetime on sub-ps and > 100 pstimescales were determined, and the former is ascribed to the solvation dynamics in the S1 state after
the ultrafast internal conversion from the S3 state and the latter to the subsequent population decayof the S1 state.

Received 19 December 2019
Accepted 23 March 2020

1. Introduction
Time-resolved photoelectron spectroscopy (TRPES) is
one of the most useful methods to explore ultrafast electronic dynamics in materials [1–8], and its application
has been extended to solution chemistry [9–12]. So far,
TRPES has been successfully performed for various liquids using the UV pump and UV probe method to
obtain valuable new insights into the electronic structure and dynamics in bulk solutions. The advantage of
UV-UV TRPES experiment is its simple implementation
and a high contrast ratio of the two-colour signal against
one-colour photoionization background signal [13]. On

Photoelectron spectroscopy;
liquid; surface; extreme
ultraviolet; ultrafast

the other hand, the observation window of UV-TRPES
is limited to the electron binding energy (eBE) up to
about 6 eV. It is also noted that the low-energy photoelectrons generated by UV probe pulses are vulnerable toinelastic scattering in the bulk liquid, and their analysis
requires careful consideration of inelastic scattering effectin the liquid [13–17]. TRPES using extreme UV (XUV)or X-ray probe pulses significantly expands the observation energy window for the entire valence electrons
and inner-shell electrons, respectively, and high-energy
photoelectrons generated by these pulses are much lessaffected by inelastic scattering than the UV-UV TRPES.

suzuki@kuchem.kyoto-u.ac.jp; suzuki.toshinori.2n@kyoto-u.ac.jp
CONTACT Toshinori Suzuki
Kyoto University, Kitashirakawa Oiwake-cho, Sakyo-Ku, Kyoto 606-8502, Japan

Department of Chemistry, Graduate School of Science,

Supplemental data for this article can be accessed here. https://doi.org/10.1080/00268976.2020.1748240© 2020 Informa UK Limited, trading as Taylor & Francis GroupKEYWORDS2

C. W. WEST ET AL.

Recent development of the high-order harmonic generation (HHG) technique enabled construction of a tabletop XUV and soft X-ray light sources with femtosecond
or attosecond pulse durations [18]. These light sources
are ideal for TRPES of liquids [9,17,19–26]. On the other
hand, since the XUV radiation induces photoemission
from all species in solution, XUV-TRPES faces several
challenging problems. First of all, since the photoemission signal is always present from the solute and solventin their ground electronic state, it is difficult to observe
the pump-probe signal at low excitation efficiency. Thisdifficulty is common with the methods such as ultrafast
electron diffraction [27–29], X-ray diffraction [30,31],
and X-ray absorption spectroscopy [32,33], which all
require a high excitation efficiency to extract clear pumpprobe signals. When one attempts to improve the excitation efficiency with a higher pump intensity, it often
leads to multiphoton ionisation of the target species. This
causes UV pump-induced generation of photoelectrons,
which creates Coulombic repulsion between the pumpinduced and probe-induced photoelectron packets and,
consequently, pump-probe delay-dependent shift of photoelectron kinetic energy. We refer to this effect as thepump-induced space charge effect (PISC) in this study.
This phenomenon has previously been identified in thepump-probe TRPES of solids and liquids, and the correction method based on the mean field model has been
proposed [19,34,35]. We employ the mean field model inthis study.
Photoelectron spectroscopy has a finite probing depth,
which depends on the electron attenuation length (EAL)
in the material at a given electron kinetic energy (eKE)[13–16,36–39]. EAL in liquid remains as a subjectof intense debate; however, the following features are
known. The electron-electron inelastic scattering in liquid water requires collision energy greater than the band
gap of liquid water (7 eV), so that inelastic scattering
at low eKE ( < 7 eV) is exclusively electron-vibron orelectron-phonon scattering. Since these processes havesmall cross-sections and small energy loss per collision,
EAL is relatively long in the low eKE region, even though
it is still in the scale of nanometers [13] owing to combinational effects from elastic and inelastic scattering. As
eKE increases above 11 eV, electron-electron scatteringand electron impact ionisation can occur, so that EAL
becomes about or less than a nanometer in the eKE rangeof tens of eV [16,36–38]. Thus, although XUV-TRPES is
not rigorously surface selective, unlike non-linear optical
spectroscopies based on the second harmonic generationor sum-frequency generation [40–42], its probing depth
is limited to on the order of a nanometer. When the solutehas an enhanced surface concentration, photoelectron

