Liquid-phase mega-electron-volt ultrafast
electron diffraction
Cite as: Struct. Dyn. 7, 024301 (2020); https://doi.org/10.1063/1.5144518
Submitted: 01 January 2020 . Accepted: 13 February 2020 . Published Online: 09 March 2020
J. P. F. Nunes
, K. Ledbetter
, M. Lin, M. Kozina
, D. P. DePonte, E. Biasin, M. Centurion, C. J.
Crissman, M. Dunning, S. Guillet, K. Jobe, Y. Liu, M. Mo
T. J. A. Wolf
, J. Yang
, X. Shen, R. Sublett, S. Weathersby, C. Yoneda,
, A. A. Cordones, and X. J. Wang
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© 2020 Author(s).
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Liquid-phase mega-electron-volt ultrafast
electron diffraction
Cite as: Struct. Dyn. 7, 024301 (2020); doi: 10.1063/1.5144518
Submitted: 1 January 2020 . Accepted: 13 February 2020 .
Published Online: 9 March 2020
J. P. F. Nunes,1
K. Ledbetter,2,3
M. Lin,4 M. Kozina,4
D. P. DePonte,4 E. Biasin,3 M. Centurion,1 C. J. Crissman,5
4
4
4
6
4
M. Dunning, S. Guillet, K. Jobe, Y. Liu, M. Mo,
X. Shen,4 R. Sublett,4 S. Weathersby,4 C. Yoneda,4
3,a)
3,4,a)
3,a)
T. J. A. Wolf,
J. Yang,
A. A. Cordones,
and X. J. Wang4,a)
AFFILIATIONS
1
Department of Physics and Astronomy, University of Nebraska-Lincoln, Lincoln, Nebraska 68588, USA
2
3
Department of Physics, Stanford University, Stanford, California 94305, USA
Stanford PULSE Institute, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA
4
SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA
5
Department of Applied Physics, Stanford University, Stanford, California 94305, USA
Department of Physics and Astronomy, Stony Brook University, Stony Brook, New York 11794, USA
6
a)
Authors to whom correspondence should be addressed: thomas.wolf@slac.stanford.edu; jieyang@slac.stanford.edu;
acordon@slac.stanford.edu; and wangxj@slac.stanford.edu
ABSTRACT
The conversion of light into usable chemical and mechanical energy is pivotal to several biological and chemical processes, many of which
occur in solution. To understand the structure–function relationships mediating these processes, a technique with high spatial and temporal
resolutions is required. Here, we report on the design and commissioning of a liquid-phase mega-electron-volt (MeV) ultrafast electron diffraction instrument for the study of structural dynamics in solution. Limitations posed by the shallow penetration depth of electrons and the
resulting information loss due to multiple scattering and the technical challenge of delivering liquids to vacuum were overcome through the
use of MeV electrons and a gas-accelerated thin liquid sheet jet. To demonstrate the capabilities of this instrument, the structure of water
and its network were resolved up to the 3rd hydration shell with a spatial resolution of 0.6 Å; preliminary time-resolved experiments demonstrated a temporal resolution of 200 fs.
C 2020 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/10.1063/1.5144518
I. INTRODUCTION
Ultrafast solution phase photochemistry is the pillar of many biological and chemical processes, such as vision, photosynthesis, and
DNA photodamage,1–3 responsible for converting light into usable
chemical and mechanical energy. The atomistic understanding of these
chemical processes requires characterization of both solute and solvent
dynamics, as the reaction environment can dictate rates, pathways,
and efficiencies of reactions. Many spectroscopic methods have been
developed to probe reaction dynamics in solution. However, these are
not directly sensitive to the position of the nuclei and often infer the
nuclear structure from changes in the valence electronic structure.4
Time-resolved scattering techniques, on the other hand, offer
direct access to structural information concerning all atom pairs in a
solution, allowing the simultaneous capture of solute, solvent, and
Struct. Dyn. 7, 024301 (2020); doi: 10.1063/1.5144518
C Author(s) 2020
V
solute-solvent interaction dynamics. Time-resolved diffuse x-ray scattering experiments have been successfully used to track structural
changes in a variety of solution-phase systems at both the picosecond5
and femtosecond6 time scales. The shallow penetration depth of electrons (typically <1 lm, even at MeV energies7) compared to hard x
rays (typically >100 lm) had, until recently, limited their use in the
study of liquid-phase samples, as excessive multiple scattering prevents
the retrieval of structural information. Previous attempts at using electrons to probe the structure of liquids had relied on slowly evaporating
films,8 thin layer vapor deposition,9,10 and liquid cells11 to generate
thin samples. However, a rapidly refreshed sample is required for
ultrafast electron diffraction (UED) measurements, as such, the sample
delivery method must allow liquid flow. Liquid jets12 and nanofluidic
flow cells13,14 show promise in offering thin, flowing samples; here, we
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adopt the thin liquid jet approach developed by DePonte et al.,
which showed great promise in static scattering experiments. Jet characterization via transmission electron microscopy showed elastic scattering to overcome the inelastic background at jet thicknesses below
800 nm, motivating the development of thinner gas-accelerated liquid
sheet jets.12
The liquid-phase ultrafast electron diffraction (LUED) instrument presented here minimizes the loss of information due to multiple
scattering through the use of mega-electron-volt (MeV) electrons and
a gas-accelerated liquid sheet jet12,15 capable of producing sample
thicknesses on the order of 100 nm. The use of relativistic electrons16,17
not only overcomes the temporal resolution penalty associated with
the velocity mismatch between the pump laser and electron probe but
also reduces the space-charge induced broadening of electron bunches.
