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ArticleExploration of Gas−Liquid Interfaces for Liquid Water and Methanol
Using Extreme Ultraviolet Laser Photoemission Spectroscopy
Yo-ichi Yamamoto, Tatsuya Ishiyama, Akihiro Morita, and Toshinori Suzuki*
Cite This: J. Phys. Chem. B 2021, 125, 10514−10526

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sı Supporting Information*

ABSTRACT: We present a study using extreme UV (EUV) photoemission spectroscopy of the valence electronic structures of aqueousand methanol solutions using a 10 kHz EUV light source based on highorder harmonic generation and a magnetic bottle time-of-ﬂight electron
spectrometer. Two aspects of the observed spectra are highlighted in thisstudy. One is variation of the vertical ionization energy (VIE) for liquids
as a function of the solute concentration, which is closely related tosurface dipoles at the gas−liquid interface. The experimental results
show that the VIE of liquid water increases slightly with increasing
concentrations of NaCl and NaI and decreases with NaOH. The VIE ofliquid methanol was also found to change slightly with NaI. On the other
hand, tetrabutylammonium iodide (TBAI) and butylamine (BA) clearly reduce the VIE for liquid water, which is attributed to theformation of an electric double layer (EDL) by segregated solutes at the gas−liquid interface. As evidence for this, when the pH of
an aqueous BA solution is reduced to protonate BA, the VIE shift gradually decreases because the protonated BA moves into thebulk to suppress the inﬂuence of the EDL. We computed the surface potentials for these solutions using molecular dynamics
simulations, and the results supported our interpretation of the experimental results. Another observation is the variation of therelative energy and shape of individual photoelectron bands for solvents, which is related to alteration of the structure andconstituents of the ﬁrst solvation shell of ionized solvent molecules. All of the solutes cause changes in the photoelectron spectra athigh concentration, one of the most prominent of which is the degree of splitting of the 3a1 band for liquid water and the 7a′ bandfor liquid methanol, which are sensitive to hydrogen bonding in the liquids. The 3a1 splitting decreases with the increasing
concentration of NaI, NaCl, and NaOH, indicating that Na+ penetrates into the hydrogen-bonding network to coordinate to anonbonding electron of a water molecule. On the other hand, TBAI and BA cause smaller changes in the 3a1 splitting. Fullinterpretation of these spectroscopic features awaits extensive quantum chemical calculations and is beyond the scope of this study.
However, these results illustrate the strong potential of EUV laser photoemission spectroscopy of liquids for exploration of interfacialand solution chemistry.

■

INTRODUCTION
The inﬂuences of solute on the hydrogen bonding andelectronic structure of liquid water have been attracting muchinterest over the years.1−7 In this study, we explore thisproblem from the viewpoint of the electron binding energy(eBE) measured using extreme UV (EUV) photoemission
spectroscopy. Photoemission spectroscopy using a liquid
microjet technique was pioneered by Faubel and colleagues,8,9and considerable progress has been made since then.10−13 The
vertical ionization energies (VIE) for liquid water, methanol,and ethanol were initially reported to be 10.92, 9.99, and 9.66
eV,8 and these values have since been reﬁned to 11.31(4),149.70(7), and 9.52(7) eV,15 respectively. Rigorously speaking,however, none of the aforementioned values are for a puresolvent. Because liquid microjets are inevitably charged by anelectrokinetic charging eﬀect,16 a small amount of electrolyte isalways added into these solvents to mitigate this eﬀect.14,17
Furthermore, as will be described later, all energy calibration
methods for liquid photoemission spectra require the addition© 2021 American Chemical Societyof electrolytes. In our most recent He(II) photoemission
spectroscopy study of methanol and ethanol,15 the concentration of the electrolyte was only 1 mM, yet its addition was
indispensable for accurate measurements. Because the VIE fora solvent is unaltered by addition of a small amount of solute,
these measurements provide accurate estimates of the VIE forpure solvents. On the other hand, at high solute concentrations, the electronic structure of a solvent will be inﬂuenced
by the solute.18,19 Moreover, if a solute is segregated at thegas−liquid interface, the local solute density becomesconsiderably higher than the bulk value.20−23 Therefore, even
Received: May 30, 2021
Revised: August 26, 2021
Published: September 8, 2021

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Articlelevel at a position distant from the liquid. This third method
requires applying a bias voltage (<−10 V) to a liquid microjet
to upshift the entire electron kinetic energy (eKE) distribution
because electrons with energies close to 0 eV are neverdetected without a bias voltage. The bias voltage also ensuresthat the work function of the analyzer does not dictate thecutoﬀ energy. The energy diﬀerence between the cutoﬀ energy
and the measured eKE provides the absolute eKE withreference to the local vacuum level. Thus, the third method
enables measurement of photoemission spectra of liquids forarbitrary solute concentrations without using gaseous reference
samples. Very recently, Thürmer et al. presented a detailed
discussion on these calibration methods and indicated that the
VIE for a liquid is invariant with NaI addition, but it decreases
with tetrabutylammonium iodide (TBAI) addition.19
An important question regarding photoemission spectroscopy of liquids is its probing depth. As reviewed in theliterature,11,28 the probing depth for photoemission spectroscopy of liquids has not been established for an electron kinetic
energy lower than 100 eV, even for the most fundamental
liquid of water. This is due to the lack of precise estimation of
the electron scattering cross sections in liquids either
theoretically or experimentally in this energy region.11,29
However, the probing depth in this study is crudely estimatedto be on the order of a nanometer; therefore, our EUV
photoemission spectroscopy results are sensitive to, albeit not
exclusively, the gas−liquid interface. When solutes aresegregated at the surface, the photoemission signal is produced
unambiguously from several molecular layers near the liquidsurface. For example, we have previously performed time- andangle-resolved photoelectron spectroscopy (TARPES) of
charge transfer to solvent (CTTS) reactions from 1,3diazabicyclo[2.2.2]octane (DABCO) or TBAI to liquidwater. Clear photoelectron angular anisotropy was observedfor DABCO solution, indicating that the photoelectron signals
are from molecules segregated at the gas−liquid interface.21
On the other hand, photoelectrons from an aqueous TBAI
solution exhibited much weaker anisotropy, indicating that I−
resides at a larger depth than DABCO.21
The electronic structure and dynamics at the gas−liquidinterface have been investigated extensively by using nonlinear
optical spectroscopy approaches such as second harmonic
generation (SHG) and sum-frequency generation (SFG).30−33
In these methods, surface-selective observations were ensured
by focusing on the second-order nonlinear susceptibility that
becomes nonzero in the spatial region where inversion
symmetry is broken. On the other hand, when a strong local
electric ﬁeld is present at the gas−liquid interface, for example
by adsorption of charged amphiphilic compounds, theinversion symmetry is broken for a depth greater than a single
molecular layer. Then, the third-order nonlinear response
involving the local electric ﬁeld creates SHG and SFG signals
not only from the top molecular layer at the gas−liquidinterface but a deeper spatial region.34 Under these circumstances, the probing depths of nonlinear spectroscopy and
EUV photoemission spectroscopy are expected to be similar,
allowing complementary information to be obtained on the
structure and dynamics of the interfacial region.
In this study, we investigate the variation of the electronic
structure of liquid water and methanol using EUV photoemission spectroscopy. Liquid water is of paramountimportance in chemical, biological, and environmental
sciences, and it is the central target of our study. Methanol

