Design and characterization of amagnetic bottle electron spectrometerfor time-resolved extreme UV and X-ray
photoemission spectroscopy of liquid
microjets
Cite as: Struct. Dyn. 8, 034303 (2021); https://doi.org/10.1063/4.0000107
Submitted: 04 April 2021 . Accepted: 17 May 2021 . Published Online: 09 June 2021
Naoya Kurahashi,
Yoshihiro Ogi,

Stephan Thürmer, Suet Yi Liu,
Takuya Horio, andYo-ichi Yamamoto, Shutaro Karashima, Atanu Bhattacharya,

Toshinori Suzuki

Struct. Dyn. 8, 034303 (2021); https://doi.org/10.1063/4.0000107© 2021 Author(s).

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scitation.org/journal/sdy

Design and characterization of a magnetic bottle
electron spectrometer for time-resolved extreme
UV and X-ray photoemission spectroscopy ofliquid microjets
Cite as: Struct. Dyn. 8, 034303 (2021); doi: 10.1063/4.0000107
Submitted: 4 April 2021 . Accepted: 17 May 2021 .
Published Online: 9 June 2021
€ rmer,1
Naoya Kurahashi,1,a) Stephan Thu
Suet Yi Liu,2,b) Yo-ichi Yamamoto,1Shutaro Karashima,1
1,c)
2
1,d)
1,2,e)
Atanu Bhattacharya, Yoshihiro Ogi, Takuya Horio,
and Toshinori Suzuki
AFFILIATIONS
1

Department of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa-Oiwakecho, Sakyo, Kyoto 606-8501, Japan2

Molecular Reaction Dynamics Research Team, RIKEN Center for Advanced Photonics, 2–1 Hirosawa, Wako 351-0198, Japana)

Present address: The institute for Solid State Physics, The university of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japan
Present address: TOYAMA corporation, 3816-1 Kishi, Yamakita-machi, Ashigarakami-gun, Kanagawa 258-0112, Japanc)Present address: Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, Indiad)Present address: Department of Chemistry, Faculty of Science, Kyushu University, 744 Motooka, Nishi-ku,
Fukuoka 819-0395, Japane)
Author to whom correspondence should be addressed: suzuki@kuchem.kyoto-u.ac.jpb)

ABSTRACTWe describe a magnetic bottle time-of-ﬂight electron spectrometer designed for time-resolved photoemission spectroscopy of a liquid microjet using extreme UV and X-ray radiation. The spectrometer can be easily reconﬁgured depending on experimental requirements and theenergy range of interest. To improve the energy resolution at high electron kinetic energy, a retarding potential can be applied either via astack of electrodes or retarding mesh grids, and a ﬂight-tube extension can be attached to increase the ﬂight time. A gated electron detector
was developed to reject intense parasitic signal from light scattered off the surface of the cylindrically shaped liquid microjet. This detector
features a two-stage multiplication with a microchannel plate plus a fast-response scintillator followed by an image-intensiﬁed photon detector. The performance of the spectrometer was tested at SPring-8 and SACLA, and time-resolved photoelectron spectra were measured for an
ultrafast charge transfer to solvent reaction in an aqueous NaI solution with a 200 nm UV pump pulses from a table-top ultrafast laser andthe 5.5 keV hard X-ray probe pulses from SACLA.
C 2021 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://V

creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/4.0000107

I. INTRODUCTION
A magnetic bottle time-of-ﬂight (MBTOF) spectrometer is, despite
its very simple design,1 a highly capable device for photoemission (PE)spectroscopy, featuring extremely high electron collection efﬁciency andsimultaneous detection of the full spectral energy range on a per-shotbasis. These advantages make the spectrometer a preferable choice overa classical hemispherical electron energy analyzer (HEA) for ultrafast
pump-probe photoemission spectroscopy with pulsed light sources.
Although the energy resolution (DE) obtainable with MBTOF spectrometers is usually considerably lower than that with a modern HEA,
the energy resolution required for ultrafast PE spectroscopy, especially

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Vfrom liquids, is often relatively low (on the order of 0.1 eV), and thus
this disadvantage is reduced. While several designs of MBTOF spectrometers for ultrafast laser pump-probe PE spectroscopy of a liquid
microjet have been reported,2–5 some speciﬁc design considerations are
required for employing a MBTOF spectrometer in similar experiments
using an X-ray free electron laser (XFEL). Here we report a spectrometer and a gated electron detector designed for experiments at SACLA(SPring-8 Angstrom Compact Free Electron Laser).6
In recent years, X-ray free electron lasers have been attracting great
attention as ultrashort pulsed light sources in the hard X-ray region (areview comparing each facility can be found in Ref. 7). It is noted,