signal predominantly originates from molecules at theinterface.
In the present study, we apply XUV-TRPES to aqueous aniline solution to examine the performance of thismethod and explore photo-induced dynamics of aniline in aqueous environment. Aniline is one of the mostfundamental aromatic amines. The electronic dynamics
of aniline in the gas phase have been studied by many
workers [43–51] with particular interest to a possible
role of S2 (π , σ *) state in an electronic deactivation process [43–47,52]. According to the most recent studies
by Fielding and colleagues [49], aniline has a three-state
conical intersection among S3 (π , π *), S2 (π , σ *) andS1 (π , π *) states, which facilitates ultrafast internal conversion to the latter two states in the gas phase. On theother hand, the dynamics seem significantly altered inaqueous solution: Fárník and colleagues have investigated the N-H bond rupture upon 243 nm photoexcitation of aniline embedded on the surface of large water
clusters, (H2 O)430 , and they found that dissociation via
S2 (π , σ *) state is largely suppressed in the cluster [50].
Shizuka and coworkers have performed transient absorption spectroscopy of aqueous aniline solution using 266
or 308 nm pump pulses [51], and they observed formation of a long-lived S1 state ( > 1 ns). The formationof hydrated electrons within picosecond time scales has
also been reported [51]. In our XUV-TRPES of ultrafast dynamics of aniline in aqueous solution, we foundthat PISC appears considerably more strongly than similar aqueous solutions studied in our laboratory. The fact
led us to explore an enhanced surface concentration ofaniline as the origin of strong PISC observed for aqueous
aniline solution.

2. Experimental methods
A one-box Ti-sapphire regenerative amplifier (35 fs,
800 nm) with a repetition rate of 1 or 10 kHz was used asa driving laser for an optical parametric amplifier (OPA)and HHG. OPA generated the 240 nm pump pulses,
which were focused onto a liquid microjet using an Al
concave mirror (f = 1000 mm); the pump pulse duration was expected to be about 100 fs. The spot size atthe liquid microjet was 120 μm (FWHM). We employed
several different HHG configurations driven by the fundamental (ω) or second harmonic (2ω) laser pulses. The
2ω-driving system was superior to ω-driving in complete isolation of the single harmonic order. The 2ωlaser pulses were produced with a 0.3 mm thick beta
barium borate (BBO) crystal. The driving laser pulses
were focused into a Kr gas cell using a quartz lens(f = 500 mm). Single order harmonic was isolated from

MOLECULAR PHYSICS

a number of generated harmonic radiation using a pair ofmultilayer mirrors [53] or a time-preserving monochromator [54] as shown in Figure S1 in the supplementary material (SM). In the ω-driving setup, the 19thorder harmonic (19ω: 42 nm, 29.45 eV) was employed.
With the 2ω-driving setup, the 18th order (18ω: 44 nm,
27.9 eV) or 14th order (14ω: 57 nm, 21.7 eV) harmonic
was selected using the grating system. The diameter ofthe XUV beam at the microjet was about 50 and 100 μm
FWHM using the multilayer mirrors and the grating system, respectively. The maximum XUV photon flux was
2 × 109 photon/s for 19ω. The 240 nm pump and XUVprobe pulses were introduced into a magnetic-bottle
TOF photoelectron spectrometer with a small crossing
angle (1.5°). The magnetic-bottle guided more than 50%of the photoelectrons emitted from the sample to thedetector. Our TOF spectrometer is capable of changing its electric potential to alter the electron pass energy
through the spectrometer, and we utilised the variable
electric potential to reject low-energy electrons. Photoelectrons were detected using a Chevron microchannel
plate (MCP; 38 mm ∅) and a preamplifier, and electron
counts were accumulated using a multi-channel scaler.
The overall energy resolution of our XUV-TRPES was
0.3–0.4 eV.
Aniline in aqueous solution has absorption maxima
around 230 and 280 nm for S3 and S1 states, respectively,
as shown in Figure S2 in SM. The 240 nm pump pulse
used in this study can access the S3 state. The aqueous 6–200 mM aniline solution was discharged from afused silica capillary with an inner diameter of 25 μm