a low bulk concentration can cause a large impact on theproperties at the gas−liquid interface probed by photoemission
spectroscopy. The magnitude and speciﬁcity of such inﬂuences
are of interest.
Before we discuss the photoemission spectra of aqueous andmethanol solutions, we would like to clarify the nomenclatureused in our discussion. We refer to the molecular orbitals
relevant to photoemission spectra using the orbital names forwater vapor and methanol molecules. This conventional
nomenclature is practical in that the energetic ordering ofmolecular orbitals is unchanged upon condensation. However,it should be kept in mind that this is not rigorous from theviewpoint of group theory, as the system is no longer
represented by a point group. For example, the splitting of
the 3a1 (HOMO−1) orbital of liquid water will be discussed
later, even though a1 is the notation used for a nondegenerate
orbital in an isolated water molecule; the splitting is due todonor−acceptor interactions caused by hydrogen bonding andconjugation of molecular orbitals.24 We refer to the generalkinetic energy distribution of electrons emitted by lightirradiation as the photoemission spectrum. The term photoelectron spectrum is speciﬁcally used to describe the
distribution of primary electrons emitted by the photoelectriceﬀect. All spectra discussed in this paper are of thephotoelectrons, except for a low-energy background signal.
An early study by Weber et al. using soft X-ray photoemission spectroscopy indicated that the photoemission
spectra of aqueous alkali metal halide (NaX) solutions areunchanged up to rather high concentrations (12 M for NaI).25
This notion, however, was questioned by Olivieri et al., whoshowed that the O(1s) ionization energy for liquid water varies
with the concentration of butylamine (BA).23 Pohl et al. haverevisited the photoemission spectra of aqueous NaI solution
for concentrations up to 8 M, and they showed that the 1b2
and 3a1 band positions relative to the 1b1 band vary only
slightly with electrolyte concentration;18 however, they did not
address the concentration dependence of the absolute
ionization energy for liquid water because of the technicaldiﬃculties described below. Thus, the claim by Olivieri et al.
has not been fully addressed.
Photoemission spectroscopy requires careful calibration of
the absolute kinetic energy of electrons. Calibration has mostoften been performed by using gaseous samples for which
ionization energies are precisely known. The gas calibration
method employs adjustment of the electrolyte concentration ofa solution, as discussed by Kurahashi et al., to equalize thevacuum levels between the ionization points for a liquid and areference gas.14 Because of this strict requirement, it was notpossible to perform precise measurements of the VIE for
arbitrary electrolyte concentrations. A similar calibration
method using a reference gas proposed by Olivieri et al. wasto apply a small bias voltage to a liquid microjet to adjust thevacuum levels between the ionization positions for a liquid anda gas,23 which in principle enables measurements at any
arbitrary concentration. However, the results so far obtainedwith this method have not been fully veriﬁed, and further
examination seems necessary. The third method is to use thelow-energy cutoﬀ of a photoemission spectrum of a liquid as
the reference energy,19,23,26 which has previously been
employed in photoemission spectroscopy of solid samples.27
An essential diﬀerence between this method and the ﬁrst two
methods is that the cutoﬀ energy corresponds to the local
vacuum level just outside the liquid rather than the vacuum
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Articleliquid photoemission spectrum. The energy resolution of theelectron energy analyzer and the overall energy resolution
including the EUV radiation bandwidth are estimated to be
0.05 and 0.1 eV, respectively, based on the photoelectron
spectra of gaseous samples. The photoemission spectrum of aliquid consists of a well-resolved valence band structure and abroad background that increases in intensity toward zeroeKE.48 This background signal is due to inelastically scatteredphotoelectrons, low-energy photoelectrons produced byintermolecular Coulombic decay,49 and secondary electrons
from electron impact ionization induced by photoelectrons
traveling in the liquid. The spectral shape of the backgroundcan be well expressed by using a Gaussian function. Thus, weanalyzed each spectrum by using least-squares ﬁtting based on
the valence bands and the broad background. A typical ﬁt isshown in Figure 1. The spectra presented later are those after

is a variant of water created by replacing a hydrogen atom witha methyl group; while liquid water has a tetrahedral hydrogenbonding network provided by a double proton donor anddouble proton acceptor structure, methanol has a onedimensional chain structure of hydrogen bonds with a single
donor and single acceptor conﬁguration. Unlike our previous
studies on these solvents using a hemispherical electron energy
analyzer (HEA) and (quasi-)continuous radiation from asynchrotron radiation facility24 or a He discharge lamp,15 weemploy a magnetic bottle time-of-ﬂight (MBTOF) electronenergy analyzer35 and a table-top EUV laser based on highorder harmonic generation (HHG),36−39 which are thestandard instruments in time-resolved photoemission spectroscopy (TRPES). HEA is superior to a MBTOF analyzer interms of energy resolution, especially at high electron kinetic
energies in the X-ray region. On the other hand, a MBTOF
analyzer oﬀers an extremely high electron collection eﬃciencyof over 50% and the capability of measuring the entire
photoemission spectrum on a shot-to-shot basis (the TOF
method requires a pulsed light source). These features make a
MBTOF spectrometer an ideal instrument for TRPES;
moreover, the combination of a MBTOF analyzer and a 10
kHz pulsed laser provides an extremely high data acquisition
rate. Our experimental results reveal that some solutes cause anenergy shift of the entire photoemission spectrum of a solution
and that all solutes induce some spectral changes at highconcentrations. To study the spectral energy shift, we compare
the experimental results with those for the liquid surface
potential computed by using classical molecular dynamics
simulations.40−43 To our knowledge, such a comparison has
not been attempted previously.

■

Figure 1. Photoemission spectrum of 50 mM aqueous NaCl solution
and least-squares ﬁtting results. The notations 1b1, 3a1, and 1b2
indicate the electron orbitals for water molecules with a C2v point
group. Bg indicates the background photoemission signal.

EXPERIMENTAL METHODS
All measurements were performed by using a MBTOF
spectrometer44 and a table-top EUV laser.45 We havedetermined the photon energy accurately to be 27.5 eVusing photoemission spectroscopy of Xe and nitric oxide. This
photon energy is of the ninth harmonic of the 400 nm driving
laser and is a typical probe photon energy in our EUV-TRPES
studies of aqueous solutions. We applied a negative bias
voltage of −10.0 V to a liquid microjet to shift the entire eKE
distribution into the region where a uniform transmission
eﬃciency through the spectrometer is ensured. We measured
the photoemission spectrum and determined the true eKE bytaking the diﬀerence between the measured kinetic energy andthe cutoﬀ energy. The cutoﬀ energy was determined by using
the inﬂection point found by taking the second derivative of
the photoemission spectrum in the cutoﬀ region (see theSupporting Information for more details). However, thespectrum thus measured contained signals from gaseous
species. Therefore, we also measured a spectrum by applying
a lower bias voltage, typically −35.0 V, to the liquid microjet toremove the signals from the gaseous species.15,19,46,47 This
provides a greater energy shift in the photoemission spectrumof a liquid, while photoelectrons from a gas undergo much lessacceleration because the electric potential outside the liquid
rapidly converges to 0 V at the entrance skimmer for the
MBTOF spectrometer. We applied a retarding potential of−25.0 V to the ﬂight tube inside of MBTOF to decelerate theincoming electrons and enhance the kinetic energy resolution.
The drawback of the latter method is a slightly lower accuracy
in determining the cutoﬀ energy. Thus, we combined these
two methods (−10.0 V or −35.0/−25.0 V) to analyze thesubtraction of the broad background. The analysis of the 1b2band is inevitably less accurate because of the overlap with thebroad background signal and the inﬂuence of inelastic
scattering eﬀects. Here we present the positions of 1b2 bands
but exclude them from our detailed discussion. We will focus
on the concentration dependence of the 1b1 band, which isoften used as an internal energy standard in photoemission
spectroscopy of aqueous solutions, and on the variation in the
3a1 band, which is sensitive to the hydrogen-bonding structureof liquid water.
A 10 kHz one-box Ti−sapphire regenerative ampliﬁer (35 fs,
800 nm (ω)) was used to produce EUV pulses. The
fundamental output of the ampliﬁer was converted to 2ω
laser pulses in a 0.3 mm thick β-barium borate (BBO) crystal
and focused into a Kr gas cell by using a quartz lens (f = 500mm) to induce HHG. A single-order harmonic was isolated byusing a time-preserving monochromator.50 The full width at
half-maximum (FWHM) of the EUV beam at the microjet was
about 100 μm. A continuous liquid microjet was introduced
into a photoionization chamber through a 25 μm i.d. graphitecoated fused silica capillary at a ﬂow rate of 0.5 mL/min. The
capillary was connected to a stainless steel nozzle assembly,
and a bias voltage was applied to the entire nozzle assembly.
The microjet discharged from the nozzle generally underwent
disintegration within a traveling distance of 10 mm owing to
ampliﬁcation of microscopic turbulence. The disintegrated
liquid droplets were collected with a liquid-nitrogen-cooled10516

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Articleand the real-space part of the Ewald sum were taken to be 10
Å. A simulation for at least the time period of 60 ns wasnecessary to reach an equilibrium state. The subsequent 10 ns
production run was conducted with 32 parallel computation
starting from diﬀerent initial conﬁgurations; hence, a total
ensemble average was taken for 320 ns. The GROMACS
package (ver. 5.1.2)61 was employed for these trajectory
calculations.
The surface potential (χ) is deﬁned as the diﬀerence in theelectric potential ϕ given by the following formula:trap. A SmCo permanent magnet with a soft iron cone wasplaced 2 mm away from the liquid microjet in thephotoionization chamber to provide a strong magnetic ﬁeldof about 1 T. A weak magnetic ﬁeld (1 mT) was applied to theﬂight tube along its ﬂight axis by using a solenoid coil (300turns/m, 3 A). The combination of the strong and weakmagnetic ﬁelds provided a magnetic bottle eﬀect to collect
more than 50% of the photoelectrons emitted from the liquid.
The entire photoionization chamber and the energy analyzer
were magnetically shielded against the terrestrial magnetic ﬁeld
with a permalloy inner layer placed inside the vacuum. A
variable retardation potential was applied to the ﬂight tube to
decelerate photoelectrons, and the electrons were detected byusing a Chevron microchannel plate (MCP) with a 38 mm
eﬀective diameter and a preampliﬁer, and electron counts were
integrated by using an analog-to-digital converter. The electron
ﬂight length was 1.2 m. A more detailed description of the
MBTOF spectrometer can be found in a previous paper.44

χ = ϕ(z1) − ϕ(z 0) = −z1∫z dz′ Ez(z′)0Ez(z) =

1
ε0

(1)

z

∫z dz′ ρq (z′)0(2)

where z0 and z1 represent arbitrary positions in the vacuum
and the liquid, respectively, so that the integral of eq 1
encompasses the interfacial region. ρq(z) and Ez(z) are thecharge density and the normal component of the electric ﬁeld
at z, respectively, and ε0 is the permittivity of free space. The
absolute value of χ depends on the computational method andthe molecular potential; for example, an estimation based on
ab initio calculations predicts χ for liquid water to be +3.1 V,
while a classical simulation based on atomic partial point
charges yields about −0.5 V.62 However, in this study, we
focused on the relative shift of χ from the value for pure waterby addition of solutes, which is free from any ambiguity in theabsolute value of χ. The orientation distribution for watermolecules is deﬁned as ρ⟨cos θ⟩, where ρ is the number
density for water molecules and ⟨cos θ⟩ is the average direction
cosine (θ is the angle between the OH bond and the surface
normal unit vector). When this value is negative, the OH bondis pointing toward the bulk on average.