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however, that pump-probe PE spectroscopy using XFELs is considerably more challenging than using a common table-top UV laser system.
For example, SACLA was operated at only 30–60 Hz at the time of ourexperiments, which is orders of magnitude lower than the kHz repetition rate of table-top lasers. Other XFELs like PAL,8 SwissFEL,9 or
LCLS,10 with the still-common nonsuperconducting accelerator technology have similar low repetition rates of up to 120 Hz; only very
recently was this barrier lifted by using superconducting accelerators,
like at the European XFEL or the LCLS-II upgrade facilities.7 PE spectroscopy needs to avoid the so-called space-charge effect caused by
Coulombic repulsion between photoelectrons when the density ofsimultaneously released electrons is high, which energy shifts and/or
broadens the electron kinetic energy (eKE) distribution.11 Therefore,
although XFELs are capable of producing extremely intense X-ray
pulses, their pulse energy must always be attenuated for PE spectroscopy. This inevitably diminishes one of the advantages of XFELs overother X-ray sources; however, the temporal duration of the hard X-ray
pulses generated by XFELs is unrivaled and considerably shorter thanthose from other X-ray sources. To mitigate the difﬁculty from the low
repetition rate, it is crucial to use a MBTOF design with its excellent collection efﬁciency of up to almost 100% of electrons emitted from a gaseous target and as much as 50% (the half-sphere facing the entrance
aperture) of electrons emitted from a liquid microjet. Furthermore,
spectroscopy using hard X-ray radiation requires to cope with very highabsolute kinetic energies (>100 eV) and a wide energy range of the photoelectrons. It is practically impossible to observe the full energy range
with a sufﬁcient energy resolution (DE) even with the time-of-ﬂight(TOF) method, and it is of great assistance to apply a retardation potential to shift the kinetic energy (E) of interest to a lower energy where sufﬁciently high-energy resolution is obtainable. Previously, Mucke et al.12
have reported a short (0.6 m) MBTOF spectrometer with a retardation
potential for measurements in the energy region around 13 eV, and similar MBTOF spectrometers with retardation potentials have been
employed for experiments using extreme UV lasers.4 Hikosaka et al.13
tested MBTOF-based PE spectroscopy in a higher electron energy range
up to 700 eV using Ar 2p and He 1s photoemission at UVSOR, andthey demonstrated that the resolving power (E/DE) of the spectrometer
is improved from 35 to 200 with a retardation potential. For achieving ahigh energy resolution, an alternative approach is to employ a very long
TOF tube;14–16 however, this approach often conﬂicts with the ﬁnite
experimental space inside the radiation-sealed hutches at XFEL
beamlines.
In this paper, we present two design approaches for retarding theincoming electrons, a multistage electrostatic lens system and a mesharray, each with its unique advantages. Both are easily swappable
depending on the experimental requirements. Additionally, our design
features a drop-in extension which, together with the compact mesharray, more than doubles the ﬁeld-free ﬂight length of the TOF tube
(to up to 1.8 m) for improved energy resolution. Although a greater
TOF tube length is expected to improve the energy resolution further,
the practical length allowed for the spectrometer was limited by thesize of an experimental hutch at SACLA and for reasons of ease ofoperation. The well-established liquid-microjet technique minimizes
the gas-pressure from highly volatile liquids and the fast liquid ﬂow
minimizes radiation damage of the sample. However, the chamberpressure on the order of 104 Torr must be further reduced to operate
the microchannel plate (MCP) detector at a pressure of 106 Torr or

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lower. Differential pumping of the MBTOF spectrometer is achievedby constricting the spectrometer oriﬁce down to about 1–2 mm via agraphite-coated conical skimmer. Because of the cylindrical shape ofliquid jets, inevitably some part of the pump and probe light pulses arescattered into the TOF tube toward the detector surface. As the MCP
is operated at a relatively high gain to mitigate a low count rate of theelectron signal of interest, the scattered light can cause the signal output to exceed reasonable limits which also quenches the detector until
the excess charge on the MCP is dissipated, and the MCP may even bepermanently damaged. We developed a gated MCP which can beswitched off on a per-pulse basis for a freely determined time window.
This enables us to maximize the detector gain for the weak electron
signal of interest while rejecting other intense signals including thescatter light and the associated background electrons. Our MBTOF
design is thus highly ﬂexible and can be utilized for a wide range ofexperimental applications in the extreme UV (XUV) to hard X-ray
energy range.
II. APPARATUS
A. MBTOF
The principle of an MBTOF spectrometer has been described indetail by Kruit and Read;1 contemporary designs usually use a permanent magnet4,12,13 instead of the electromagnet described in theirpaper. The essence of the MB principle is the parallelization of electron
trajectories in a magnetic ﬁeld gradient produced by a combination ofa strong and a weak magnet. Electrons are forced into a helical motion
around the ﬁeld lines, and when the variation of the magnetic ﬁeld
gradient from the high to low ﬁeld region is sufﬁciently gradual, i.e.,
adiabatic, then the angular momentum of the electron is conserved.
The conservation of the angular momentum reduces the transverse
velocity component of the electrons and thus provides parallelizationof the electron trajectories in the direction of the ﬂight tube. Thisfavorable condition is quantitatively characterized by the so-called
adiabaticity parameter given by1
 
rﬃﬃﬃﬃ pﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ  
2pmv dBz m 2E½eV  dBz 
¼ 2p
(1)
v¼
 dz ;eB2z  dz eB2z
where m and v are the electron mass and speed, respectively, and e isthe elementary charge. E is the eKE in eV, Bz is the magnetic ﬁeld
strength, and z is the distance from the ionization point along theﬂight axis. Maintaining an adiabatic condition (small value of v)
requires a low eKE and a high absolute magnetic ﬁeld while keeping
the ﬁeld gradient along the ﬂight path small.
Our magnetic bottle is formed by a strong magnetic ﬁeld, createdby a permanent magnet that is placed in the vicinity of the ionization
point, and a weak homogenous ﬁeld induced in the solenoid coil inside
the TOF analyzer tube. The strong permanent magnet is assembled
using a stack of four Sm2Co17 magnets, each of which is disc-shapedwith a diameter of 20 mm and a thickness of 5 mm. A 10-mm longsoft iron cone with a half angle of 45 is placed on top of the magnet
stack to focus the magnetic ﬂux. The magnetic ﬁeld strength at theface of the permanent magnet was measured to be 0.44 T, from which
the local ﬁeld strength at the tip of the iron cone was calculated to be
1.60 T using the simulation software Finite Element Method
Magnetics (FEMM: www.femm.info). Thus, we estimate the magnetic
ﬁeld strength at the ionization region to be approximately 1 T. Our