3

at a flow rate of 0.5 mL/min to generate a liquid microjet. For reducing electrokinetic charging of the microjet,
we added NaI (60–75 mM) in initial experiments, whilelater measurements were performed by replacing NaI
with NaBr, which does not absorb 240 nm light. The
temperature in the sample reservoir was 295 K.
The cross-correlation of the UV and XUV laser pulses
and the time origin were estimated by a global fit of theaniline pump-probe time energy map, as described inpart VI of the SM, from which the time-resolution of
our experiment was determined to be 130 fs. The energy
calibration of the photoelectron spectrometer was performed using XUV one-colour photoelectron spectrumof Xe, and the 2 P1/2 and 2 P3/2 doublet of Xe+ was used.

3. Results and discussion
3.1. Enhanced surface density of aniline
Figure 1(a) compares the one-colour photoelectron spectra, measured using the 29.5 eV probe pulses, of an aqueous NaI (50 mM) solution (blue) and a mixed solution ofaniline (90 mM) and NaI (75 mM) (red). The two peaks
at 7.4 (3b1 ) and 8.5 eV (1a2 ) seen for the aniline solution
are of aniline molecule; their band positions are in good
agreement with 7.49 and 8.62 eV by Tentscher et al. [55]and 7.6 ± 0.1 and 8.7 ± 0.2 eV by our unpublished soft
X-ray photoelectron spectroscopy performed at SPring8 BL17SU beam line. The photoemission signal fromI− in aqueous solution are expected at 8.03 and 8.96 eV
as reported previously [56–58], and they were observed

Figure 1. (a) Comparison of one-colour photoelectron spectra of mixed aqueous solution of aniline (90 mM) and NaI (75 mM) (red) andaqueous NaI (50 mM) solution (blue) measured using the 29.5 eV probe pulses only. (b) Comparison of one-colour photoelectron spectraof aniline (200 mM) and NaBr (75 mM) solutions in water (red) and methanol (blue) measured using the 27.9 eV probe pulses only. Lower
panels show the enlarged views.

4

C. W. WEST ET AL.

Figure 2. (a) Surface tension measurements of aqueous solution of aniline (Black circles), phenol (red diamonds), and pyrazine (green
triangles). (b) The surface concentration calculated from the result shown in (a).

very weakly for the aqueous 50 mM NaI solution. It issomewhat surprising that aniline exhibits considerablystronger photoemission than I− at similar molar fractions in aqueous solution. Since it is unlikely that thephotoemission cross-section of aniline is far greater thanthat of I− , the result indicates that aniline is segregated on
the liquid surface more than I− , which is also known tohave an enhanced concentration at the gas-liquid interface of an aqueous solution [59]. Figure 1(b) compares
the photoelectron spectra of aniline solutions in waterand methanol; in both cases, the concentration of aniline
was 200, and 75 mM of NaBr was added for controlling the electrokinetic charging of microjets. The photoemission signal from aniline in methanol solution is far
weaker than that in aqueous solution. Since a large difference is not expected for a photoemission cross-sectionof aniline in these solutions, the result indicates that aniline is well solvated in methanol while segregated on thesurface of water. The results in Figure 1(a) and (b) indicate that the strong photoemission signal of aniline in
an aqueous solution originates from its enhanced surface
concentration.
For a more quantitative analysis, we have measured
the surface tension of aqueous aniline solution using theWilhelmy plate method and estimated the surface concentration. Figure 2(a) is the change in surface tension
measured as a function of molar concentration of aniline in aqueous solution. In Figure 2(b) shown is the plotof surface concentration () calculated using the Gibbs
adsorption equation [60],
=−