■

COMPUTATIONAL METHOD
We performed molecular dynamics simulations on aqueous
solutions using the SPC/E model7 for water. We employed thegeneral AMBER force ﬁeld51 for TBA+, BA, and BAH+
(protonated BA) with partial charges determined by a
restrained electrostatic potential ﬁt52 to the potential calculated
at the B3LYP/6-31G(d, p) level of theory and the force ﬁeld
model for ions compatible with the SPC/E model (the partial
charges employed for simulations are listed in the Supporting
Information).53 Initial conﬁgurations were prepared in thefollowing way. A rectangular simulation box was prepared witha slab geometry. The box dimensions Lx × Ly × Lz were 40 ×
40 × 250 Å3 for a system with 3000 water molecules and 40 ×
40 × 370 Å3 for that of 6000 water molecules, and the interface
was set parallel to the x−y plane. Initially, water and solute
molecules were placed at random positions and with randomorientations in the middle of the simulation cell (see Figure 2)■

RESULTS
Aqueous NaX Solutions (X = Cl, I, and OH). Figure 3
shows the photoelectron spectra of aqueous solutions of (a)NaCl, (b) NaI, and (c) NaOH at various concentrations. The
signal intensity in each spectrum is normalized by the 1b1(HOMO) band for liquid water. The axes at the bottom and
top indicate the photoelectron kinetic energy (PKE) and theeBE, the latter being the diﬀerence between the photon energy
and the PKE. These strong electrolytes completely dissociateand dissolve in an aqueous solution, and in the case of NaI,
iodine is known to have a higher surface density compared tothe bulk value.6 In all cases, the position of the 1b1 band in thephotoelectron spectrum changed only slightly with concentration. The relative band positions observed for aqueous NaI
solutions agree with Pohl et al.18 The intensities of the 3a1 and1b2 bands increase and the 3a1 bandwidth clearly decreases
with increasing the NaI concentration. The band associatedwith I− is well separated from those for liquid water, indicatingclear spin−orbit splitting (5p 2P3/2 and 2P1/2). The photoelectron spectra of aqueous NaOH solution, shown in Figure
3c, exhibit a similar concentration dependence, except that thevariation in the intensity of the 1b2 band is smaller than thatfor aqueous NaCl and NaI solutions, and the OH− band is
weaker than the Cl− and I− bands for a given concentration. Itshould be noted that the eﬀect of NaOH is slightly diﬀerent
from that of NaCl and NaI in that it changes the pH of thesolution; for example, addition of 1 M NaOH changes the pHfrom 7 to 14.

Figure 2. Snapshots of initial conﬁguration (top) and conﬁguration
after equilibration (bottom) of aqueous TBAI solution containing
3000 water molecules and 50 solute molecules (ion pairs).

by using the PACKMOL package.54 The steepest descentmethod was used to remove initial overlaps of molecules.
Newton’s equation of motion was solved by using the leapfrog
algorithm,55 with a time step Δt = 2.0 fs with threedimensional periodic boundary conditions, where the bonds
with H were treated by using a linear constraint solver.56 In theequilibration step, a constant number, volume, and temperature (NVT) ensemble simulation was carried out, where thesystem temperature was kept at T = 298.15 K by using a
Nosé−Hoover thermostat.57,58 The long-range Coulombic
interactions were treated by using the particle-mesh Ewald
method.59,60 The cutoﬀ length of the Lennard-Jones potential
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Articledetermined in this analysis. The VIE for liquid water, shownin Figure 4a, shows only a slight dependence on theconcentration of electrolytes. With increasing concentration,
NaOH causes a slight reduction in VIE, while NaCl and NaI
cause a slight increase, although the changes are comparable tothe experimental energy resolution. The results conﬁrm that
the VIE of liquid water (11.3 eV) is the most convenientinternal energy standard for photoemission spectroscopy of
aqueous solutions at low concentrations (<1 M). The precise
determination of the positions of solute bands is rather diﬃcult
due to their small intensities and spectral overlap with theintense 1b1 band; however, our results suggest that they shift
only slightly with the concentration.63
The relative energies for the 3a1 and 1b2 bands with respect
to the 1b1 band are summarized in Figures 4c and 4d,
respectively. In the 3a1H orbital, there is an in-phaseinteraction between the 3a1 orbital of the ionized moleculeand the nonbonding orbitals (3a1 and 1b1) of proton acceptor
molecules in the ﬁrst hydration shell. In the 3a1L orbital, there
is an out-of-phase interaction with the orbitals of the proton
acceptors; therefore it is less stable. The experimental results
indicate that VBE of 3a1H is almost invariant with theconcentration, while 3a1L gradually shifts to higher eBE (moreenergetically stabilized). Consequently, the degree of 3a1 band
splitting decreases with increasing solute concentration in allcases. Figure 5 shows the dependence of the FWHM of the

Figure 3. Photoelectron spectra showing valence bands for aqueous
solutions of (a) NaCl, (b) NaI, and (c) NaOH at various
concentrations. The signal intensity of each spectrum is normalized
to the 1b1 (HOMO) band of liquid water.

We performed least-squares ﬁtting of these spectra byassuming Gaussian functions for each band; the representativeﬁts are seen in Figure 1. As described in the Introduction, the
3a1 band for liquid water consists of two components (3a1H
and 3a1L), which are split as a result of hydrogen bonding.24 In
the ﬁtting process, we assumed that the 3a1H and 3a1L bands
have the same Gaussian shape and intensity (Figure 1). The
uncertainties in the band positions estimated by using theleast-squares ﬁtting were typically tens of meV, which weremuch smaller than the spectral resolution (0.1 eV). Figure 4
summarizes the estimated shifts of photoelectron bands

Figure 5. Solute concentration dependence of FWHM for 1b1,
anions, 3a1, and 1b2 bands. The FWHM values estimated for the 1b2band were all ±0.065 eV, but for clarity the error bar is only shown forthe 6 M case.

photoelectron bands on the solute concentration. The 1b1band exhibits an increase in FWHM with increasing
concentration for all electrolytes. The 3a1 band exhibits adecrease in FWHM. We examined the validity of the FWHM
estimated for the 1b1 and 3a1 bands by evaluating theirintegrated intensities (areas). We found that the 3a1/1b1 ratio
diminishes to 83% at 7M compared to low concentration

Figure 4. Photoelectron band positions for (a) 1b1, (b) anions, (c)3a1, and (d) 1b2 at various solute concentrations. VIE: vertical
ionization energy; VBE: vertical electron binding energy. The
experimental uncertainty is ±0.05 eV.
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owing to the continuous growth of the 1b1 bandwidth; theintensity of the 3a1 band remained almost constant. Thus, it islikely that the growth of the FWHM of 1b1 is overestimated inour results.
Figure 6 shows the dependence of the photoelectron band
intensity (integrated area estimated by the curve ﬁtting) for

Articlecomparison of the experimental results with molecular
dynamics simulations provides useful insights for understanding interfacial chemistry.
Aqueous Solution of Butylamine (BA). As described in
the Introduction, Olivieri et al. investigated the soft X-ray
O(1s) photoelectron spectrum of an aqueous 0.05 M NaClsolution with a small bias voltage (a few volts) applied to a
microjet.23 The purpose of the bias voltage was to compensate
for any inﬂuence of the potential diﬀerence around themicrojet. When the vacuum level around the jet becomes ﬂat,the O(1s) photoelectron band for water vapor can be used forenergy calibration of the photoemission spectrum of liquidwater. Using this procedure, they found that the O(1s) VBE
for liquid water was 538.21 eV. An interesting ﬁnding by
Olivieri et al. was that addition of 0.15 M BA shifted the O(1s)band for liquid water to 537.78 eV, from which they
determined that the ionization energy for an aqueous solution
depends on the composition of the solution.23 The observed
shift was striking because the O(1s) orbital is not expected to
be much involved in intermolecular interactions. This led tothe question of why the band was shifted.
Shown in Figure 7a are EUV photoelectron spectra of a 0.05
M aqueous NaCl solution with BA added at various

Figure 6. Solute concentration dependence of photoelectron intensity
for Cl−, I−, and OH− ions. The intensities are normalized to that of I−
at 7 M.