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simulations (see below) indicated that the magnetic ﬁeld at the ionization point can be increased by 24% by cutting the tip by 1 mm andmoving the magnet closer to the ionization point. We employed sucha cut cone for hard X-ray experiments. The solenoid coil is made of asilver-plated, PTFE-insulated copper wire with a conductor diameterof 1 mm and shielded with an outer metal shielding layer. The wire
was wrapped around the 1.3 m long drift tube (a nonmagnetic
stainless-steel tube) with 300 turns. The typical coil current is 3 A, producing a homogenous ﬁeld strength of 0.9 mT inside the drift tube.
After 10 h of operation in vacuum, the coil temperature reaches a plateau of 78  C. A higher coil current of 10 A would reduce v to half thevalue reached when using a coil current of 3 A; however, it would
require water cooling of the wire for stable operation.
We employed FEMM to calculate the magnetic ﬁelds created byour permanent magnet and solenoid-coil combination (see details ofSec. II B below). The resulting adiabaticity parameter according toEquation (1) and magnetic ﬁeld BZ along the center ﬂight axis is plotted in Fig. 1. In the simulation, the magnet-skimmer distance was set
to 4 mm, and in Fig. 1, the horizontal axis is plotted starting from theassumed position of the liquid-jet target, which is placed exactly halfway between the magnet tip and skimmer. The simulation indicatesthat v of our apparatus increases monotonically from the ionization
point up to 0.18 (in multiples of E1/2) at about 60 mm along theﬂight axis for a coil current of 3 A and then declines slowly. For
designing and modifying the apparatus, it is desirable to carry out electron trajectory calculations at high eKEs, where the adiabaticity tends
to decline. We imported the calculated magnetic ﬁeld data into the
SIMION 3D software package (Scientiﬁc Instrument Services) to carryout electron trajectory calculations. This is achieved by exporting themagnetic ﬁeld values from FEMM, which then get dynamically loaded
via a LUA-script into a magnetic array in SIMION. The magnetic ﬁeld
gradient is insufﬁcient for full parallelization of high-energy electrons,
so that a bundle of electron trajectories exhibit focusing and defocusing in a wavy pattern along the ﬂight axis in the ﬁeld-free drift region

FIG. 1. Evolution of the adiabaticity parameter v (red and green; left scale with values for E ¼ 1 eV) and magnetic ﬁeld strength along the TOF ﬂight direction BZ(blue; right scale) vs distance from the target (i.e., the liquid-jet nozzle) which isassumed to be placed halfway between the magnet tip and the skimmer oriﬁce.
The position of the two retarding meshes are marked with dashed lines.

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scitation.org/journal/sdy(in this paper, we refer to the “ﬁeld-free drift region” as the part of theelectron ﬂight path inside the TOF tube where both the electric potential and the magnetic ﬁeld strength is constant). When the diameter ofthis bundle becomes greater than the detector diameter, the electron
detection efﬁciency exhibits oscillation with the kinetic energy as someelectrons will inevitably miss the electron detector. With our 20 mm
diameter detector described below, the oscillation begins to impact thetransmission efﬁciency at around 300 eV.
We designed the TOF energy analyzer to be able to change its
electric potential and the electron pass energy. The variable potential isuseful for both decelerating high-energy electrons to increase the adiabaticity as well as the energy resolution and accelerating low-energy
electrons to improve their transmission efﬁciency through the spectrometer; usually, the detection efﬁciency of low-energy electrons
declines below 0.5 eV with MBTOFs. Therefore, when we measureslow electrons near zero kinetic energy, we apply at least þ0.5 V to theﬂight tube to increase the electron pass energy through the spectrometer and ensure an uniform detection sensitivity.
B. Chamber
Figure 2 shows a schematic drawing of our liquid-microjet
MBTOF apparatus where the photoelectron spectrometer is conﬁgured with a stack of retardation electrodes and features the standard(not elongated) 1.3-m ﬂight tube. It consists of (i) the main chamber(where the laser-liquid interaction occurs), (ii) a liquid-nitrogen trap
chamber, and (iii) the TOF electron energy analyzer. The main chamber is evacuated using a turbomolecular pump (TMP, 1400 L/s,
Pfeiffer Vacuum, TMU1601P). Pumping of water vapor is strongly
boosted by a large (surface area of 1385 cm2) liquid-nitrogen-cooled
trap; the liquid jet is collected in a second liquid-nitrogen cooled
catcher tube below the main chamber. The TOF analyzer is separated
from the main chamber with a skimmer oriﬁce of 1–2 mm in diameterand pumped by a TMP with a pumping speed of 2650 l/s (for N2 gas,
Edwards, STP-XA2703C). The rather large TMP attached to the TOF
tube is needed to enable the use of a relatively large skimmer oriﬁce;
MBTOF spectrometers designed for UV and EUV radiation usuallyemploy an entrance skimmer of 1 mm diameter. Chang and colleagues
employed a 3m long TOF tube and a 0.15 mm pinhole for the electron
sampling aperture and achieved E/DE of about 200 without a retarding
ﬁeld, while this sacriﬁced the electron collection efﬁciency down to
7%.16 On the contrary, our instrument is aimed at a higher electron
collection efﬁciency. Our simulations indicate that a diameter of theentrance skimmer smaller than 1.0 mm clips electron trajectories at anelectron kinetic energy higher than 150 eV, while the 2 mm diameterskimmer increases this energy limit up to 300 eV. With the 1 mmskimmer and when running a 25-lm diameter liquid-water microjetat a ﬂow rate of 0.5 ml/min, the pressure in the main chamber and the
TOF analyzer are 2.8  104 Torr and 1.7  106 Torr, respectively.
The entire regions of the main chamber and the TOF analyzer are
shielded with a Permalloy inner layer to prevent penetration of the terrestrial magnetic ﬁeld into the chamber at the photoionization point
and the TOF analyzer, which enables linear-drift operation without
the magnets. A pair of sliding rails are installed to mechanically support the entire TOF analyzer and to move the device out of the way
easily for inspection, servicing and cleaning of the TOF system andmain chamber and to install the extension drift tube from the front(see below). For pumping purposes, the drift tube holding the coil of