1 dγ
,
RT dlnC(1)

where γ , C, R, and T are the surface tension (Jm−2 ), bulk
concentration (mol L−1 ), gas constant (JK−1 mol−1 ), andtemperature (K), respectively. For comparison, the resultson phenol and pyrazine are also presented. These results

show that the surface concentration of aniline and phenol
are quite similar at a given bulk concentration, as is easily
anticipated from their molecular structures.
Figure 3 overlays the photoelectron intensities of aniline and phenol measured in the present work and theirsurface concentrations shown in Figure 2(b). The relative photoemission intensity closely follows the surface
concentration of aniline and phenol in aqueous solution
which tends to saturate at around 0.1 M. Both experimental and theoretical studies have shown that, at theinterface of aqueous phenol solution, phenol molecule
points its OH group toward the bulk water [61–64].
It is expected that the aniline molecule at the water
surface also points the NH2 group towards the bulk
water side. Interestingly, even though aniline is segregated at the liquid-gas interface, its experimental eBE isin agreement with the value calculated for fully hydrated
aniline using quantum mechanics/molecular mechanics(QM/MM) method within 0.2 eV [55]. We did not findnoticeable spectral shift of aniline peak in the concentration range examined in this study.
3.2. Time-dependent photoelectron energy shifts at
moderate pump intensity
TRPES using XUV or soft X-ray radiation tend to cause
the pump-probe delay-dependent space charge effects
due to multiphoton ionisation of the sample by the pumppulses. For demonstrating this effect, Figure 4(a) presents
the time-energy map of the raw photoelectron spectra
measured with a moderate pump pulse energy of 0.5 μJ. It
is noted that there is no strong photoelectron bands fromgaseous water molecules in this particular spectrum. Thisis because we applied −10 V to the microjet and shifted
the liquid photoelectron bands to a higher eKE region.
While the applied potential also shifts the gaseous waterbands, their shifts are smaller and dependent on theMOLECULAR PHYSICS

Figure 3. Concentration dependence of photoelectron intensityfrom aniline and phenol in aqueous solution compared withthe surface concentration estimated by surface tension measurements. Photoelectron intensities probed by 27.9 eV (green
crosses) and 21.7 eV (orange triangles) in aniline and 21.7 eV inphenol (blue pluses) are plotted. NaBr of 75 mM was added toreduce electrokinetic charging of the microjet in the photoemission measurements. Black circles and red diamonds are thesurface concentration of aniline and phenol in aqueous solution,
respectively, shown in Figure 2 (b). It was experimentally conﬁrmed that the surface tension of aqueous aniline solution was
unaﬀected by the addition of NaBr that is water-soluble electrolyte. Note that the photoelectron intensity is scaled appropriately to ﬁt the surface concentration.

ionisation position, because the electrostatic potential
rapidly varies with the distance from the jet. Consequently, the gaseous water bands are flattened and shiftedaway from the liquid bands. The time-energy map of photoelectron bands of liquid thus measured clearly exhibits
time-dependent energy shifts, manifesting PISC.
PISC has been previously discussed for TRPES ofsolids (VO2 , SrTiO3 , and graphite) by Oloff et al. [34,35]
and aqueous solutions by Al-Obaidi et al. [19]. For solids,
eKE of the probe photoelectron increases in the presenceof pump photoelectrons for the entire pump-probe timedelay owing to Coulombic repulsion between the pumpand probe photoelectrons. (An exception was observedin photoemission from highly oriented pyrolytic graphite
at high pump pulse intensity [35].) For liquids, eKE in
the presence of pump-induced photoemission increases
at around the pump-probe time origin similarly withthe solid case, while eKE gradually diminishes to exhibit
a negative shift with respect to one-colour XUV photoemission at pump-probe delay time on the order ofseveral hundred picoseconds [19]. The negative eKE shift
is caused by the influence of positive charge created in
the liquid by pump-induced photoemission. While thepositive charge is also created in the solid, it dissipates
instantaneously. In the case of liquid, the positive charge

5

Figure 4. (a) Time-energy map of photoelectron spectra of aqueous 60 mM aniline solution measured with 240 nm pump and29.45 eV probe pulses. The three selected spectra are shown on
the right to illustrate the pump-probe delay-dependent energy
shifts. (b) The spectra after correction for PISC. The broken lines
indicate the eBE of some valence orbitals of liquid water andaniline.

remains for more than 100 ps [19]. Close examinationof our result shown in Figure 4 (a) reveals that eBE
gradually diminishes as the delay time increases from−30 to 0 ps, while eBE increases clearly from 0 to 40 ps.
This characteristic behaviour is in agreement with theprevious study on PISC of a liquid sample [19]. Whenthe pump-probe delay time becomes several hundred
picoseconds, the pump photoelectrons are at far distantplaces from the liquid when the probe pulse creates photoelectrons. Therefore, PISC is dominated by the positive
charge remaining in the liquid at large pump-probe delay
time. This positive charge gradually dissipates with time,making PISC diminishing with time. Consequently, eKEtakes the minimum (for eBE, the maximum) at a certain
positive delay time [19]. Since the delay time scanned inthis study is only up to 40 ps, eBE shown in Figure 4(a) did
not reach the maximum. The magnitude of the eBE shift
is well over 1 eV at this moderate pump pulse intensity,
illustrating the significant influence of PISC.
Oloff et al. have shown that a simple mean field model
reproduces PISC rather well; more rigorous treatment