anions on the solute concentration; as mentioned earlier, theseband intensities were normalized by the 1b1 (HOMO) bandfor liquid water. The OH− band intensity is clearly weaker thanthose of I− and Cl−. The weakness may be partly due to thelower surface activity of OH− than that of Cl− and I−; however,a smaller photodetachment cross section for OH− is anotherlikely cause. Although the photodetachment cross sections forthese anions are unknown in aqueous solutions, they may becrudely estimated from the values in the gas phase64−66 andthose for isoelectronic rare gas atoms.67 The cross sections
thus estimated for I−, Cl−, and OH− are 70, 30, and 10 Mb for
a PKE of 10 eV, respectively. Therefore, it is possible that the
weaker photoelectron signal associated with OH− ions is dueto the smaller photodetachment cross section. The diﬀerencein photoelectron angular anisotropy plays a negligible role in
the above discussion, as we employed a MBTOF spectrometer.
Ishiyama and Morita performed molecular dynamics
simulations of the gas−liquid interface for aqueous NaI and
NaCl solutions using a ﬂexible polarizable water model.43
Their calculations indicated that the surface potential for thesesolutions is about −0.4 V, which is almost the same as thevalue for pure water (−0.421 V). The results of EUV
photoemission spectroscopy presented here are consistent
with their calculations. Close examination of the experimental
results, however, suggests that NaI and NaCl induce a small
upshift of VIE of liquid water while NaOH provides an
opposite trend, although these shifts are detectable only at highconcentrations. These tendencies are consistent with surface
enrichment of I− and Cl− and a small depletion of OH−
density with respect to Na+;68 thus, the directions of surface
dipoles due to an electric double layer (EDL) are diﬀerent
between I−/Cl− and OH−. As presented later, the observed
eﬀect of NaI is also opposite from that of TBAI which createsan opposite direction of the surface dipole. The results showthat the photoemission spectroscopy of liquids is sensitive
enough to detect EDLs at the gas−liquid interface, and a

Figure 7. (a) Photoelectron spectra of 0.05 M aqueous NaCl solution
with diﬀerent BA concentrations. (b) Example of subtraction of BA
signal for 0.5 M BA solution. The photoelectron spectrum of BA in
solution (blue) was estimated by convoluting the spectrum (red) of
BA vapor with a Gaussian function and subtracted from the overall
spectrum of the solution. See the text. (c) Spectra after subtraction of
BA and background photoemission signals.

concentrations. A small amount of NaCl was dissolved in thesolution to stabilize the streaming potential of the liquid
microjet. It can be seen that the VIE for liquid water clearlydecreases with increasing BA concentration and is reduced by
0.4 eV for a BA concentration of 0.15 M. This result is
consistent with that of Olivieri et al., who found that the O(1s)photoelectron band position depended on the BA concentration, indicating that the entire photoemission spectrum
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Articlebe seen that all VBE values decrease with increasing BA
concentration. These shifts seem to be almost proportional tothe interfacial BA concentration, estimated from the photoelectron intensity, shown in Figure 8b, and they graduallysaturate. BA is an amphiphilic compound, and it is expected tosegregate at the gas−liquid interface. The relative shift fromthe 1b1 band position is plotted in Figure 8c, and the FWHM
values obtained by ﬁtting are shown in Figure 8d.
Given that BA is segregated at the gas−liquid interface,photoelectrons emitted from the solvent water may beinelastically scattered by this interfacial BA. Although thiscannot be ruled out, inelastic scattering would be expected toreduce the PKE and increase the apparent eBE at higher BA
concentration, which is opposite to what we observe.
Therefore, the upshift of the PKE with increasing BA
concentration is not due to scattering but is an electronic
eﬀect. We performed molecular dynamics simulations for a BA
aqueous solution, in which the bulk BA concentration was
assumed to be 10, 20, or 24 mM, and the surface concentration
was calculated to be ∼5 M. For these bulk concentrations, thesurface potential was estimated to be −0.66, −0.68, and −0.74
V, which are greater than that for pure water (−0.63 V) by−0.03, −0.05, and −0.11 V, respectively. The magnitude andpolarity of the computed surface potentials are in reasonableagreement with the −0.1 eV shift in VIE experimentallyobserved for a BA concentration of 50 mM.
Because BA is an amine, it is of interest to change the pH toproduce the protonated form (BAH+) and examine its
inﬂuence. The pKa value for a conjugated acid of BA in liquidwater is 10.8. As shown in Figure 9, the pH of an aqueous

including the inner-shell and valence bands for liquid water isshifted by the potential of the solution. It is also noted herethat the probing depth of photoemission spectroscopy depends
on the electron kinetic energy. Olivieri et al. employed aphoton energy greater than 800 eV, which provided a PKE of
about 300 eV, while our experiment gave a PKE of about 20
eV. According to previous estimates, the electron attenuation
length in liquid water is diﬀerent by at least a factor of 2−4 foreKEs of 20 and 300 eV.11 However, a similar eBE shift is seen
independent of the probing depth because the surface dipole
moment created by segregated BA is located outside the bulk
solution, as shown in Figure 15.
For a more detailed spectroscopic analysis of solvent waterin aqueous BA solutions, we attempted subtracting thespectrum of hydrated BA from the observed photoelectron
spectra of BA solutions. Because the photoelectron spectrumof hydrated BA cannot be measured separately, we estimated itfrom the spectrum of gaseous BA in the following way. As seenin Figure 7a, the ﬁrst photoelectron band of hydrated BA is
isolated at VBE of about 9 eV. Therefore, we measured thephotoelectron spectrum of gaseous BA, as shown in Figure 7b,
and compared the spectral bandwidth of the ﬁrst band between
aqueous BA solution and gaseous BA. This comparison
indicated that the spectral broadening in solution is expressedwith a Gaussian function with a FWHM of 1 eV. Thus, weconvoluted the spectrum of BA vapor with the Gaussian topredict the entire photoelectron spectrum of hydrated BA as
shown in Figure 7b. (The relative band positions andintensities were assumed to be unaltered by solvation.) Finally,
we subtracted this spectrum from the photoelectron spectrumof the BA solution to extract the spectral features of solvent
water as shown in Figure 7c. The band positions of liquidwater thus estimated are summarized in Figure 8a, where it canFigure 9. Calculated (a) pH value and (b) fraction of BAH+ as afunction of HCl concentration.

solution of 0.5 M BA and 50 mM NaCl is expected to decrease
from 11 to 6 as the HCl concentration is increased from 0.1 to
0.5 M, and the fraction of BAH+ in an aqueous solution of 0.5
M BA and 50 mM NaCl is expected to increase from 0.2 toabout 1.0 as the HCl concentration is increased from 0.1 to 0.5
M. Figure 10a shows photoelectron spectra measured forvarious HCl concentrations. The spectra shown in Figure 10b
were obtained by subtracting the contributions of BA andBAH+ by least-squares ﬁtting. In our analysis, BA and BAH+
were not diﬀerentiated, and the bands were ﬁt by shifting the
aforementioned broadened photoelectron spectrum of gaseous
BA. As seen in Figure 10c, the VIE for both liquid water and
BA gradually increases with increasing HCl concentration. The

Figure 8. (a) VBE measured for diﬀerent bands of aqueous BA
solutions. 50 mM of NaCl was added. (b) BA band intensity relative
to that for 1b1. (c) Band position relative to that for 1b1. (d) FWHM
for each band. The experimental uncertainty is ±0.05 eV, which is the
instrumental resolution.
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Figure 10. (a) Measured photoelectron spectra of aqueous solutionsof 0.5 M BA and 50 mM NaCl as a function of HCl concentration.
(b) Spectra in (a) after subtraction of solute and background signals.
(c) Spectral shifts of BA/BAH+ and 1b1 bands at each HClconcentration. (d) Band positions relative to 1b1 band. (e) FWHM
of individual bands. In (c−e), the error bars are indicated only when
the ﬁtting errors exceed ±0.05 eV; otherwise, the experimental
uncertainty is ±0.05 eV, which is the instrumental resolution.

ArticleFigure 11. Variation of photoelectron spectrum of aqueous TBAI
solution. (a) Photoelectron spectra, (b) VBE for individual bands, (c)relative energy shift from 1b1 band, and (d) FWHM. In (b−d), theexperimental uncertainty estimated from the instrumental resolution
is ±0.05 eV. The estimated values vary smoothly with concentration,although the values for 20 and 30 mM are slightly oﬀ. The
photoelectron band of TBA+ is anticipated at around the 1b2 band,
but it is not considered in the ﬁtting. Therefore, the 1b2 band is
excluded from these plots.