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FIG. 2. The schematic drawing of our liquid-microjet MBTOF apparatus (side view) equipped here with the standard 1.3-m length TOF photoelectron spectrometer and inserted
electrostatic lens system. Indicated in blue is a Permalloy inner layer to shield the apparatus from an external magnetic ﬁeld.

the electromagnet is perforated with holes of Ø4 mm (not shown inthe ﬁgure).
Liquid samples are injected using a fused silica capillary with a
15–25 lm inner diameter. A constant liquid ﬂow is supplied using ahigh-pressure liquid chromatography (HPLC) pump (Jasco PU2089)
at a typical ﬂow rate of 0.5 ml/min. Laminar ﬂow is observed for about
3–4 mm after ejection from the liquid nozzle oriﬁce and before the liquid beam spontaneously breaks up into a stream of droplets. After
traveling a distance of about 110 mm, the liquid is frozen in a liquidnitrogen-cooled trap with an entrance-aperture diameter of 73 mm.
The ionizing laser crosses the laminar ﬂow region within 1 mm downstream from the nozzle. For energy calibration of the spectrometer, agas sample is leaked into the main chamber via 1/1600 PEEK (polyether
ether ketone) tube from a different port. Both the permanent magnet
and the liquid jet assemblies are mounted separately on XYZ manipulators for ﬁne positioning relative to the light-interaction point and theentrance of the TOF analyzer (the skimmer). The distance between
the skimmer and the magnetic tip was usually adjusted to be 3–4 mm(compare Fig. 6). The vacuum components are graphite coated to
avoid any uneven potentials and to make their work functions equal.
C. Electrostatic lensFigure 3(a) shows the cross-sectional view of our electrostaticlens system, which is composed of stacked circular aluminum ring
electrodes, and PEEK spacers were used to insulate them from one

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another. For a retardation ﬁeld of –3000 V, the ﬁrst eight electrodes[#1–#8 in Fig. 3(a)] have a potential of 63.3% (–1900 V), 83.3%(–2500 V), 66.7% (–2000 V), 0.3% (–8 V), 10.9% (–327 V), 68.2%(–2045 V), 72.7% (–2180 V), 80% (–2400 V) of the total retardation
voltage (–3000 V), respectively. The last electrode (the drift tube
itself) and the front mesh of the MCP were set to be at equal potential, which is set 100% of the total voltage (–3000 V), to provide aﬁeld-free drift region. A screening mesh is placed in front of the
MCP assembly in order to prevent leakage of electric potentialfrom the assembly into the ﬁeld-free drift region. Voltages of theelectrodes, the drift tube, and the MCP mesh are independently set
using a computer-controlled multiport power supply (MBS, A-1
Electronics; 612 kV max). The entrance skimmer and the magnet
are grounded so that the electric potential gradient created by theelectrodes does not affect electron trajectories in the main chamber.
Figure 3(b) shows the transmission efﬁciency simulated withSIMION. It can be seen that our electrostatic lens in addition to themagnetic bottle enhances electron transmission through the ﬂight
tube and increases the detection efﬁciency.
Although not exploited in our experiments at SACLA andSPring-8, a highly useful application of the electrostatic lens is thefocusing of electron trajectories onto the active area of the electron
detector in PE anisotropy measurements. In such measurements, themagnetic ﬁelds in the ionization region and TOF analyzer are turned
off and the photoelectron ﬂux is measured as a function of the polarization direction of the ionizing radiation with respect to the electron

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FIG. 3. (a) Cross-sectional view of our electrostatic lens system (all units in millimeter). All electrodes were supported onfour PEEK insulating rods separated by PEEK insulating
spacers (green). (b) Simulated transmission efﬁciency (percentage of emitted electrons from the target which passes
the entire TOF region and hit the / 42 mm MCP) as a function of the initial kinetic energy of electrons with a 2 mm
entrance skimmer (blue: with magnetic bottle only; orange:
with additional applied retardation voltage to the electrostaticlens system); here, electrons were assumed to be emitted
randomly within a cone angle of 690 (half-sphere) from thesample toward the TOF tube.