This allows the liquid jet, typically held at 104 Torr, to be far from the
electron source, held at ultra-high vacuum, while still preserving a
temporal resolution of sub-200 fs. The use of a continuous flow gasaccelerated liquid jet ensures that the sample volume is refreshed after
every shot at sample thicknesses amenable to scattering experiments
using MeV electrons. Preliminary studies of liquid water using the
MeV LUED instrument show spatial and temporal resolutions identical to those reported in MeV UED studies in the gas phase.18–20
Moreover, liquid-phase UED holds the promise of providing structural
information similar to, and in some cases, complementary to, that
obtained from x-ray scattering experiments. The sensitivity of electrons to the total charge distribution (Coulomb potential) of the
sample21 may be of particular importance in the study of photochemical mechanisms mediated by electron and/or proton transfer events.
In combination with the ability to achieve both sub-200 fs full width at
half maximum (FWHM) temporal resolution and momentum transfer
ranges in excess of 10 Å1, this makes liquid-phase MeV UED a
method to probe solution phase photochemistry with the potential to
resolve effects, e.g., from hydrogen bonding.
In this manuscript, we report on the design and commissioning of
an MeV LUED instrument for optical pump-electron probe studies of
liquid-phase samples, enabling the use of the MeV UED in liquid samples for the first time. The MeV LUED instrument, depicted schematically in Fig. 1, uses an MeV electron beam to probe the structure of
molecules in a thin liquid sheet produced by a gas-accelerated liquid jet.
Electrons scattered by the 100 nm thick liquid sheet are detected as diffraction patterns several meters downstream of the interaction point. In
Sec. II, we present commissioning results that demonstrate the spatial
and temporal resolution of the LUED instrument as well as the properties of the liquid jet. In particular, we report on the static diffraction signal for liquid water and its plasma lensing response upon illumination
with 800 nm light. An outlook onto the future development of the technique is presented in Sec. III. The design of the instrument is presented
in Sec. IV in three parts: (A) Integration with the SLAC MeV UED
beamline, (B) Sample chamber, and (C) Sample delivery.
II. RESULTS
A. Static electron diffraction of pure water
Electron diffraction for liquid water was acquired as part of the
LUED instrument commissioning. In Figs. 2(a) and 2(b), we present
an average diffraction pattern and the corresponding azimuthally averaged scattering signal for static water, integrated over 350 s and
expressed as a function of momentum transfer, s,
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FIG. 1. Schematic representation of the liquid-phase MeV UED experimental setup,
illustrating a MeV electron beam (A) traversing a thin liquid sheet (D) and the scattered electron (E) being recorded as a diffraction pattern (G) at the detector (F).
Species in the liquid sheet are excited by an optical pump (B) made to travel colinearly to the electron beam by a 90 holey-mirror (C). A detailed description of our
MeV LUED instrument can be found in Sec. IV.
s ¼ ð4p=kÞ sin ðh=2Þ;
(1)
where k is the de Broglie wavelength of the electron, 0.3 pm in the
case of our 3.7 MeV electron beam, and h is the angle between
scattered and unscattered electrons. The experimentally available
momentum transfer range is s ¼ 0.4–10 Å1; based on the inverse relationship between s and real-space distances, the spatial resolution is
2p=smax ¼ 0:6 Å.