relative band positions shown in Figure 10d reveal that thedegree of 3a1 band splitting decreases with increasing HClconcentration. The BA band position shifts slightly withincreasing HCl concentration, which is ascribed to protonationof BA. The FWHM’s calculated by the least-squares ﬁtting arealso shown in Figure 10e.
Aqueous Solution of Tetrabutylammonium Iodide(TBAI). An aqueous TBAI solution is known to form EDL ofsegregated TBA+ and dragged I− ions at the gas−liquidinterface. N(1s) photoemission spectroscopy of aqueous TBAI
solution revealed that the density of TBA+ is 66 times higher at
the gas−liquid interface than that in the bulk,20 and the I−
photoemission intensity for an aqueous TBAI solution was
found to be 70 times higher.22 Figure 11a shows photoelectron
spectra of aqueous TBAI solutions for various concentrations
up to 50 mM. Given the above-mentioned surface density
increase of about 70 times, the highest bulk concentration of
50 mM gives a surface TBA+ concentration of 3.5 M. The
spectra clearly indicate that the VIE for liquid water decreases
with increasing TBAI concentration. At similar concentrationsof 3−5 M, an aqueous NaI solution does not exhibit as large a
VIE shift as seen here. An aqueous TBAI solution exhibits aphotoelectron band due to TBA+ in the vicinity of the 1b2band, which makes spectral analysis of this region more
complex and less reliable. We focus on the I−5p(2PJ), 1b1, and3a1 bands here, similarly with other cases. Figure 11b shows
the absolute VBE values for these bands. All bands shift tolower VBE values with increasing TBAI concentration,
indicating that the entire photoemission spectrum is shifteddue to the TBAI EDL. These shifts become saturated as the I−
density in the interfacial region saturates. To examine theindividual band shifts, Figure 11c presents the relative band
positions measured from the 1b1 band. Figure 11d shows the
FWHM of these bands. Although the values at 20 and 30 mM

are slightly oﬀ, the estimated values change smoothly with theconcentration.
The observation of a spectral shift for aqueous TBAI
solutions warrants reconsideration of our previous TRPES
results for CTTS reactions in aqueous TBAI solutions.20,21
Previously, we compared TARPES results for CTTS reactions
in aqueous 0.5 M NaI and 50 mM TBAI solutions and foundthat the eBE values in the latter were much lower than in theformer.21 We then investigated CTTS in aqueous TBAI
solutions in more detail for a wider concentration range andfound that the dynamics in the TBAI concentration range of
0.1−1 mM were essentially the same as those in NaI solutions
but that the CTTS kinetic rate constants and the eBE values
for the CTTS state and hydrated electrons (eaq−) change
dramatically at higher concentrations.20 We attributed the
reduction in the VBE for eaq− to electron trapping in regionswith high solute concentration. However, the present study
indicates that the entire photoemission spectrum of anaqueous TBAI solution is shifted, including the bands forliquid water and I−. Thus, the spectral shift for eaq− in aqueous
TBAI solutions at high concentration is most likely due to theeﬀect of the surface dipole moment.69 The clear photoemission
signal observed for I− indicates that the probing depth of our
EUV photoemission spectroscopy is comparable to or greaterthan the average depth of I−. Because the electric potential of
the liquid steeply changes between these layers of TBA+ andI−, chemical species located at smaller depths than I− will
exhibit smaller VBE shifts than that of I−; the VBE shift of10521

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ArticleTBA+, measurable by using N(1s) photoemission spectroscopy, is possibly smaller than that of I−. As shown in Figure 12,

Figure 12. VBE shift for I−, 1b1 of water, and hydrated electron bands
in aqueous TBAI solutions with various concentrations. The values
for hydrated electron are taken from ref 20.
Figure 13. Variation of photoelectron spectrum of aqueous sodium
butyrate solution. (a) Photoelectron spectra, (b) relative energy shift
from 1b1 band, and (c) FWHM. Pale color symbols in (b) are band
positions for NaX (X = Cl, I, OH). In (b, c), the error bars areindicated only when the ﬁtting errors exceed ±0.05 eV; otherwise, theexperimental uncertainty is ±0.05 eV, which is the instrumental
resolution.

the VBE shifts observed for 1b1 of liquid water and eaq− 20 are
similar. The results suggest that the observed signals of waterand eaq− do not originate from the region at much smaller
depths than that of I−. We are currently revisiting the CTTS
reaction in aqueous TBAI solutions using EUV-TRPES to
conﬁrm the VBE shift of eaq− determined by UV-TRPES.
Aqueous Sodium Butyrate Solution. Because TBA+ andBA are cationic and neutral compounds, it is interesting to
examine anionic compounds. Sodium butyrate (SB) dissociates
into Na+ and butyrate anions. The photoelectron spectra ofaqueous SB solutions are shown in Figure 13a. The results
indicate that the spectral shift is very small. As seen in Figure
13b,c, the inﬂuence of this solute on the photoelectron spectraof liquid water is quite small.
Methanol Solutions of NaI. Figure 14a shows photoelectron spectra of methanol solutions of NaI up to aconcentration of 3 M. The 7a′ band reveals the mostprominent spectral variation with changing concentration,
reminiscent of the 3a1 band of liquid water. Thus, we analyzed
the 7a′ band as a combination of two split bands. Similar to the
1b2 band of liquid water, the experimental result on the 5a′band is less accurate because of an overlap with a low-energy
background, so that we exclude it from our discussion and only
indicate its estimated band position. The 6a′/1a″ band is taken
into consideration to evaluate the 7a′ band positions
accurately, in which the 6a′/1a″ band is approximated as asingle band. More detailed discussion about the valence bandsof liquid methanol can be found elsewhere.15 We performed
least-squares ﬁtting of the spectra by assuming the same
Gaussian shape for both of the split 7a′ bands. Figure 14b−fshows the concentration dependence of the photoelectronband position obtained by this analysis. The VIE value
determined from the 2a″ band position was almost constantfor diﬀerent NaI concentrations (Figure 14c). The degree of
splitting of the 7a′ band decreases with increasing concentration, similar to the 3a1 band for liquid water; however, bothof the 7a′H and 7a′L shift in this case (Figure 14d). The
FWHM values estimated by the least-squares ﬁtting haverelatively large uncertainties, and they are indicated as coloredbands in Figure 14g. However, the ratio of the integrated

intensities 7a′/2a″ estimated by our analysis was constant in
the entire concentration range, so that the results arereasonable from that point of view. The low solubility of
NaCl in methanol prevented us from performing similar
measurements, so the eﬀect of the halogen anions could not be
examined for methanol.

■

DISCUSSION
Overall Energy Shift. When the surface potential χ in eq 1
moves toward the negative direction, electrons emitted fromthe liquid surface gain an additional kinetic energy uponejection into the vacuum. This energy shift is the same for all
electrons irrespective of the nature (inner-shell or valence) ofan electron orbital or the emitter (solute or solvent), except forphotoelectrons generated above the EDL. Our EUV photoemission spectroscopy results for the valence bands of liquidwater support the earlier observations by Olivieri et al. that theO(1s) photoelectron energy is shifted due to the addition of
BA. The present study demonstrates that these energy shifts
are consistent with the surface potentials computed by using
molecular dynamics simulations.
Strong electrolytes, such as NaCl and NaI, do not induce
appreciable energy shifts in the photoemission spectrum evenat high concentrations on the order of a few mol/L. Solutes
such as TBAI and BA, segregated or preferentially adsorbed at
the interfacial region, cause an energy shift that is almost
proportional to the surface solute density. To clarify the originof these surface potentials, we present a few representative
depth proﬁles of solutes in Figure 15a−f and calculated surface
potentials in Figure 15g, h. In the case of BA, its alkyl group ishydrophobic, and it induces segregation of BA at the liquidsurface with NH2 embedded in the bulk and the alkyl group
exposed to the vacuum. Thus, the dipole moment of BA (0.294

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ArticleFigure 15. Computed structures and electric potential at the gas−liquid interfaces of aqueous 24 mM BA and 49 mM TBAI solutions.
(a, b) Snapshot of molecular dynamics simulation at the gas−liquidinterface, (c, d) depth proﬁles of solutes, (e, f) enlarged view of depth
proﬁles, (g, h) electric potential, and (i, j) ρ⟨cos θ⟩ computed for
aqueous BA and TBAI solutions, respectively. The dashed vertical line
indicates the Gibbs dividing surface. In (a, b), H, O, C, N, and I−
atoms are indicated in white, red, cyan, blue, and green, respectively.

Figure 14. (a) Photoelectron spectra of methanol solutions of NaI.
Concentration dependence of spectral shift for (b) I− (light blue), (c)2a″ (black), (d) 7a′ (red), (e) 1a″ and 6a′ (blue), and (f) 5a′ (green).
(g) Variation of FWHM with concentration. The FWHM values areassociated with relatively large uncertainties, and they are indicated ascolored bands. In (b−f), the error bars are indicated only when theﬁtting errors exceed ±0.05 eV; otherwise, the experimental
uncertainty is ±0.05 eV, which is the instrumental resolution.