detection axis. Under these conditions, the detection solid angle is limited by the diameter (42 mm) of the electron detector and its distance(1.3 m) from the ionization point, because a certain number of electrons that pass the entrance aperture of the TOF tube will still miss theelectron detector. However, the electrostatic lens inside the TOF tube
can focus the electron trajectories onto the detector to increase thedetection solid angle, which is ultimately only limited by the solid
angle formed by the electron sampling skimmer in front of the TOF
tube. We utilized this capability in ultrafast time- and angle-resolved
photoemission spectroscopy of liquid samples in our laboratory in thepast.17
D. Retarding mesh
While the electrostatic lens system enables ﬂexible control of theelectron pass energy and trajectories depending on the experimental
requirements, a drawback is its length of about 50 cm that diminishes
the ﬁeld-free ﬂight length after the lens stack and, consequently, theenergy resolution. Therefore, for utilizing the full length of the TOF
tube for energy analysis, we replaced the electrostatic lens system withsimple retarding meshes (two pieces with 90% open area), similar to
those previously employed by Hikosaka et al.,13 at the entrance of theﬂight tube behind the skimmer (Fig. 4). We also added an extension to
the ﬂight tube to increase the ﬁeld-free drift region to a total length ofabout 1.8 m (Fig. 5). In this conﬁguration, trajectory calculations indicate that extreme retardation (more than 93% reduction of the initial
kinetic energy) introduces inaccuracies in the measured PE spectra
due to band shape distortions; such an extreme condition is, however,
never reached in actual measurements. Another drawback of the retardation mesh is that we observed an additional background signal
which was absent with the electrostatic lens system described above.

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We attribute this disturbance signal to secondary photoemission after
electron impact onto the retarding mesh material. In principle, thiscan be mitigated by using meshes with a larger open area ratio.
E. Gated electron detector
In most of our ultrafast PE-spectroscopic measurements using
table-top lasers, we employ a standard chevron-type (dual) MCP
detector of 42 mm effective diameter with an electric current readoutfrom the anode in combination with a preampliﬁer (FAST Comtec,
1.5 GHz) and either a multichannel scaler (FAST Comtec, P7887) witha bin width of 250 ps or an analog-to-digital converter (Acqiris,
U1084A-002) for data acquisition. The former is employed at low electron count rates where only one or less than one electron arrives
within the response time (several nanoseconds) of the detection system, while the latter is employed at higher count rates. Since the signal
ampliﬁcation in the detector is stochastic, the individual pulse height
upon an electron arrival is not uniform; therefore, the multichannel
scaler is preferred as long as the count rate is sufﬁciently low. At
SACLA with an operating repetition rate of only 30–60 Hz, the UVpump and hard X-ray probe pulse intensities have to be relatively
high. Light scattered off the liquid jet onto the detector surface of the
MCP caused an intense signal spike, overwhelming the MCP at thehigh gain settings needed to detect the desired signal. To circumvent
this, a gated electron detector was employed in SACLA experiments.
To time-gate an electron detector one needs to switch the high-voltage
input for the microchannel plates. However, switching of high voltages
causes AC noise in the output current from the MCP anode, which
deteriorates the TOF data or even damages the preampliﬁer circuit. To
reject this noise, the signal was electrically isolated from the output byinserting a photoconversion step into the detector assembly, i.e., the8, 034303-5

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FIG. 4. Photos of the retarding meshes: (a) mesh cap and (b) mesh inset. The arrows indicate the diameter of the exposed mesh surface in each case. In the assembled conﬁguration these meshes have a distance of 8 mm (compare Fig. 5). The mesh cap is screwed onto the coil support while the mesh inset is placed into the bulkhead front plate;both are electrically insulated from each other.

signal is converted to light ﬂashes in the ﬁrst multiplier array and thislight signal is then converted back into an electric signal by a second
MCP stage. The bottom-left inset of Fig. 5 shows the schematic diagram of our gated detector; only the isolated ﬁrst MCP stack isswitched. We employed a phosphor screen with a short decay time
(1–4 ns) to transform electron signals ampliﬁed by a MCP(Hamamatsu, F2222–21PF282) to light pulses in the ﬁrst stage andthen observe the light with an image-intensiﬁed photomultiplier(Hamamatsu, R3809U-50-MOD) with a response time shorter than
0.3 ns in the second stage. This safely rejected any electric noise fromthe high-voltage switching of the electron detector. The gate-switchingis triggered by a synchronization signal from the light source, which
can be additionally offset with a time delay to ﬁnd the optimal cutout
window for the signal. Because of the limited size available for thephosphor screen, the effective diameter of the detector was restricted
to 20 mm. An 88-mm-diameter metal plate with a 20-mm-diameter

FIG. 5. TOF spectrometer in the elongated conﬁguration with the retarding meshes and the gated electron detector. A 520 mm inset brings the total drift tube length to1763 mm (inner mesh to detector) for improved resolution. The gated electron detector consists of two MCP stacks of which the ﬁrst can be switched by an external trigger.
The conversion to photons in the intermediate step ensures electric decoupling between the ﬁrst MCP and second MCP (for signal output), thus preventing the introduction ofelectric noise caused by the gate pulse.