A method analogous to those employed in the analysis of gas
electron diffraction (GED)23 is here employed in the retrieval of
structural information from the electron diffraction pattern of liquid water. Assuming elastic scattering, the total scattering intensity, I(s), for a sample of randomly oriented molecules can be
expressed as the sum of the atomic, Iat ðsÞ, and molecular, Imol ðsÞ,
scattering terms: I(s) ¼ Iat ðsÞ þ Imol ðsÞ. The contribution of the
atomic scattering to the overall scattering intensity is simply given
as the sum of all the elastic scattering amplitudes for all atoms in
the system,
Iat ðsÞ ¼
N
X
jfi ðsÞj2 ;
(2)
i¼1
where N is the number of atoms in the system and fi ðsÞ is the elastic scattering amplitude for the ith atom. The elastic scattering
amplitude for an MeV electron can be calculated using the
ELSEPA program.24 Note that Iat ðsÞ [Eq. (2)] does not contain
structural information. The molecular scattering term, on the
other hand, can be expressed as a sum of interference terms for all
atom pairs in the system,
Imol ðsÞ ¼
N X
N
X
i¼1 j6¼1
jfi ðsÞjjfj ðsÞj cos ðgi gj Þ
sin ðsrij Þ
;
srij
(3)
where fi ðsÞ and fj ðsÞ are the elastic scattering amplitudes of the ith and
jth atom, respectively, and gi ðsÞ and gj ðsÞ are their corresponding
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FIG. 2. Panel (a) shows an average diffraction pattern and panel (b) the azimuthally averaged scattering signal for liquid water at 290 K, acquired over 350 s. The temperature
of the water was determined using the method described in Sec. II C. The background signal (orange line) in panel (b) was acquired by switching off the flow of water and
maintaining a constant flow of helium. Panels (c) and (d) show UED, x-ray and simulated scattering curves, and sM(s) curves for liquid water, respectively. The simulated
sM(s) is generated under the independent atom model (IAM) approximation and assuming a water temperature of 290 K. Residual experimental background response contributions were subtracted from both the UED and x-ray scattering curves [panel (c)] using a third order polynomial curve fitted over the entire s range to obtain sM(s) curves [panel
(d)]. The x-ray scattering data were measured for liquid water at the European Synchrotron Radiation Facility, using the conditions described in Ref. 22. Panel (e) shows UED,
x-ray, and simulated pdf(r) curves for liquid water.
phases. Both elastic scattering amplitudes and phases are calculated
using the ELSEPA package.24 rij is the internuclear separation between
the ith and jth atoms.
Sinusoidal modulations imparted to the scattering intensity by
the interference term are made clearer through the use of a modified
scattering intensity, sM(s), defined as
sMðsÞ ¼
Imol ðsÞ
s:
Iat ðsÞ
(4)
The experimental modified scattering intensity for liquid water was
calculated using an adaptation of the method developed by Ihee
et al.,25 in which sM(s) is expressed as
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sMðsÞ ¼
I exp Ibkg
s;
Iat ðsÞ
(5)
where I exp is the experimental scattering intensity and Ibkg is a smooth
experimental background response, which includes elastic atomic scattering, inelastic scattering, and system-specific background. Iat is the
theoretical atomic scattering intensity for the sample [see Eq. (2)]. In
the case of our water diffraction, the experimental background
response contribution to the total scattering can be approximated by
fitting a smooth power curve (Ibkg ¼ Asn ) to I exp .
Theoretical sM(s) curves for liquid water were determined with
the aid of molecular dynamics (MD) simulations and a method developed by Dohn et al.,26 which allows the calculation of theoretical
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scattering intensities from MD generated pairwise radial distribution
functions. Under the independent atom model (IAM) approximation,
the atomic and molecular elastic scattering intensities of a system in
the liquid phase are given by
X
Nl jfl ðsÞj2 ;
(6)
Iat ðsÞ ¼
l
Imol ðsÞ ¼
X
l;m
Nm ðNl dm;l Þ
4p
jfm ðsÞjjfl ðsÞj
V
ðR
0
r 2 gl;m ðrÞ
sin ðsrÞ
dr;
sr
(7)
where Nl and Nm are the number of occurrences of atom types l and
m, which in our application correspond to different elements.