solution, each water molecule interacts with at least one of Na+
or I−. Thus, a fraction of intact water molecules with a double
donor and double acceptor conﬁguration considerably
diminish at high concentrations; the splitting of 3a1 nevervanishes while its splitting magnitude changes in alteredhydrogen-bonding structures. The change of intermoleculardistances is another possible source for variation of the 3a1
splitting width that is caused by intermolecular orbital
interactions. However, molecular dynamics simulations seemto indicate that the radial distribution functions of the O−O
distance in an aqueous NaCl solution is not noticeably alteredfrom that in pure water.43,72 A full interpretation of theconcentration dependence of the photoelectron spectra ofsolutions requires detailed quantum chemical simulations ofsolute−solvent interactions and photoemission processes,
which is beyond the scope of this study. Previously, Pohl etal. performed theoretical simulations of photoelectron spectraof aqueous NaI solutions by using quantum chemicaltreatment of an ionized water molecule and its ﬁrst hydration
shell surrounded by a dielectric continuum.18 Further extensive
quantum mechanical simulations of photoemission of solutions
are indispensable for extracting as much information as
possible from the experimental results.
TBAI induced much smaller changes in the 3a1 band
splitting. One possible explanation for it is that TBA+ has aweaker inﬂuence than Na+ on the hydrogen-bonding structureof liquid water. Another possibility is that there is a large
contribution of the photoelectron spectrum of liquid waterfrom the deeper region, where TBAI concentration is much
lower than that in EDL. The clear observation of thephotoelectron signal for I− indicates that the probing depthof EUV photoemission spectroscopy is comparable with or
greater than the width of EDL. (Olivieri et al. estimated theD) creates an electric ﬁeld that is a primary source for thenegative potential of the liquid. When HCl is added to an
aqueous BA solution, BA is protonated, and its spatial
distribution extends into the bulk. The Cl− counterions are
distributed beneath the BAH+. The EDL orients the watermolecules in this region, which compensates in part for the
dipole moment created by the EDL. The orientational
distribution of water molecules is shown in Figure 15i, j, andthe degree of orientation is seen to be stronger in aqueous
TBAI than in a BA solution.
Band Shift and Splitting. EUV photoemission spectra ofsolutions revealed a clear variation of the relative energy andshape of photoelectron bands as a function of soluteconcentration. Because the photoelectron bands for solutes
are very weak and are only observable in the eBE region lower
than the VIE for the solvent, only a small number of solutebands could be observed. Nevertheless, energy shifts wereidentiﬁed for the photoelectron bands for solutes.
The photoelectron spectra of aqueous NaCl, NaI, andNaOH solutions exhibited a reduced splitting width of the 3a1
band. A similar feature was also seen for the 7a′ band of liquidmethanol in NaI solution. As a solute concentration increases,
a greater number of solvent molecules are involved in thesolvation shell of ions. In an aqueous NaI solution, forexample, each Na+ ion binds 5−6 water molecules on averagein its ﬁrst hydration shell70 and similarly each I− ion has 6−7
water molecules;43,71 therefore, in the aqueous 5 M NaI
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Articlenot precisely known and is estimated to be about a nanometer.
When the probing depth is determined more accurately,
photoemission spectroscopy will provide further insights into
interfacial molecular structures and distributions. Previously,
we reported TARPES using UV probe pulses. Although
TARPES using EUV radiation is technically rather demanding,it will be highly useful for studying gas−liquid interfaces. The
use of a tabletop soft X-ray laser will also expand theopportunities of this approach.

inelastic mean free path (IMFP) of an electron in aqueous NaI
solution for eKE between 65 and 1500 eV, in which IMFP was
estimated to be less than 1 nm in the energy region less than
100 eV.73) We speculate that both of the above two factors areoperative here. In the case of BA, BA resides outside of liquid,so that BA shifts the entire photoemission spectra while theinﬂuence on the spectral features of liquid water is weak.
In this study, we have discussed the valence bands of thesolvents and solutes by subtracting the low-energy backgroundsignal from the observed spectrum, as shown in Figure 1.
However, it is worth noting that the relative intensity of thelow-energy background signal to the valence bands increased
with a solute concentration in all cases (e.g., Figures 7a and10a). This fact implies that the presence of solute alters theeﬃciency of dynamical processes, such as elastic/inelastic
scattering and intermolecular Coulombic decay, responsible forthe background signal. Explanation of the concentrationdependent background signal will be challenging but helpful
for deepening our understanding of electron transport inliquids.
The inﬂuence of ions on the hydrogen-bonding network is
phenomenologically related to the Hoﬀmeister series for
precipitation of proteins and the arguments of structure
maker and breaker for liquid water. It is of great interest to
further explore ion−solvent interactions and also hydrophobic
hydration by using photoemission spectroscopy and quantum
chemical calculations.

■

ASSOCIATED CONTENT* Supporting Information
sı

The Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acs.jpcb.1c04765.

■

Energy calibration method and molecular dynamics
simulation (PDF)

AUTHOR INFORMATION

Corresponding Author

Toshinori Suzuki − Department of Chemistry, Graduate
School of Science, Kyoto University, Kyoto 606-8502, Japan;
orcid.org/0000-0002-4603-9168; Email: suzuki@
kuchem.kyoto-u.ac.jp
Authors

Yo-ichi Yamamoto − Department of Chemistry, Graduate
School of Science, Kyoto University, Kyoto 606-8502, Japan
Tatsuya Ishiyama − Department of Applied Chemistry,
Graduate School of Science and Engineering, University of
Toyama, Toyama 930-8555, Japan; orcid.org/00000003-2388-7727
Akihiro Morita − Department of Chemistry, Graduate Schoolof Science, Tohoku University, Sendai 980-8578, Japan;
Elements Strategy Initiative for Catalysts and Batteries(ESICB), Kyoto University, Kyoto 615-8530, Japan;
orcid.org/0000-0002-2104-0605■

CONCLUSION
We investigated photoemission spectra of aqueous solutions ofvarious electrolytes and a methanol solution of NaI. Strong
inorganic electrolytes do not signiﬁcantly alter the VIE forsolvents even at high concentrations, while they inﬂuence thephotoelectron bands for electron orbitals relevant to thehydrogen-bonding network of solvent molecules. Hydrophobicorganic solutes such as TBA+ and BA are concentrated at theinterfacial region, and they induce an appreciable VIE shift for
liquid water at relatively low concentrations. These eﬀects are
due to the formation of surface dipoles, but the mechanisms
are slightly diﬀerent between BA and TBAI. BA is electricallyneutral and has an amino group embedded in the liquid and abutyl group on the vacuum side to orient its permanent dipole
nearly normal to the surface. BA does not cause strong
orientation of water molecules. TBA+ is at the surface of theliquid, and it drags its counterion I− underneath the TBA-richlayer to form an EDL. Water molecules in the EDL are strongly
oriented with their permanent dipole moment antiparallel tothe electric ﬁeld and the hydrogen atoms pointing away fromthe surface. The unquenched electric ﬁeld in the EDL shifts thesurface potential of aqueous solutions. The observed shifts ofeBE values agree with classical molecular dynamics simulations. The splitting of the 3a1 band of liquid water and the 7a′band of liquid methanol indicates the inﬂuence of a solute on
the hydrogen-bonding network of the solvents. Quantum
chemical calculations are necessary to elucidate the atomistic
mechanisms that alter the photoelectron spectra.
This study showed that photoemission spectroscopy ofliquids can provide useful information for investigating theelectronic structures and molecular interactions at a gas−liquidinterface. It is desirable to improve the energy resolution of a
MBTOF spectrometer by the use of a larger ﬂight length, andsuch modiﬁcation of our apparatus is in progress. At present,
the probing depth of photoemission spectroscopy of liquids is

Complete contact information is available at:https://pubs.acs.org/10.1021/acs.jpcb.1c04765
Notes

The authors declare no competing ﬁnancial interest.

■

ACKNOWLEDGMENTS
This work was supported by the JSPS KAKENHI Grants (Nos.
21H04970, 21H01878, 20H00368, and 18H05265), theMitsubishi Foundation, the Fugaku Supercomputer Project(No. JPMXP1020200308) from the Ministry of Education,
Culture, Sports, Science and Technology of Japan, and the
Asahi Glass Foundation. Some of the molecular dynamics
simulations in this study were performed by using thesupercomputers at the Research Center for Computational
Science of National Institutes of Natural Sciences and
MASAMUNE-IMR at Center for Computational Material
Science of Tohoku University. We thank Professor Nobuhiro
Kosugi for his helpful comments on the manuscript. The data
presented here are available on request sent to suzuki@
kuchem.kyoto-u.ac.jp.

■

REFERENCES(1) Marcus, Y. Effect of Ions on the Structure of Water: Structure
Making and Breaking. Chem. Rev. 2009, 109, 1346−1370.

10524

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J. Phys. Chem. B 2021, 125, 10514−10526