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screening mesh in the center was placed in front of MCP to even out
the electric potential seen by incoming photoelectrons. The drawbacksof this gated detector are the small effective diameter (only 20 vs
42 mm), being limited by the size of a phosphor screen, and the slower
response (at best 1–1.5 ns vs 0.4 ns) as compared to the standard
detector.
F. Alignment
Since the propagation direction of the hard X-ray beam at
SACLA cannot be manipulated, the entire photoelectron chamber has
to be moved in order to bring the X-ray light to the desired interaction
position. For this, the entire apparatus was placed on a precision XYZtranslation stage of 1.2  2 m2 size (AINO Sangyo Co. Ltd.) andmoved by computer-controlled stepper motors. For precise alignmentof the chamber to the X-ray radiation, we used a thin Ce:YAG rod tovisualize the focal spots of the UV and hard X-ray pulses. The imageof the Ce:YAG rod was observed via a teleobjective lens through theskimmer oriﬁce from the far end of the TOF tube to precisely center
the rod with respect to the TOF tube axis. Then, we illuminated therod with the UV and the hard X-ray pulses and monitored their focal
positions from the rod’s ﬂuorescence response with three CCD cameras from different directions. This ensured precise alignment of the
deep-UV pump beam to the X-ray beam on the center axis of the TOF
tube. Finally, the liquid discharging nozzle was moved to the position
marked by Ce:YAG rod, and ﬁnal adjustments were made by maximizing the actual photoelectron signal. The liquid microjet and permanent magnet positions were adjusted using vacuum-compatible
manipulators; a few sample images are shown in Fig. 6.
III. RESULTS
A. Estimation of energy resolution
The kinetic energy resolution of our spectrometer was examinedusing PE spectroscopy of gaseous Ar and Xe with hard X-rays (5.5 keV)from SACLA; here, the conﬁguration of the extended 1.8-m ﬂight tube
and the retarding meshes were employed. For Ar, valence (eBE¼15.8 eV), KLL Auger (eKE ¼ 2660.5 eV), and K(1s) photoemission

ARTICLE

scitation.org/journal/sdy(eBE ¼ 3205.9 eV) were measured, while for Xe, valence (eBE
¼ 12.1 eV), M shell (eBE ¼ 676.4 – 1148.7 eV), LMM Auger (eKE
¼ 2500–4000 eV) photoemission were examined.18,19 Figure 7(a) shows
the photoelectron spectra of Xe shifting with the applied retardation
voltage; it is noted that each peak seen in these spectra are comprised
not necessarily of a single line but quite possibly an unresolved clusterof lines. The multiple peaks observed below an eKE of 2700 eV do not
originate from photoelectron or Auger emission from Xe and are of
unclear origin. The peaks shift 1:1 to lower eKEs with increased retardation voltage, while at the same time the peak’s FWHM decreases. The
intensity appears to decrease with increased retardation voltage, whichis ascribed to a nonuniform transmission efﬁciency at these high energies. As we explained in the Methods section, a part of electron ﬂux
misses the electron detector owing to wavy oscillations of electron trajectories when the kinetic energy in the ﬂight tube exceeds 300 eV; thus,
the detection efﬁciency varies with the electron pass energy in the tube.
The retardation potential changes the electron pass energy in the TOF
tube, which may then lead to a somewhat reduced probability for anelectron to hit the detector. Figure 7(b) shows the FWHMs of the measured gas peaks along with the simulated values for the extended TOF
tube conﬁguration in blue. Inpred
the theoretical resolution is shown,
ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
22
þ DESACLAwithcalculated by using DE exp ¼ DETOF
2Dt
3
DETOF ¼ pﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ E2 ;L m=2e(2)where Dt is the overall time-resolution of our instrument (assuming
an average of 1.25 ns here), L is the length of the electron path, and
DESACLA is the energy resolution of the SACLA beamline (assuming
1.3 eV here). The measured FWHMs are in good agreement with andeven smaller than the values predicted using the simple approximation
above. Figure 7(c) compares experimental results for the conﬁguration
with the standard 1.3-m ﬂight tube length and the electrostatic lenssystem in comparison with simulations of both the standard andextended ﬂight-tube lengths. Again, the experiment is in good agreement with the simulation, and it can be seen that the ﬂight-tube extension leads to a signiﬁcant increase in resolution at higher eKEs. For

FIG. 6. The liquid jet between the permanent magnet (to the left in both images) and skimmer oriﬁce (to the right) is observed from different directions via CCD cameras; here,
the side view aligned 135 to the TOF axis (left image) in the horizontal plane and the top view elevated 45 above the X-ray propagation axis (right image) is shown.

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obtaining an energy resolution sufﬁcient to detect chemical shifts on
the order of several eV, the absolute value of the kinetic energy needs
to be retarded to be less than 300 eV.

B. X-ray photoelectron spectra

FIG. 7. (a) Photoelectron and Auger electron signals measured for Xe with 5.5 keV
X-ray radiation and various retardation voltages using the extended (1.8 m) TOF
conﬁguration with retarding meshes at SACLA. (b) Comparison of FWHM values
achieved for Xe and Ar gases (black) with simulation results (blue dots), and an
approximation (red curve) for the extended TOF design. The inset is an enlarged
view of the vicinity of the origin. (c) Comparison of average FWHM values achievedfor Kr gas at an earlier SACLA experiment using the standard (1.3 m) TOF conﬁguration and electrostatic lens system with simulation results of the standard (red) andextended [blue, same as in (b)] TOF conﬁgurations using a mesh; the dashed lines
are again an approximation. The extended TOF design achieves a considerable
improvement in resolution.

