Extreme Ultraviolet Laser Photoelectron Spectroscopyof Flat Liquid Jet Generated Using Microfluidic Device
Yo-ichi Yamamoto, Hiroto Yano, Shutaro Karashima, Ryuta Uenishi,
Natsumi Orimo, Junichi Nishitani, and Toshinori Suzuki*
We have developed a microfluidic chip for liquid-liquid collisions and confirmed thegeneration of a thin (0.7 ¯m) flat liquid jet with a large cross-sectional area. The flatjet enhanced the photoelectron signal intensity by a factor of 8 compared to that for a
microjet under the same laser focusing conditions, which enables quick and accurate
measurement of photoelectron spectra of liquids.

REPRINTED FROM

Vol.96 No.9 2023 p.938–942
September 15, 2023

The Chemical Society of Japan

Document type: Article

BCSJ Award Article
Extreme Ultraviolet Laser Photoelectron Spectroscopyof Flat Liquid Jet Generated Using Microfluidic Device
Yo-ichi Yamamoto, Hiroto Yano, Shutaro Karashima, Ryuta Uenishi,
Natsumi Orimo, Junichi Nishitani, and Toshinori Suzuki*
Department of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan
E-mail: suzuki@kuchem.kyoto-u.ac.jp
Received: July 5, 2023; Accepted: July 29, 2023; Web Released: August 4, 2023

Toshinori Suzuki
TS received a Doctor of Science from Tohoku University in 1988. He joined the Institute for Molecular
Science (IMS) as a technical associate in 1988 and became a research associate in 1989. From 1990 to
1992, he conducted research at Cornell University and UC Berkeley as a JSPS Postdoctoral Fellow for
Overseas Research. In 1992, he was appointed at IMS as an associate professor and started his researchgroup. He moved to RIKEN in 2001 as Chief Scientist and Director of the Chemical Dynamics Laboratory,
and since 2008 he has been a full professor of chemistry at Kyoto University.

Molecular aggregation and chemical interactions at gasliquid interfaces are of great importance for environmental
sciences, synthetic chemistry and various other ﬁelds. The
primary experimental means to study these interfaces have so
far been nonlinear spectroscopic methods such as secondharmonic generation and sum-frequency generation techniques.1,2 On the other hand, extreme UV (EUV) photoelectron
spectroscopy (PES) is also promising as a complementary
approach for studying gas-liquid interfaces.3 Because the electron escape depth for a material is about 1 nm or less in theelectron kinetic energy (eKE) range of 30­100 eV,4,5 EUV-PES
provides high interfacial sensitivity. For example, EUV-PES
enables in situ measurements of surface excess of solutes undervacuum conditions.6 Most importantly, EUV-PES provides
insights into electronic structures at gas-liquid interfaces.
PES studies of volatile liquids have generally been performed using a cylindrically shaped liquid microjet ³20 ¯m indiameter.7­10 The microjet can be created by discharging aliquid from a thin glass capillary or a metal aperture, which iseasy to implement. However, one of the drawbacks of this technique is that the jet diameter is smaller than the typical focal
spot size (100­200 ¯m) for a laser beam, so that the photon ﬂux
cannot be fully utilized, which limits the data acquisition eﬃciency. The electron escape depth is extremely small (³1 nm)

for a liquid, so the EUV photoelectron signal is inherently
weak. Moreover, because the number density of a solute is typically three orders of magnitude smaller than that of a solvent,
the signal from a solute is extremely weak. Consequently, dataacquisition in time-resolved EUV-PES of a solute typicallytakes well over 24 h. This diﬃculty can be mitigated by increasing the repetition rate of the EUV laser to the submegahertz range using a Yb-based laser.11 To further maximize
the photoelectron signal, it is also important to reduce the size
mismatch between the laser focal spot and the liquid target.
This can be achieved by increasing the area of the liquid target
for EUV-PES measurements. The liquid must travel in vacuum
at high speed for the following two reasons. One is to continuously bring a fresh sample to the focal point of the laser
operated at a repetition rate >10 kHz. The speed of a liquid
ﬂow must be higher than 3 m/sec to avoid two consecutive
laser pulses in the 10 kHz pulse train from exciting the same
portion of the liquid. Our experimental condition provides a
speed higher than 20 m/sec. The other is to transport the
sample liquid into the laser interaction region before extensive
evaporative cooling occurs under vacuum.
There have been at least three diﬀerent methods reported
for generating a traveling liquid ﬁlm, or a ﬂat jet.12 The ﬁrst
employs liquid-liquid collisions; two liquid microjets collide
under vacuum, resulting in the merged liquid being squeezed
into a thin ﬁlm. This method has been implemented for soft Xray absorption spectroscopy,13 high-order harmonic generation(HHG)14, soft X-ray transient absorption spectroscopy,15 andphotoelectron spectroscopy.16 The second method employs aslit-shaped nozzle to discharge a liquid into a vacuum;17 a very
narrow slit can be fabricated using photolithography to generate a submicron-thick liquid ﬁlm.18,19 The advantage of this938 | Bull. Chem. Soc. Jpn. 2023, 96, 938–942 | doi:10.1246/bcsj.20230151© 2023 The Chemical Society of Japan