The Journal of Physical Chemistry B

pubs.acs.org/JPCB(2) Ohtaki, H.; Radnai, T. Structure and Dynamics of Hydrated
Ions. Chem. Rev. 1993, 93, 1157−1204.
(3) Frank, H. S.; Wen, W. Y. Structural Aspects of Ion-Solvent
Interaction in Aqueous Solutions: A Suggested Picture of Water
Structure. Discuss. Faraday Soc. 1957, 24, 133−140.
(4) Omta, A. W.; Kropman, M. F.; Woutersen, S.; Bakker, H. J.
Negligible Effect of Ions on the Hydrogen-Bond Structure in Liquid
Water. Science 2003, 301, 347−349.
(5) Zhang, Y. J.; Cremer, P. S. Interactions between Macromolecules
and Ions: The Hofmeister Series. Curr. Opin. Chem. Biol. 2006, 10,
658−663.
(6) Jungwirth, P.; Tobias, D. J. Specific Ion Effects at the Air/Water
Interface. Chem. Rev. 2006, 106, 1259−1281.
(7) Berendsen, H. J. C.; Grigera, J. R.; Straatsma, T. P. The Missing
Term in Effective Pair Potentials. J. Phys. Chem. 1987, 91, 6269−
6271.
(8) Faubel, M.; Steiner, B.; Toennies, J. P. Photoelectron
Spectroscopy of Liquid Water, Some Alcohols, and Pure Nonane inFree Micro Jets. J. Chem. Phys. 1997, 106, 9013−9031.
(9) Winter, B.; Faubel, M. Photoemission from Liquid Aqueous
Solutions. Chem. Rev. 2006, 106, 1176−1211.
(10) Dupuy, R.; Richter, C.; Winter, B.; Meijer, G.; Schlögl, R.;
Bluhm, H. Core Level Photoelectron Spectroscopy of Heterogeneous
Reactions at Liquid-Vapor Interfaces: Current Status, Challenges, andProspects. J. Chem. Phys. 2021, 154, 060901.
(11) Suzuki, T. Ultrafast Photoelectron Spectroscopy of Aqueous
Solutions. J. Chem. Phys. 2019, 151, 090901.
(12) Buttersack, T.; Mason, P. E.; McMullen, R. S.; Schewe, H. C.;
Martinek, T.; Brezina, K.; Crhan, M.; Gomez, A.; Hein, D.; Wartner,
G.; et al. Photoelectron Spectra of Alkali Metal-Ammonia Microjets:
From Blue Electrolyte to Bronze Metal. Science 2020, 368, 1086−
1091.
(13) Jordan, I.; Huppert, M.; Rattenbacher, D.; Peper, M.; Jelovina,
D.; Perry, C.; von Conta, A.; Schild, A.; Wörner, H. J. Attosecond
Spectroscopy of Liquid Water. Science 2020, 369, 974−979.
(14) Kurahashi, N.; Karashima, S.; Tang, Y.; Horio, T.; Abulimiti, B.;
Suzuki, Y. I.; Ogi, Y.; Oura, M.; Suzuki, T. Photoelectron
Spectroscopy of Aqueous Solutions: Streaming Potentials of NaX(X = Cl, Br, and I) Solutions and Electron Binding Energies of Liquid
Water and X−. J. Chem. Phys. 2014, 140, 174506.
(15) Thürmer, S.; Shinno, T.; Suzuki, T. Valence Photoelectron
Spectra of Liquid Methanol and Ethanol Measured Using He II
Radiation. J. Phys. Chem. A 2021, 125, 2492−2503.
(16) Faubel, M.; Steiner, B. Strong Bipolar Electrokinetic Charging
of Thin Liquid Jets Emerging from 10 μm PtIr Nozzles. Ber. Bunsen.
Phys. Chem. 1992, 96, 1167−1172.
(17) Preissler, N.; Buchner, F.; Schultz, T.; Lübcke, A. Electrokinetic
Charging and Evidence for Charge Evaporation in Liquid Microjets of
Aqueous Salt Solution. J. Phys. Chem. B 2013, 117, 2422−2428.
(18) Pohl, M. N.; Muchová, E.; Seidel, R.; Ali, H.; Sršeá, S.;
Wilkinson, I.; Winter, B.; Slavíček, P. Do Water’s Electrons Care
About Electrolytes? Chem. Sci. 2019, 10, 848−865.
(19) Thürmer, S.; Malerz, S.; Trinter, F.; Hergenhahn, U.; Lee, C.;
Neumark, D. M.; Meijer, G.; Winter, B.; Wilkinson, I. Accurate
Vertical Ionization Energy and Work Function Determinations ofLiquid Water and Aqueous Solutions. Chem. Sci. 2021, 12, 10558−
10582.
(20) Karashima, S.; Suzuki, T. Charge-Transfer-to-Solvent Reaction
in a Hydrophobic Tetrabutylammonium Iodide Molecular Layer in
Aqueous Solution. J. Phys. Chem. B 2019, 123, 3769−3775.
(21) Yamamoto, Y.; Suzuki, Y. I.; Tomasello, G.; Horio, T.;
Karashima, S.; Mitríc, R.; Suzuki, T. Time- and Angle-Resolved
Photoemission Spectroscopy of Hydrated Electrons near a Liquid
Water Surface. Phys. Rev. Lett. 2014, 112, 187603.
(22) Winter, B.; Weber, R.; Schmidt, P. M.; Hertel, I. V.; Faubel, M.;
Vrbka, L.; Jungwirth, P. Molecular Structure of Surface-Active Salt
Solutions: Photoelectron Spectroscopy and Molecular Dynamics
Simulations of Aqueous Tetrabutylammonium Iodide. J. Phys. Chem.
B 2004, 108, 14558−14564.

Article(23) Olivieri, G.; Goel, A.; Kleibert, A.; Cvetko, D.; Brown, M. A.
Quantitative Ionization Energies and Work Functions of Aqueous
Solutions. Phys. Chem. Chem. Phys. 2016, 18, 29506−29515.
(24) Nishizawa, K.; Kurahashi, N.; Sekiguchi, K.; Mizuno, T.; Ogi,Y.; Horio, T.; Oura, M.; Kosugi, N.; Suzuki, T. High-Resolution SoftX-Ray Photoelectron Spectroscopy of Liquid Water. Phys. Chem.
Chem. Phys. 2011, 13, 413−417.
(25) Weber, R.; Winter, B.; Schmidt, P. M.; Widdra, W.; Hertel, I.
V.; Dittmar, M.; Faubel, M. Photoemission from Aqueous AlkaliMetal-Iodide Salt Solutions Using EUV Synchrotron Radiation. J.
Phys. Chem. B 2004, 108, 4729−4736.
(26) Tissot, H.; Gallet, J. J.; Bournel, F.; Olivieri, G.; Silly, M. G.;
Sirotti, F.; Boucly, A.; Rochet, F. The Electronic Structure of
Saturated NaCl and NaCl Solutions in Contact with a Gold Substrate.
Top. Catal. 2016, 59, 605−620.
(27) Ishii, H.; Sugiyama, K.; Ito, E.; Seki, K. Energy Level Alignment
and Interfacial Electronic Structures at Organic Metal and Organic
Organic Interfaces. Adv. Mater. 1999, 11, 605−625.
(28) Jablonski, A.; Powell, C. J. Effective Attenuation Lengths forDifferent Quantitative Applications of X-Ray Photoelectron Spectroscopy. J. Phys. Chem. Ref. Data 2020, 49, 033102.
(29) Shinotsuka, H.; Tanuma, S.; Powell, C. J.; Penn, D. R.
Calculations of Electron Inelastic Mean Free Paths. XII. Data for 42
Inorganic Compounds over the 50 eV to 200 keV Range with the Full
Penn Algorithm. Surf. Interface Anal. 2019, 51, 427−457.
(30) Shen, Y. R. Fundamentals of Sum-Frequency Spectroscopy;
Cambridge University Press: Cambridge, 2016.
(31) Eisenthal, K. B. Liquid Interfaces Probed by Second-Harmonicand Sum-Frequency Spectroscopy. Chem. Rev. 1996, 96, 1343−1360.
(32) Richmond, G. L. Molecular Bonding and Interactions at
Aqueous Surfaces as Probed by Vibrational Sum Frequency
Spectroscopy. Chem. Rev. 2002, 102, 2693−2724.
(33) Nihonyanagi, S.; Mondal, J. A.; Yamaguchi, S.; Tahara, T.
Structure and Dynamics of Interfacial Water Studied by HeterodyneDetected Vibrational Sum-Frequency Generation. Annu. Rev. Phys.
Chem. 2013, 64, 579−603.
(34) Zhao, X. L.; Ong, S. W.; Wang, H. F.; Eisenthal, K. B. New
Method for Determination of Surface pKa Using Second Harmonic
Generation. Chem. Phys. Lett. 1993, 214, 203−207.
(35) Kruit, P.; Read, F. H. Magnetic Field Parallelizer for 2π
Electron-Spectrometer and Electron-Image Magnifier. J. Phys. E: Sci.
Instrum. 1983, 16, 313−324.
(36) Ojeda, J.; Arrell, C. A.; Longetti, L.; Chergui, M.; Helbing, J.
Charge-Transfer and Impulsive Electronic-to-Vibrational Energy
Conversion in Ferricyanide: Ultrafast Photoelectron and Transient
Infrared Studies. Phys. Chem. Chem. Phys. 2017, 19, 17052−17062.
(37) Engel, N.; Bokarev, S. I.; Moguilevski, A.; Raheem, A. A.; AlObaidi, R.; Möhle, T.; Grell, G.; Siefermann, K. R.; Abel, B.; Aziz, S.
G.; et al. Light-Induced Relaxation Dynamics of the Ferricyanide Ion
Revisited by Ultrafast XUV Photoelectron Spectroscopy. Phys. Chem.
Chem. Phys. 2017, 19, 14248−14255.
(38) Hummert, J.; Reitsma, G.; Mayer, N.; Ikonnikov, E.; Eckstein,M.; Kornilov, O. Femtosecond Extreme Ultraviolet Photoelectron
Spectroscopy of Organic Molecules in Aqueous Solution. J. Phys.
Chem. Lett. 2018, 9, 6649−6655.
(39) Nishitani, J.; Yamamoto, Y.; West, C. W.; Karashima, S.;
Suzuki, T. Binding Energy of Solvated Electrons and Retrieval of True
UV Photoelectron Spectra of Liquids. Sci. Adv. 2019, 5, aaw6896.
(40) Matsumoto, M.; Kataoka, Y. Study on Liquid Vapor Interfaceof Water. I. Simulational Results of Thermodynamic Properties andOrientational Structure. J. Chem. Phys. 1988, 88, 3233−3245.
(41) Wilson, M. A.; Pohorille, A.; Pratt, L. R. Comment on “Studyon the Liquid Vapor Interface of Water. I. Simulation Results ofThermodynamic Properties and Orientational Structure. J. Chem.
Phys. 1989, 90, 5211−5213.
(42) Sokhan, V. P.; Tildesley, D. J. The Free Surface of Water:
Molecular Orientation, Surface Potential and Nonlinear Susceptibility.
Mol. Phys. 1997, 92, 625−640.
10525