6

C. W. WEST ET AL.

using many-body trajectory calculations is considerably
more complex and time-consuming [34]. Thus, we haveattempted to correct the photoelectron time-energy map
using the mean field model, which takes into account thepositive charge. (See the SM for further details) Briefly,
the model calculates the kinetic and potential energies ofprobe electron at the instant of its generation and calculates the asymptotic kinetic energy at the analyser. Thisis readily justified for an electrostatic energy analyser.
In the case of a time-of-flight analyser, it measures theflight time instead of kinetic energy, so that, in principle,
we need to consider the time consumed for deceleration and acceleration of a probe electron in overtaking
pump electrons. However, its influence on the overall
flight time is negligible under our experimental conditions; thus, the numerical treatment is the same as thecase of electrostatic energy analyser. As seen in Figure4(b) after the correction, three valence bands of liquidwater are seen at energies consistent with the literature
values (1b1 : 11.31 eV, 3a1 (H): 13.08 eV, 3a1 (L): 14.47 eV,
1b2 : 17.41 eV in eBE) [58]. The positions of the aniline
peaks in the straightened spectra are also stable within±0.1 eV. However, the bands such as 1b2 at higher eBE are
not perfectly aligned against the pump-probe time delay
even in the corrected time-energy map.
Figure 5(a) displays the spectral region of the water
valence bands extracted from the experimental data in
Figure 4(b), in which the one-colour background has
been subtracted from the entire time-energy map. At
the time origin, the liquid photoelectron bands diminish their intensities, which is followed by the appearance of three positive bands indicated with red colour.
The binding energies of these positive peaks agree withthose of gaseous water (1b1 : 12.62 eV, 3a1 : 14.74 eV, 1b2 :18.51 eV) [65]. One possible cause of this enhanced signal is photo-induced evaporation of the surface layers ofliquid, increasing the density of gaseous water molecules
at the liquid-vacuum interface [66]. The water molecules
evaporated by the pump-induced heating will fly for only
a short distance from the microjet within tens of picoseconds, so that their photoelectron spectrum will not be
broadened or shifted away with the electric potential
applied to the liquid. An alternative explanation is thatthe correction against PISC using the mean field modelis imperfect for this moderate pump intensity and subtraction of the background spectrum created artificial
positive bands.
A minor detail in the data shown in Figure 5(a) is
an appearance of photoemission signal induced by 21ω;the liquid water 1b1 band created by 21ω is indicatedby ‘21ω’ in Figure 5(a). This particular data set was
obtained using the 19th-order harmonic (19ω) isolatedwith the multilayer mirror system, which was unable toreject the neighbouring order completely (the reflectivity of the neighbouring order radiation is roughly 10%of that for the main order at each mirror). The gratingbased monochromator isolates the 19ω probe pulse andavoids the contributions from residual neighbouring harmonics (17ω and 21ω). Therefore, the grating setup
was used in the main measurements to observe aniline pump-probe signals described below. With a timepreserving monochromator, photoemission signal was
identified only for the isolated single-order harmonic asshown Figure S3.
3.3. The pump-probe signal of aniline at low pumpintensity
In view of the complexity caused by PISC described in
the previous section, it is desirable to perform TRPES at
as low pump and probe intensities as possible to minimise
PISC, except that one is interested in electron dynamics under the extreme conditions driven by intense laser
pulses. It has been shown that the influence of PISC is
proportional to the charge density of the pump photoelectrons [19,35]. Figure 5(b) shows the pump-probe
signal in the eBE region of 1–5.5 eV extracted from experimental data measured at a pump pulse intensity of 0.3 μJ.
PISC has been corrected using the mean field model for
Figure 5(b). Since pump-induced photoionization of aniline is a two-photon process, a 60% reduction of thepump pulse intensity reduced PISC by approximately afactor of 3; the spectra measured at 0.3 μJ shows significantly less influence from PISC (0.26 eV shift at time
origin) than 0.5 μJ (0.77 eV shift). It is also noted nospectral contamination from neighbouring harmonics(as shown in Figure S3) was observed, because the grating
monochromator was employed for these measurements.
The main distribution is seen between 2.5 and 4.3 eV,
while a weaker distribution extends up to 5.5 eV. The
global fit of the time-energy map in this region provides
two time constants of τ1 = 760 ± 130 fs and τ2 > 100 ps
and the decay associated spectra (DAS) shown in Figure 6(see the SM for details). Both of these spectra have eBE
consistent with photoionization from the S1 state, inwhich the DAS associated with τ2 has a higher eBE thanthat associated with τ1 .
In order to confirm the results obtained by XUVTRPES at 0.3 μJ, we examined the UV pump (5.2 eV)– UV probe (3.8 eV) TRPES, which is free from PISC.
Figure 7 shows (a) the 2D time-energy map of the photoelectron spectra, (b) the decay profile, and (c) the decay
associated spectra obtained for aqueous 90 mM aniline
solution. Due to the reduced probe photon energy, only
features up to eBE of 3 eV were observed. The resultis explained using two time constants of ca. 1 ps andMOLECULAR PHYSICS