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Prior to the experiments at SACLA, we performed PE spectroscopy of both gaseous and liquid samples at the BL29 beamline ofSPring-8 using the so-called H-mode, and we tested the performanceof our MBTOF spectrometer, the alignment method of hard X-ray
synchrotron radiation, and the measurement procedure. The H-modeof SPring-8 provides a single bunch at 208.9 kHz and 11/29 ( 38% of
the remaining ring) ﬁlled with a distribution of bunches; the former
was used for our measurements. The single-bunch X-ray pulse arrives
with a time delay of 1487 ns before/after the continuous 11/29-bunchtrain (i.e., there is a time window of 2974 ns containing only the single
bunch in its center). Figure 8(a) shows a retardation test using the signal from photoionization of Kr gas, which demonstrates the feasibility
and usefulness of electron retardation with meshes for the SACLA
experiments (see Sec. I.). Then, PE spectroscopy was performed on
Fe(1s) photoemission with 7.3 keV photons using 0.2 M aqueous solutions of K3[Fe(CN)6] and K4[Fe(CN)6] introduced as 25-lm diameterliquid microjets. Figure 8(b) shows the Fe(1s) signals of both solutions
measured using a retardation voltage of –100 V. The Fe(1s) photoemission signals were observed at 88.9 eV (FWHM 2.6 6 0.1 eV) forFe3þ and at 91.0 eV (FWHM 2.2 6 0.1 eV) for Fe2þ, i.e., exhibiting achemical shift of 2.1 eV. This shift is in reasonable agreement with thevalues previously estimated from X-ray absorption ﬁne-structure spectra by Bianconi et al. (1 eV)20 and by Reinhard et al. (2 eV).21 This
demonstrated that the resolution is high enough to spectrally separate
these two species with our apparatus and can thus be used in the
future for pump-probe experiments to study ultrafast Fe3þ $ Fe2þcharge-transfer processes. Figure 9 shows similar L-shell spectra ofiodine measured from 10 mM TBAI (Tetrabutylammonium iodide)aqueous solution with 5.5 keV photons and retardation voltages
between –50 and –700 V. PE peaks close to 1 keV were observed, andthe energy resolution was improved as retardation is increased (lower
voltage).
MCP gating was tested using photoelectrons from Kr, where thecontinuous light from the 11/29 bunches following the single bunch in
the H-mode generates a large photoelectron background (see Fig. 10).
By activating the MCP gate, which was triggered in synchronization
with the storage ring’s acceleration cavity, it was possible to precisely
reject a freely selectable part of the photoelectron signal from the multibunch train. As a result, almost no photoelectron signals wereobserved while the MCP was turned off. At Spring-8, the gating significantly reduced the overall signal intensity (see larger noise ﬂoor in
Fig. 10) since the gating trigger stage is limited to 1 kHz (optimized forthe low repetition rate of SACLA), which thus only utilizes 1/209 of
the total intensity from the single pulse repeating with 209 kHz. We
also noticed residual trigger noise when the MCP was turned back on,leading to parasitic spikes in the spectrum; no trigger spikes wereobserved without the gating. Since these noise peaks did not shift evenwhen a retardation voltage was applied, it was easy to distinguish themfrom true photoelectron signals and remove their contribution.

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FIG. 8. Representative spectra taken at the BL29XU beamline of SPring-8. (a) Kr gas measured at 14.6 keV with retardation voltages between 0 V and –250 V. The retardation
bias is translated 1:1 into a PKE reduction, while the resolution increases (lower FWHM). (b) 0.2 M iron hexacyanide aqueous solutions measured at 7.3 keV and with –100 V
retardation bias voltage. Both species are spectrally separable with the reached resolution of the device.

C. UV pump and hard X-ray probe experiment
Finally, we performed UV pump and hard X-ray probe PE spectroscopy at SACLA using a synchronized femtosecond laser system.
The pump-probe photoemission spectra measured for a 15-lm diameter liquid microjet of 0.5 M NaI aqueous solution are shown in Fig.
11. The spot sizes of the UV (200 nm, 5.0 lJ) and the hard X-ray(5.5 keV, 0.1 mJ) light were 200  100 lm and 16  24 lm, respectively. The UV spot size was made intentionally larger than that of thehard X-rays to mitigate unavoidable pointing instabilities of the UV
beam, which is delivered from a laser system placed outside of theexperimental hutch. The UV pump pulses promote a valence electronof I to a metastable CTTS (charge transfer to solvent) state, which

FIG. 9. L-shell spectra of 10 mM TBAI (tetrabutylammonium iodide) aqueous solution measured with 5.5 keV and retardation voltages between –50 and –700 V.

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spontaneously decays by detaching the electron to bulk water.3,22–27
The aqueous I solution exhibits two CTTS absorption bands corresponding to I(2P3/2) and I(2P1/2) ﬁne-structure levels, and the absorption at 200 nm is resonant with the transition to the CTTS(2P1/2)
band; however, UV photoexcitation at 200 nm produces I(2P3/2) andan excess electron within 300 fs and no signature of I(2P1/2) has been
observed.28 A great number of studies have been performed on this
CTTS reaction primarily by observation of the excess electron, while

FIG. 10. Comparison of photoelectron time-of-ﬂight spectra of Kr gas measured
with 14.6 keV photon energy at BL29 beamline of SPring-8 when the MCP gate is
activated (blue line; right intensity scale) and deactivated (red line; left intensity
scale). The signal drops to zero in the time window where the gating is active. The
gating at the maximum frequency of 1 kHz utilizes only 1/209 of the total photon
ﬂux of the H-mode at Spring-8, leading to a vastly reduced overall signal (see thetext for details).