Abstract
We present photoelectron spectroscopy of liquid ﬁlms generated in a vacuum with microﬂuidic devices based on liquidliquid or gas-liquid collisions. The results are compared tothose for a standard liquid microjet technique.
Keywords: Photoelectron spectroscopy j Microﬂuidics j LiquidFigure 1. Microﬂuidic chip and ﬂat liquid jet. (a) Design of
ﬂow channels (units: mm). The depth of the channel is 20¯m. (b) Microﬂuidic device made from Si (units: mm). (c)Formation of stable liquid ﬁlm at ﬂow rate of 1.5 mL/min.
The simulated ﬁlm shape is shown on the right-hand side(units: mm). (d) Thickness of ﬂat liquid jet estimated using
interference and extinction methods.

method is that it provides a well-deﬁned liquid thickness thatis crucial for photoabsorption spectroscopy. The third method
uses gas-liquid collisions in a gas-dynamic nozzle, in which aliquid microjet is aerodynamically ﬂattened by two gas jets
impinging from opposite sides.20 The advantages of thismethod are that the ﬁlm thickness can be controlled by thegas jet pressure and that very thin ﬁlms (<100 nm) can begenerated using strong gas jets.
In the present study, we primarily employ the ﬁrst of thesethree methods. However, instead of using two independentliquid microjet nozzles, we employ a single microﬂuidic device
to discharge two precisely aligned liquid jets. This method
allows liquid jets to collide precisely with each other, generating a stable liquid ﬁlm over a long period of time. We fabricated a microﬂuidic device of our own design. In the following,
we describe the performance of this device for EUV-PES measurements. We also brieﬂy compare the results of a performance test using the third method.
We designed the ﬂow channels based on a past study on
liquid-liquid collision.21 Our design is shown in Figure 1(a).
We chose a crossing angle of 70° between the two jets, with theliquid-liquid collision point 0.1 mm downstream from the endof the microchip. The two ﬂow channels are mirror images ofeach other, which ensures a well-balanced liquid ﬂow rate forthe two microjets. The ﬂow channels have a rectangular crosssection with dimensions of 100 © 20 ¯m almost entirely toprevent the pressure loss in the channels. However, the width
decreases gradually from 3 mm upstream of the exits to 20 ¯mat the exit to increase the ﬂow speed. The collision of highBull. Chem. Soc. Jpn. 2023, 96, 938–942 | doi:10.1246/bcsj.20230151