https://doi.org/10.1021/acs.jpcb.1c04765
J. Phys. Chem. B 2021, 125, 10514−10526

The Journal of Physical Chemistry B

pubs.acs.org/JPCB

ArticleAqueous Acid and Base Solutions Combined with Electronic
Structure Calculations. J. Am. Chem. Soc. 2006, 128, 3864−3865.
(64) Radojević, V.; Kelly, H. P.; Johnson, W. R. Photodetachment ofNegative Halogen Ions. Phys. Rev. A: At., Mol., Opt. Phys. 1987, 35,
2117−2121.
(65) Hlavenka, P.; Otto, R.; Trippel, S.; Mikosch, J.; Weidemüller,
M.; Wester, R. Absolute Photodetachment Cross Section Measurements of the O− and OH− Anion. J. Chem. Phys. 2009, 130, 061105.
(66) Mandl, A. Electron Photodetachment Cross Section of
Negative Ion of Fluorine. Phys. Rev. A: At., Mol., Opt. Phys. 1971, 3,
251−255.
(67) Swanson, J. R.; Armstrong, L. Multiconfiguration Hartree-FockCalculation of Photoionization Cross-Sections of Rare-Gases. Phys.
Rev. A: At., Mol., Opt. Phys. 1977, 15, 661−667.
(68) Mucha, M.; Frigato, T.; Levering, L. M.; Allen, H. C.; Tobias,
D. J.; Dang, L. X.; Jungwirth, P. Unified Molecular Picture of the
Surfaces of Aqueous Acid, Base, and Salt Solutions. J. Phys. Chem. B
2005, 109, 7617−7623.
(69) Alloway, D. M.; Hofmann, M.; Smith, D. L.; Gruhn, N. E.;
Graham, A. L.; Colorado, R.; Wysocki, V. H.; Lee, T. R.; Lee, P. A.;
Armstrong, N. R. Interface Dipoles Arising from Self-Assembled
Monolayers on Gold: UV-Photoemission Studies of Alkanethiols andPartially Fluorinated Alkanethiols. J. Phys. Chem. B 2003, 107,
11690−11699.
(70) Galib, M.; Baer, M. D.; Skinner, L. B.; Mundy, C. J.;
Huthwelker, T.; Schenter, G. K.; Benmore, C. J.; Govind, N.; Fulton,
J. L. Revisiting the Hydration Structure of Aqueous Na+. J. Chem.
Phys. 2017, 146, 084504.
(71) Fulton, J. L.; Schenter, G. K.; Baer, M. D.; Mundy, C. J.; Dang,
L. X.; Balasubramanian, M. Probing the Hydration Structure of
Polarizable Halides: A Multiedge XAFS and Molecular Dynamics
Study of the Iodide Anion. J. Phys. Chem. B 2010, 114, 12926−12937.
(72) Zhu, S. B.; Robinson, G. W. Molecular-Dynamics Computer
Simulation of an Aqueous NaCl Solution: Structure. J. Chem. Phys.
1992, 97, 4336−4348.
(73) Olivieri, G.; Parry, K. M.; Powell, C. J.; Tobias, D. J.; Brown, M.
A. Quantitative Interpretation of Molecular Dynamics Simulations for
X-Ray Photoelectron Spectroscopy of Aqueous Solutions. J. Chem.
Phys. 2016, 144, 154704.

(43) Ishiyama, T.; Morita, A. Molecular Dynamics Study of GasLiquid Aqueous Sodium Halide Interfaces. I. Flexible and Polarizable
Molecular Modeling and Interfacial Properties. J. Phys. Chem. C 2007,
111, 721−737.
(44) Kurahashi, N.; Thürmer, S.; Liu, S. Y.; Yamamoto, Y.;
Karashima, S.; Bhattacharya, A.; Ogi, Y.; Horio, T.; Suzuki, T. Design
and Characterization of a Magnetic Bottle Electron Spectrometer for
Time-Resolved Extreme UV and X-Ray Photoemission Spectroscopyof Liquid Microjets. Struct. Dyn. 2021, 8, 034303.
(45) Nishitani, J.; Karashima, S.; West, C. W.; Suzuki, T. Surface
Potential of Liquid Microjet Investigated Using Extreme Ultraviolet
Photoelectron Spectroscopy. J. Chem. Phys. 2020, 152, 144503.
(46) Nishitani, J.; West, C. W.; Suzuki, T. Angle-Resolved
Photoemission Spectroscopy of Liquid Water at 29.5 eV. Struct.
Dyn. 2017, 4, 044014.
(47) Siegbahn, H. Electron Spectroscopy for Chemical Analysis of
Liquids and Solutions. J. Phys. Chem. 1985, 89, 897−909.
(48) Malerz, S.; Trinter, F.; Hergenhahn, U.; Ghrist, A.; Ali, H.;
Nicolas, C.; Saak, C. M.; Richter, C.; Hartweg, S.; Nahon, L.; et al.
Low-Energy Constraints on Photoelectron Spectra Measured from
Liquid Water and Aqueous Solutions. Phys. Chem. Chem. Phys. 2021,
23, 8246−8260.
(49) Mucke, M.; Braune, M.; Barth, S.; Förstel, M.; Lischke, T.;
Ulrich, V.; Arion, T.; Becker, U.; Bradshaw, A.; Hergenhahn, U. A
Hitherto Unrecognized Source of Low-Energy Electrons in Water.
Nat. Phys. 2010, 6, 143−146.
(50) Frassetto, F.; Cacho, C.; Froud, C. A.; Turcu, I. C. E.; Villoresi,P.; Bryan, W. A.; Springate, E.; Poletto, L. Single-Grating
Monochromator for Extreme-Ultraviolet Ultrashort Pulses. Opt.
Express 2011, 19, 19169−19181.
(51) Wang, J. M.; Wolf, R. M.; Caldwell, J. W.; Kollman, P. A.; Case,
D. A. Development and Testing of a General Amber Force Field. J.
Comput. Chem. 2004, 25, 1157−1174.
(52) Bayly, C. I.; Cieplak, P.; Cornell, W. D.; Kollman, P. A. A WellBehaved Electrostatic Potential Based Method Using Charge
Restraints for Deriving Atomic Charges - the RESP Model. J. Phys.
Chem. 1993, 97, 10269−10280.
(53) Joung, I. S.; Cheatham, T. E. Determination of Alkali and
Halide Monovalent Ion Parameters for Use in Explicitly Solvated
Biomolecular Simulations. J. Phys. Chem. B 2008, 112, 9020−9041.
(54) Martínez, L.; Andrade, R.; Birgin, E. G.; Martínez, J. M.
PACKMOL: A Package for Building Initial Configurations for
Molecular Dynamics Simulations. J. Comput. Chem. 2009, 30,
2157−2164.
(55) Allen, M. P.; Tildesley, D. J. Computer Simulation of Liquids;
Clarendon Press: Oxford, 1987.
(56) Hess, B.; Bekker, H.; Berendsen, H. J. C.; Fraaije, J. LINCS: A
Linear Constraint Solver for Molecular Simulations. J. Comput. Chem.
1997, 18, 1463−1472.
(57) Nosé, S. A Unified Formulation of the Constant Temperature
Molecular Dynamics Methods. J. Chem. Phys. 1984, 81, 511−519.
(58) Hoover, W. G. Canonical Dynamics: Equilibrium Phase-Space
Distributions. Phys. Rev. A: At., Mol., Opt. Phys. 1985, 31, 1695−1697.
(59) Darden, T.; York, D.; Pedersen, L. Particle Mesh Ewald: An N·
log(N) Method for Ewald Sums in Large Systems. J. Chem. Phys.
1993, 98, 10089−10092.
(60) Perera, L.; Essmann, U.; Berkowitz, M. L. Effect of theTreatment of Long-Range Forces on the Dynamics of Ions in
Aqueous-Solutions. J. Chem. Phys. 1995, 102, 450−456.
(61) Van der Spoel, D.; Lindahl, E.; Hess, B.; Groenhof, G.; Mark, A.
E.; Berendsen, H. J. C. GROMACS: Fast, Flexible, and Free. J.
Comput. Chem. 2005, 26, 1701−1718.
(62) Kathmann, S. M.; Kuo, I. F. W.; Mundy, C. J.; Schenter, G. K.
Understanding the Surface Potential of Water. J. Phys. Chem. B 2011,
115, 4369−4377.
(63) Winter, B.; Faubel, M.; Hertel, I. V.; Pettenkofer, C.; Bradforth,
S. E.; Jagoda-Cwiklik, B.; Cwiklik, L.; Jungwirth, P. Electron Binding
Energies of Hydrated H3O+ and OH−: Photoelectron Spectroscopy of10526

https://doi.org/10.1021/acs.jpcb.1c04765
J. Phys. Chem. B 2021, 125, 10514−10526