Similarly, fm ðsÞ and fl ðsÞ correspond to the elastic scattering amplitude
of elements l and m. gl;m is the pair radial distribution function for the
l and m elements, dm;l is the Kronecker delta, and R is the radius of the
coherence volume, V, in the sample. The derivation of Eqs. (6) and (7)
can be found in Ref. 26. Classical MD simulations of 4054 water molecules (50 50 50 Å box) were carried out using the TIP4P-Ew
force-field27 at constant temperature and pressure. TIP4P-Ew is a
good general-purpose model for water, purposely tuned to reproduce
the bulk-density and enthalpy of vaporization of liquid water28
and with predicted structural properties (O–O radial distribution
functions) in good agreement with those observed using x-ray
scattering.27,29 The MD simulations were carried out over 1000 ps in 2
fs time steps using the GROMACS package.30 The resulting MD trajectories were processed into time averaged pairwise radial distribution
functions using the VMD package.31 The temperature of the MD simulations was varied between 250 and 400 K using the Berendsen thermostat32 and using Eqs. (6) and (7), a theoretical scattering curve was
generated for each temperature. As the position of the first diffraction
peak of water is strongly dependent on the temperature of the water,33
the position of the first peak in our experimental data was used to
determine the temperature of the sample and select the adequate simulation temperature. Using this method, the experimental data presented in this section were determined to correspond to water at
290 K. A more detailed description of the sample temperature determination method can be found in Sec. II C. The experimental and simulated modified scattering intensities for liquid water at 290 K are
presented in Fig. 2(d). The sine transform of the modified scattering
intensity can be used to retrieve structural information in the form of
a pair distribution function, pdf(r),
ð smax
pdf ðrÞ ¼
sMðsÞ sin ðsrÞ exp ðks2 Þds;
(8)
0
where k is a damping factor used to suppress high frequency artifacts
generated by the truncation of sM(s) at s ¼ 10 Å1.
The experimental and simulated pair distribution functions for
liquid water are shown in Fig. 2(e). The positions of features in the
pdf(r), corresponding to ensembles of similar internuclear distances,
match those predicted by our IAM simulations and observed in x-ray
scattering experiments.34–37 Therefore, we are able to assign these features to the structural motifs of liquid water. The peak at 1.0 and
shoulder at 1.8 Å correspond to the bonded O–H and hydrogen
bonded O H internuclear distances in the water’s first hydration
shell, respectively. The ability to resolve the hydrogen bonded O H
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is of particular importance, given the lack of sensitivity of x rays to
hydrogen nuclei. The lack of electron density around the nucleus of a
bonded hydrogen atom results in a rather weak signal, not detectable in
most cases. The peak at 2.9 Å corresponds to the non-bonded O O
internuclear distances between two neighboring waters, while the broad
peaks centered at 4.4 and 6.9 Å correspond to internuclear distances
across the 2nd and 3rd hydration shell of water, respectively.
To ensure that our comparison of UED and x-ray pdf(r)
remained unbiased from theoretical input, we relied on rather simple
power and polynomial fits to remove the experimental background
response. As a result, instances in which the background response cannot be adequately modeled by either a power or low order polynomial
functions lead to background contributions to the pdf(r). These are
believed to be the source of the discrepancies between the experimental and simulated pdf(r) amplitudes. On the other hand, discrepancy
between experimental and simulated total scattering, most noticeable
in the low s region [Fig. 2(b)], has been attributed to the breakdown of
the independent atom model and will be discussed in a follow up
publication.
B. Temporal resolution
In the absence of fast features with a known temporal profile,
only the upper limit of the time resolution can be estimated, from the
full width at half maximum (FWHM) of the fastest feature observed in
time-resolved experiments. We estimate the time resolution from the
duration of the low-s difference signal resulting from plasma lensing
of the electron beam by the laser-ionized sample, as used in gas-phase
UED to find temporal overlap.38
In pure water pumped at 800 nm, a fast feature is observed in the
difference scattering integrated from s ¼ 0.45 to 0.85 Å1. The low-s
region of difference scattering is shown in Fig. 3. The signal was fit to
the sum of a Gaussian, representing the plasma lensing signal, and a
step function convolved with the same Gaussian, representing the
small structural signal, which appears after t0. The FWHM of the
Gaussian is used as an upper-limit estimate of the experimental time
resolution. For a laser pump pulse fluence of 1.1 J/cm2 (64 fs FWHM
pulse duration measured before one lens and window), the signal is
visible after a single scan (9000 shots per pump-probe delay, 45 delays,
23 min of lab time). During stable operation of the electron gun, the
minimum single-scan FWHM of this feature was 180 6 20 fs. When
averaged over six scans, there was some broadening to 209 6 4 fs, due
to a slow drift of temporal overlap by 60 fs over 4 h of lab time.