speed liquid jets creates a large liquid ﬁlm. The outer V-shapeof the chip is to avoid spatial interference with the permanent
magnet and electron sampling skimmer that are placed a few
millimeters away from the liquid ﬁlm in the vacuum chamber.
We employed photolithography to produce grooves in a Si
wafer with a thickness of 1.5 mm, bonded the grooved surfaceof this wafer to a ﬂat Si wafer to form enclosed channels, andthen used laser-dicing to isolate individual chips. A photographof a diced chip is shown in Figure 1(b). Each microchip wasthen coated with graphite to prevent charge-up. The microﬂuidic chip is held between a ﬂat plate and a holder madeof PEEK (polyether ether ketone). A ﬂow channel is drilled
through the holder to enable liquid supply. An O-ring is placedaround the º0.3-mm entrance hole of the chip to connect thechip and the holder without any leak at a high liquid pressure.
A 1/16¤¤ PEEK tube connects the inlet of the holder and an
HPLC pump. An in-line ﬁlter with 2-¯m pore size, positioned
between the holder and the pump, removes small particles toprevent clogging in the chip. Figure 1(c) shows a photographof a ﬂat jet generated by this device in air. The total liquid ﬂow
rate was 1.5 mL/min. Also shown is the predicted shape basedon a previously derived equation21 (See Supporting Information for details). The observed width of the liquid ﬁlm
agrees very well with the prediction, while the length is slightly
larger than predicted. The ﬁlm is formed as a chain of mutually orthogonal leaves (the leaf orientation varies alternately
between the x and y directions, if the ﬂow axis is z), and theﬁrst (and largest) leaf shown in Figure 1(c) was used for PES
measurements. The dimensions of the ﬁrst leaf were 0.3 © 1.0
mm for a liquid ﬂow rate of 1.0 mL/min, and 0.7 © 2.2 mm at
1.5 mL/min. These dimensions are much larger than the laser
focal spot size. Figure 1(d) shows the thickness of a ﬁlm
measured in air for three diﬀerent chips (named A­C) using
white-light interferometry12,17 and extinction spectroscopy, thelatter of which employed a dye (Rhodamine 610) solution and532-nm light (See Supporting Information for details). These
two methods provided consistent results, as seen in the plot; thethickness of the liquid ﬁlm in the middle of the ﬁrst leaf-shaped
liquid ﬁlm is 0.7 ¯m for a ﬂow rate of 1.5 mL/min. The rimof a leaf is much thicker. The traveling speed of the ﬂat liquid
jet is estimated to be over 20 m/s for a total ﬂow rate of 1.5
mL/min.
We introduced the Si chip into a photoelectron spectrometerand illuminated the ﬁrst leaf with femtosecond EUV pulses tomeasure EUV-PE spectra. The ﬂow rate was 1.0­1.5 mL/min.
The liquid discharged into vacuum was entirely frozen in aliquid N2-cooled trap placed downstream. The EUV pulses
were generated by HHG in Ar using the second harmonic(1 kHz, 400 nm, 0.13 mJ) of a Ti:sapphire laser.22 A singleorder harmonic (EUV radiation of 27.7 eV) was selected using
a time-preserving monochromator with a diﬀraction grating(150 grooves/mm).23 The Si chip was held at room temperature. Since the EUV pulses also ionize gaseous water molecules evaporated from the liquid ﬁlm, a negative bias voltage(¹30 V) was applied to the microchip to increase the kinetic
energy of photoelectrons emitted from the liquid. This enabled
diﬀerentiation of the electrons emitted from the liquid andthose from the evaporated gas. The photoelectrons were detected using a magnetic-bottle time-of-ﬂight (MBTOF) electron© 2023 The Chemical Society of Japan | 939

Figure 2. Photoelectron spectroscopy using ﬂat liquid jet.
(a) Geometry of experiment. (b) Photoelectron spectra of
10 mM aqueous NaI solution measured using Si microﬂuidic chip. A liquid ﬂow rate of 1.0 mL/min, an excitation point 1 mm downstream from the nozzle, and various
excitation-light incidence angles were employed. The
black line indicates the results obtained using a microjet.
The inset shows an enlarged view of the spectra, and theband assignments are indicated. Note that the rim of aliquid ﬁlm is much thicker than the ﬁlm itself, and an experiment at the angle of 90° illuminates the rim, providing
a similar signal intensity with a microjet. (c) Dependenceof signal intensity on photoexcitation position for ª = 30°.
The expected intensities based on the leaf shape and laser
spot size are indicated by red dots. The diameter of the
EUV beam was 160 ¯m (FWHM) in these measurements.

Figure 3. Photoelectron spectra of 50 mM aqueous NaI
solution. Black circles: ﬂat jet combined with MBTOF
spectrometer and EUV laser. The ionization point was 0.6
mm downstream from the microchip and ª was 40°. The
inset shows an enlarged view of the energy region for
iodide anions. Solid lines indicate the decomposition of thevalence bands. A broad background photoelectron signal isindicated as bg. Red circles: microjet combined with HEA
and He(II) radiation.