7

Figure 5. (a) The pump-probe signal calculated from Figure 4(b) by subtraction of one-colour background signal and correction against
PISC. This spectrum was measured using the multilayer mirrors to isolate 19ω (29.45 eV) XUV pulses, and some spectral contamination is identiﬁed as photoionization with the residual 21ω (32.55 eV) XUV pulses. (b) The pump-probe spectrum below 5.5 eV extracted
from experimental data measured for aqueous 90 mM aniline solution using a pump pulse intensity of 0.3 μJ and the grating-based
monochromator.

Figure 6. (a) Time-resolved photoelectron intensity of the pump-enhanced signal shown in Figure 5(b) in the region below 5 eV (black
circles), the ﬁt of the time-resolved intensity to the kinetic model explained in SM (red line) and the component traces from the ﬁt (blueand green). (b) The decay associated spectra for each component.

Figure 7. (a) Time-energy map of the pump-probe signal measured by UV pump (5.2 eV) – UV probe (3.8 eV) TRPES of aqueous 90 mM
aniline solution. The inset shows the enlarged view around the time origin. (b) Time-resolved photoelectron intensity of the pumpenhanced signal shown in (a). (c) The decay associated spectra for each lifetime.

> 100 ps, in reasonable agreement with the XUV-TRPES.
It is noted that eBE stays constant after 1 ps up to several hundreds of picoseconds. Since the objective of thisstudy is to examine XUV-TRPES and PISC, we have notpursued the highest precision in UV-UV experiment;

the cross-correlation time was only 300 fs. Nonetheless,the consistency with PISC-corrected XUV-TRPES was
confirmed.
We interpret τ1 to be the solvation time in the S1state after the ultrafast internal conversion from the S3

8

C. W. WEST ET AL.

state, and τ2 is the population lifetime of the S1 state.
Our result is consistent with the solvation time in liquidwater, which is known to be less than 1 ps and the previously reported lifetime of S1 aniline (1 ns) in aqueous
solution [51]. Transient absorption spectroscopy by Saito
et al. [51] identified hydrated electron formation within
their experimental time resolution (20 ps) for photoexcitation at 266 nm; the quantum yield was 0.18. It is thenlikely that hydrated electrons are also formed by 240-nm
photoexcitation; however, since the eBE of the hydrated
electron is 3.7–3.8 eV [14,17], its spectrum overlaps withthat of the S1 state of aniline. Therefore, these two contributions could not be differentiated by photoelectron
spectroscopy. Nonetheless, since the reported quantum
yield of hydrated electron is only 0.18, its contribution,
if any, is expected to be small. It is also pointed outthat CTTS reaction in aqueous solution generally undergoes ultrafast geminate recombination [67,68], and thatphotoemission signal exhibits rather fast decay. However,
the overall photoemission intensity shown in Figure 6(a)does not exhibit significant intensity reduction with time;the yield of hydrated electron is estimated to be small
also from this viewpoint. Thus, we ascribed the observed
photoelectron signal to the solvation dynamics of S1 aniline, although the contribution of hydrated electron is
not completely ruled out. Saito et al. also indicated thatthe triplet state is formed with a time constant of 3.6 ns,
which is far larger than the time range of our measurements [51]. It is likely that the T 1 state is not present atappreciable levels over the course of our experiment.
Despite the pump wavelength of 240 nm being resonant with the S3 ← S0 transition, no direct signal fromthe S3 state was identified in our spectra. The lifetimeof the S3 state in the gas phase has been measured as
50 ± 10 fs [46,47,49] and it is not unreasonable to assume
that the decay of this state in the solution phase is similar. Since our time resolution is 130 fs in the present
study, if the S3 state decays within 50 fs, it is possibly difficult to identify. The S2 state was not identified either inour study. It seems our observation is consistent with theresult reported by Poterya et al. [50], who investigated theN-H bond photodissociation upon 243 nm photoexcitation of aniline embedded on the surface of large water
clusters, (H2 O)430 , and found that dissociation via theS2 (π , σ *) excited state is largely suppressed in the cluster. This suggests that the involvement of the S2 state issuppressed in the solution phase.