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spectroscopic study on the electronic structure of the iodine atom has
been scarce; a few exceptional examples are picosecond X-ray absorption spectroscopy using the L1 and L3 edges of iodine by Chergui andcoworkers.29,30 Figure 11(a) shows a series of L3 edge photoelectron
spectra measured at different delay times as well as in the absence ofthe pump pulse; the photon energy was 5.5 keV. We performed a leastsquares ﬁt of these bands using a Gaussian function to ﬁnd the band
center as indicated with arrows in Fig. 11(a); it is seen that the band
shifts to lower eKE as a function of delay time. Figure 11(b) plots theseshifts as a function of delay time between the pump and probe pulses;
the total shift of the PE signature was about 2.8 eV. Since electron
detachment from the CTTS state of I occurs on a subpicosecond
timescale, the initial spectral shift within 1 ps is primarily due to thedetachment, and hydration of the I atom that becomes hydrophobic
after the detachment is expected to occur within a few picoseconds.
Figure 11(c) shows the difference spectra between the spectra measured at 61 ps and negative delay time; the positive and negative signals
represent the signal created and depleted by UV excitation, respectively. Thus the positive and negative signals are ascribed to neutral I
and I, respectively. The resulting eKE peak positions of these two
species are estimated to be 192.7 and 187.2 eV, indicating a chemical
shift of 5.5 eV. A slower relaxation process may also be present after
20 ps, although it is not clearly seen here with the limited signal tonoise ratio. One possibility is the formation of a molecular complex
such as I(H2O).11 It is also possible that local heating of the solution
may play a role31 under the strong excitation condition employed forthese measurements. Elucidation of the mechanistic detail awaits further experimental and theoretical studies in the future; however, thepresent study clearly demonstrates the feasibility of time-resolved PES
using a synchronized ultrafast laser and a hard X-ray free electron laser
with a MBTOF spectrometer and a gated electron detector.
IV. CONCLUSION

FIG. 11. Time-resolved photoemission spectra of the iodine L3 shell measured for
0.5 M NaI aqueous solution with 200 nm pump and 5.5 keV probe pulses. (a)Photoelectron spectra measured up to a pump-probe delay time of 156 ps. Arrows
indicate the peak center determined using least squares ﬁtting with a Gaussian. (b)Time dependence of photoelectron kinetic energy of iodine L3 photoemission features. The dashed line shows the energy observed for I in the absence of the UVpump pulse. The photoelectron kinetic energy is red-shifted up to a delay time ofabout 30 ps and then remains almost constant up to 156 ps. (c) Difference spectrum
obtained by subtracting the spectrum with negative delay time from the spectrum at
61 ps delay time. The white circles are the measurement results, the dotted lines
are the results of multipeak ﬁtting of two components, and the solid red line shows
the residuals of the ﬁtting. The negative signal (blue) is ascribed to I depleted by
UV excitation and the positive component (red) is ascribed to neutral I produced byphotodetachment.

Struct. Dyn. 8, 034303 (2021); doi: 10.1063/4.0000107
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We have designed and tested a versatile and reconﬁgurable
liquid-jet photoelectron spectrometer based on the simple but powerful magnetic bottle time-of-ﬂight principle for use with hard X-rays of
the SACLA free electron laser facility. The photoelectrons are retarded
either by an electrostatic lens system or a pair of meshes installed at
the front of the ﬂight tube to increase the resolution and reject lowenergy electron background signals. While the electrostatic lenses
allow for focused retardation conditions, the beneﬁt of the meshes istheir compactness, which maximizes the ﬁeld-free drift region for ahigher resolution. The ﬂight tube can be additionally extended to a
total length of almost 1.8 m for a further boost of resolution at hard Xray energies. To reject noise from light pulses scattered onto the detector, a gated two-stage multichannel plate was developed, which allows
for switching of the electron signal in a precisely selectable interval.
The performance and features of the spectrometer were tested using
hard X-rays at BL29 of Spring-8 and the SACLA free electron laser.
Retardation test measurements of Ar, Kr, and Xe gas as well as 10 mM
TBAI aqueous solution with bias voltages of up to –700 V demonstrated the 1:1 translation of the retardation voltage to a reduction of
the photoelectron kinetic energy and an increase in resolution. The
resolution was in good agreement with simulations and can be lowered
down to spectrally separate PE peak features with a distance of only a
few electron volts at hard X-ray energies, which was demonstrated bymeasuring the Fe3þ and Fe2þ 1 s core peaks of 0.2 M K3[Fe(CN)6] and8, 034303-10

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K4[Fe(CN)6] aqueous solutions; a peak separation of 2.1 eV between
these two species was determined. We successfully observed the shiftof the iodine L3 peak feature of 0.5 M NaI aqueous solution withincreasing pump-probe delay time in the experiments using 200 nm
UV pump in combination with 5.5 keV hard X-ray-probe pulses,
which is ascribed to the creation of neutral I from the photoexcitationof I. The present work demonstrates the feasibility of observing ultrafast (subpicosecond) processes in aqueous solutions with timeresolved PES using a synchronized ultrafast laser and a hard X-ray freeelectron laser.
ACKNOWLEDGMENTS
This work was supported by the “X-ray Free Electron Laser
Priority Strategy Program” of Ministry of Education, Culture,
Sports, Science and Technology of Japan (MEXT), JSPS KAKENHI
Grant No. 15H05753 and Mitsubishi Foundation. We thank Dr. Y.
Kohmura for his assistance in the experiment at BL29 at SPring-8,
Dr. M. Yabashi, Dr. T. Katayama, and Dr. T. Togashi, andProfessor K. Misawa and Professor Y. Obara for their support forexperiments at SACLA. Experimental assistance from N. Bartlett,
M. Sato, T. Kobayashi, H. Okuyama, K. Nagashima, and Y.
Furumido is also acknowledged.
DATA AVAILABILITY
The data that support the ﬁndings of this study are available
from the corresponding author upon reasonable request.
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