energy analyzer 24 described in detail elsewhere.25 The photoelectrons were guided by a diverging magnetic ﬁeld provided
by a permanent magnet and a solenoid coil, and introduced into
the TOF analyzer through a º0.5-mm entrance skimmer locatedapproximately 2 mm from the liquid. The photoelectrons weredecelerated in the ﬂight tube to enhance the resolution of the
TOF analysis. The typical energy resolution was 0.1 eV, limitedby the resolution of the MBTOF apparatus and the laser bandwidth. When forming a ﬂat jet, the pressure in the main chamber and the MBTOF analyzer was 4 © 10¹4 and 3 © 10¹6 Torr,
respectively. The temperature of the liquid ﬁlm was crudely
estimated to be 273­283 K based on a recent study by Changet al.26
Measured photoelectron spectra for a 10 mM aqueous NaI
solution are presented in Figure 2. As shown in Figure 2(a), theelectron detection axis and the EUV propagation direction are
at 90° to each other. As shown in Figure 2(b), we varied therelative angle ª by rotating the ﬂat jet and found that theintensity was maximized at around 30°. However, the angulardependence is rather moderate, and one can obtain reasonably
high intensities also at 40° and 45°. The signal intensity thusachieved was 8 times higher than that obtained with a liquid
microjet 25 ¯m in diameter. The horizontal axes in Figure 2(b)are eKE and electron binding energy (eBE), the latter of whichis the diﬀerence between the photon energy and eKE. The
spectra clearly reveal valence photoelectron bands associatedwith liquid water, such as 1b1 (HOMO) and 3a1 (HOMO-1).
The strong peak at low eKE is due to electrons inelasticallyscattered and decelerated in the liquid, which is unavoidable
for PES of liquids. Figure 2(c) shows the dependence of thephotoelectron signal intensity on the downstream position, with
ª ﬁxed at 30°. The results are in reasonable agreement with the
expected values.
Due to insuﬃcient durability of the Si chip, we proceeded
to fabricate a similar chip using borosilicate glass and further
evaluated the utility of this device in EUV-PES. We performed
EUV-PES of NaI aqueous solutions at 40.0 eV, which is close
to the energy of He(II) radiation (40.8 eV). The time-preserving
monochromator (500 grooves/mm) provided good spectral
isolation of 40.0 eV radiation (13th harmonic); the measured
contamination from 21.6 (7th harmonic), 27.7 (9th harmonic)
and 33.9 eV radiation (11th harmonic) was 4.8, 2.4 and 1.4%,
respectively. Figure 3 presents EUV-PE spectra of a 50 mM
NaI solution obtained using our present instrument, together
with that27 obtained using a hemispherical electron energy analyzer (HEA, Gamma-data Scienta SES 100) and He(II) radiation (electron cyclotron resonance light source) with a liquid
microjet in our laboratory.28 The two spectra are seen to be inreasonable agreement with each other. The agreement with
HEA, which has a higher instrumental resolution, ensured a
suﬃciently high accuracy of the laser-MBTOF method combined with the liquid ﬁlm. Small diﬀerences are attributed inpart to the inﬂuence of photoemission anisotropy: our MBTOF
spectrometer collects photoelectrons emitted at all angles,
while the HEA has a ﬁnite solid angle for electron detection.
Another possible source of the diﬀerence is that the negative
bias voltage employed in the present study (¹36 V) was not
suﬃciently high to completely remove the signal associatedwith gaseous water. In the case of the HEA, the bias voltage
was ¹70 V, so that spectral contamination from gaseous water
could be completely neglected.27,29 Using the cut-oﬀ energy
method, we estimated the eBE for the water 1b1 band to be
11.3 eV, in excellent agreement with values reported in theliterature.30,31 The eBE values for the 3a1L, 3a1H, and 1b2

940 | Bull. Chem. Soc. Jpn. 2023, 96, 938–942 | doi:10.1246/bcsj.20230151© 2023 The Chemical Society of Japan

Figure 4. Photoelectron spectroscopy using gas-dynamic
microﬂuidic chip. (a) Photograph of commercial microﬂuidic device employed for measurements. (b) Comparisonof EUV photoelectron spectra of 10 mM NaI aqueous solution measured using three diﬀerent methods. For the ﬂatjets, the ionization point was 0.3 mm downstream fromthe nozzle and ª was 45°. The probe photon energy was
27.7 eV. The inset shows spectra normalized by the peakintensity of the 1b1 band for water.