4. Conclusion
The one-colour XUV photoemission intensity of aniline
in aqueous solution was found to be in good agreementwith the surface concentration of aniline estimated fromthe surface tension. Similar relation was also observed
between photoelectron intensity and surface concentration of phenol in aqueous solution. These results indicatethat XUV photoelectron spectroscopy has a small probing depth, and aniline and phenol molecules are segregated at the gas-liquid interface of an aqueous solution.
Our results suggests that the photoelectron signal in multiphoton ionisation photoelectron spectroscopy (MPIPES) of aqueous phenol solution [69,70] may also largely
arise from phenol molecules segregated at the gas-liquidinterface.
The segregation of aniline on the liquid surface makes
TRPES of aqueous aniline solutions to be highly sensitive
to the pump-induced space charge effect. We performed
TRPES using the 240 nm pump and XUV probe pulses. A
moderate pulse energy (0.5 μJ) of 240 nm was employedintentionally to examine time-dependent energy shifts.
Application of a simple mean field model enabled a fairly
reasonable correction of PISC; however, the correction
was unsatisfactory for detailed analysis of the ultrafast
dynamics of aqueous aniline solution. Thus, we analysed the experimental data obtained at the pump pulse
intensity of 0.3 μJ. The 240 nm pulse promoted aniline
to the S3 (π , π *) state, while a photoionization signal wasobserved only from the S1 (π , π*) state, populated byultrafast internal conversion from the S3 state. The result
suggests that the lifetime of the S3 state is considerably
shorter than our time-resolution (130 fs). The S1 state signal exhibited two time constants of sub-ps and > 100 ps,
in which the former is assigned to solvation dynamics
in the S1 state and the latter to the population decayof the S1 state. It is likely that photoexcited aniline also
decays by forming hydrated electron, which could not
be differentiated from S1 aniline because of their similareBE values. Nevertheless, we ascribed the observed signal
to S1 aniline, because of the absence of rapid photoemission decay expected for geminate recombination of ahydrated electron and aniline cation and the low quantum yield of hydrated electron previously reported. The
role of the S2 (π , σ *) state in aqueous photochemistry ofaniline could not be clarified in the present study.
A couple of improvements are highly desirable for
XUV-TRPES. One is to use a Yb-based laser system witha considerably higher repetition rate than a Ti:sapphirelaser system. Development of MHz XUV system has beenreported by several research groups [13,71–73], and its
application to XUV-TRPES of liquids will be promising.
The other is improvement of time-resolution. This is certainly possible by using ultrafast UV laser system basedon non-linear optical mixing in rare gases and optimisation of the time-preserving or compensating monochromator. Work is in progress in our laboratory in these
directions.

MOLECULAR PHYSICS

Disclosure statement
No potential conflict of interest was reported by the author(s).

Funding
This work was supported by JSPS KAKENHI [grant number
15H05753]. C. W. West was supported by a Research Fellowship P16036 awarded by the Japan Society for the Promotion of
Science.

ORCID
Junichi Nishitani
Toshinori Suzuki

http://orcid.org/0000-0003-1169-0433
http://orcid.org/0000-0002-4603-9168

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