The pump laser in this experiment was incident at a 30 angle
with respect to the electron beam, which introduced additional time
smearing. For a pump pulse spot size of 50 lm (smaller than the
electron beam), the tilted incidence is expected to broaden the time
resolution by 30 fs. The overall time resolution performance in the liquid phase is therefore comparable to that of gas phase experiments at
the facility, which reported time resolutions of 16019 to 23038 fs.
C. Jet parameters
The sample delivery for liquid-phase UED is bound by the constraint that the thickness of the sample must be less than 20% of the
electron mean free path,39 which is about 1 lm in water for 3.7 MeV
electrons, to avoid significant multiple scattering of electrons within
the sample and determine coordination numbers to an accuracy better
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FIG. 3. Upper-limit time resolution estimated from a low-s beam streaking effect in
water excited at 800 nm, 1.1 J/cm2 fluence. (a) Difference scattering signal DS as a
function of time delay and s, averaged over six 23-min scans. The negative signal at
the lowest s and positive signal up to 1 Å1 are a result of the main beam profile
becoming elongated when passing through the ionized sample. (b) Time trace (black)
of DS integrated between the dotted lines in panel (a), and fit (red). The FWHM of the
feature is 209 6 4 fs.
than 10%.39 Therefore, the sample delivery system is designed around
a gas-accelerated ultrathin sheet jet, which is able to deliver
sub-100 nm liquid sheets.15 Using a glass microfluidic chip, the initially
cylindrical liquid jet, 20 lm in diameter, is flattened from either side by
gas flow, thus producing a sub-micrometer sheet. The variable liquid
flow rate and gas pressure allow the dimensions of the liquid sheet
to be tuned. A range of pure water sheet parameters, from 0.15 to
0.25 ml/min liquid flow rate and 65–78 psi helium, were characterized
to determine optimal jet parameters. Images taken under two of these
conditions, 0.20 and 0.25 ml/min water flow and 78 psi helium, are
shown in Fig. 4, panels (a) and (b). These conditions produced the
largest liquid sheets, 425–450 lm in length and 140–160 lm in width.
The thickness of the sheets, as measured via the interferometric imaging described in Sec. IV B 4, is reported in Fig. 4(c) as a function of
distance from the nozzle. The measured thickness profile decreases
from 700 nm near the top of the jet to sub-100 nm at the bottom. The
increase in the flow rate from jet a to b manifests as a wider and longer,
but also thicker, liquid sheet. The gas pressure was set to the highest
possible pressure that still allowed for stable jet operations (<80 psi).
To assess the best UED measurement conditions, the liquid sheet
was characterized via static electron scattering at a grid of points
spaced by 50 lm in the x and y directions. Two parameters were
extracted from these scattering data. First, the transmission of
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electrons through the sheet was measured by a second detector behind
the hole in the main detector phosphor screen (see Sec. IV A), which
measured the non-scattered beam. The transmitted electron counts
were divided by average counts on the detector when no jet was present. The average count rate without the jet was considered a constant
between the two conditions shown, though changes in the background
pressure in the chamber could introduce uncertainty in comparing
absolute transmission values between the two conditions. Transmission
as high as 92% was measured for the thinnest part of the sheet
[Fig. 4(d)]. The second parameter was the ratio of high- to low-s scattering signal, which will be referred to as the multiple scattering ratio.
Singly scattered electrons are expected to scatter most strongly around
2 Å1 (the liquid peak). Multiply scattered electrons appear at all
momentum transfers, creating a background throughout the scattering
pattern. Therefore, we use the ratio of scattering integrated between
s ¼ 6.8–9.0 Å1 and s ¼ 1.6–3.1 Å1 as a relative measure of the
amount of multiple scattering present in an image. A low ratio corresponds to relatively little multiple scattering, and therefore a lowerbackground measurement of the elastic single scattering. This ratio at
each of the 25 grid points is shown as an overlay on the jet images in
Fig. 4, with a 50 lm uncertainty in the vertical position of the electron
probe relative to the jet image. The value of the ratio along the center of
the sheet is shown in panel (d).