valence bands were estimated to be 12.9, 14.4, and 17.4 eV,
respectively. These bands are slightly more sensitive to theunderlying background electron signal. The photoelectron
spectrum of I¹ is also clearly seen in the inset of Figure 3. The
acquisition time for the spectrum using a 5 kHz EUV laser was
10 min, while measurements using the HEA and He(II) radiation took at least 1.5 h.27 The high data acquisition eﬃciency isan advantage of EUV-PES using a laser-MBTOF method andthe ﬂat jet further improves the eﬃciency.
For comparison, spectra were also acquired for ﬂat liquid jets
obtained using a gas-dynamic nozzle, for which a commercial
microﬂuidic chip is available (Micronit Inc., Figure 4(a)). The
chip is made of borosilicate glass, and has a central channel
with cross-sectional dimensions of 30 © 20 ¯m for liquid ﬂow,
and two side channels with dimensions of 60 © 50 ¯m for gas
ﬂows. The crossing angle between the liquid microjet and gas
jets is 40°. Because the colliding gas considerably increases the
local pressure above the liquid ﬁlm, this type of nozzle seems
unfavorable for application to PES. However, we consideredthat it may be of use with He as a colliding gas, because He has
no internal degrees of freedom that would otherwise causerovibrational inelastic scattering of electrons. The total electron
scattering cross-section for He is less than 4 © 10¹16 cm2 at 10
eV.32 Figure 4(b) shows a comparison of EUV photoelectron
spectra for ﬂat jets obtained using a gas-dynamic nozzle withHe or CO2 as the colliding gas, and a standard liquid microjet
25 ¯m in diameter. It can be seen that the ﬂat jet with He gas
exhibited almost the same features at that for the microjet. The
spectral intensity obtainable using a gas-dynamic nozzle with
He is about two times higher than with a standard microjet,
while it is four times smaller than with the liquid-liquid collision nozzle. In contrast, when CO2 is used as the colliding
gas, the spectral intensity is about one order of magnitude
smaller than that for the microjet. This reduction in intensity is
attributed to the larger electron scattering cross-section for CO2(>13 © 10¹16 cm2 at 10 eV).33,34 The gas dynamic nozzle
inevitably increases electron scattering in the gas phase, generating a stronger background signal that peaks at a low electron
Bull. Chem. Soc. Jpn. 2023, 96, 938–942 | doi:10.1246/bcsj.20230151

kinetic energy (high electron binding energy). The backgroundsignal extends down to an eBE region of 20 eV, as seen in theinset of Figure 4. For the gas-dynamic nozzle, the liquid ﬂow
rate was 0.3 mL/min, which was much smaller than that for theliquid-liquid collision nozzle. However, the size of the liquidﬁlms generated by the gas-dynamic nozzle was rather small:
0.17 © 0.50 mm with He (0.45 MPa) and 0.21 © 0.46 mm with
CO2 (0.24 MPa).
In summary, we tested a microﬂuidic chip made of Si for
liquid-liquid collisions and generated a thin (0.7 ¯m) liquid
ﬁlm with a large cross-sectional area. We also fabricated asimilar chip from borosilicate glass for higher chemical durability. The performance as a ﬁlm generating device is independent of the material. The ﬂat jet thus generated enhanced thephotoelectron signal intensity by a factor of about 8 comparedto that for a microjet under the same laser focusing conditions.
An EUV-PE spectrum of a NaI aqueous solution at 40.0 eV wasmeasured in 10 min using a MBTOF spectrometer, an EUV
laser, and a ﬂat jet. We also found that a gas-dynamic nozzle isuseful for photoelectron spectroscopy with He as the colliding gas. Time-resolved PES measurements using ﬂat jets are
currently underway in our laboratory.
This work was supported by a JSPS KAKENHI Grant(No. 21H04970, 22K14646), Mitsubishi Foundation andToyota Riken Scholar Program. We thank Kazuki Maeda forexperimental assistance and Shunsuke Adachi for valuableadvice.
Supporting Information
Prediction of shape of ﬂat liquid jets and measurements ofthickness of liquid ﬁlms. This material is available on https://doi.org/10.1246/bcsj.20230151.
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942 | Bull. Chem. Soc. Jpn. 2023, 96, 938–942 | doi:10.1246/bcsj.20230151© 2023 The Chemical Society of Japan