The transmission of electrons monotonically follows the measured sheet thickness; however, the multiple scattering ratio exhibits a
minimum as the electrons are scanned vertically down the center of
the sheet. This effect can be attributed to scattering from the sheet
edges [see overlay in Figs. 3(a) and 3(b)]. While the sheet is thin in the
center, the edges are much thicker than the sheet itself, estimated at
10 lm. A small fraction of the electron beam scattering in the sheet
edge will not strongly affect the transmission. However, since the
thickness at the edge is many times greater than the electron mean
free path, all electrons incident on the edge can be expected to scatter
multiple times and contribute to the uniform background on the scattering detector. In the lower part of the sheet, the multiple scattering
background increases due to clipping the electron beam on the edges
of the jet.
Therefore, optimal measurement conditions are achieved at the
vertical midpoint of the sheet. The lower portion of the jet, although
thinner, cannot be used due to the prevalence of multiple scattering
induced by the thick sheet edges. Comparable signal quality could be
achieved with the higher flow rate jet, despite the sheet being thicker
by a factor of two, because the greater width allowed the whole electron beam to pass through the sheet. The transmission of electrons
alone, which is not as sensitive to the jet edges, is not a sufficient figure
of merit to determine optimal beam placement on the jet.
The temperature gradient across the liquid sheet was characterized
using the static diffraction of water. First, the scattering signal of water
was simulated for temperatures ranging between 250 and 400 K, using
the method described in Sec. II. This revealed a strong dependency
between the position first diffraction peak and the temperature of the
water, with lower temperatures resulting in the shifting of the first diffraction peak toward lower momentum transfer s values, as shown in
panel (a) of Fig. 5. This trend agrees with a previous experiment by
x-ray scattering.33 The evolution of the first diffraction peak position as
a function of temperature was fitted with a third order polynomial,
resulting in the calibration curves shown in panel (b) of Fig. 5. A series
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FIG. 4. (a) and (b) Images taken at 30 from normal of liquid jet at 0.20 and 0.25 ml/min liquid flow rate, respectively, and equal He pressure; liquid flow is from top to bottom.
Overlay: multiple scattering ratio, as defined in text, for a grid of electron beam positions. The scale bar is 100 lm; the FWHM electron beam size is also shown. (c) Thickness
of jets (a) (blue) and (b) (red) as a function of distance from the nozzle, as measured by thin-film interference. Error bars represent 12.5 lm resolution of camera; the dotted
line represents region where the jet is thinner than the sensitivity of the interference measurement (102 nm) as described in Sec. IV B 4. (d) Transmission and multiple scattering ratio at the center of the jet as a function of jet thickness for the two jet conditions. The shaded region corresponds to 650 lm uncertainty in the vertical position of electrons on the jet.
of diffraction patterns were recorded at different points along the liquid
sheet and the position of the first scattering peak was extracted and
compared to the calibration curve to produce a temperature estimate.
Sheet temperature estimates generated from this comparison are shown
in panel (d) of Fig. 5. The top of the sheet, nearest the chip, is considerably warmer than room temperature due to radiative heating of the chip
and chip holder by the heated catcher. The temperature decreases down
the sheet, as the liquid undergoes rapid evaporative cooling. While cooling occurs in all jet conditions, the absolute temperature values are
expected to depend heavily on the distance between the catcher and
chip, the solvent used, and the thickness of the jet. Considering the
13 m/s speed of the jet, the temperature gradient implies cooling on the
order of 106 K/s. This is roughly an order of magnitude faster than
reported values for supercooled water in 5 lm cylindrical jets40 and two
orders of magnitude faster than in 12 lm droplets.33,41
Jet stability cannot be evaluated on a shot-by-shot basis due to
the integrating mode of the detector (generally 5 s per image).
However, image-to-image stability on the time scale of minutes to
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hours can be addressed. The intensity of the transmitted beam
without a sample, measured by the secondary detector, has 6% rms
fluctuations over 2.3 min of lab time. In comparison, the beam
transmitted through the jet exhibits 7% rms fluctuations on the
same time scale. However, on longer timescales, slow changes in
the transmission (corresponding to the thickness of the jet) can be
observed, with a 19% decrease in transmission over 4 h observed in
a pure water jet.
The microfluidic chips were used continuously for as long as 40 h
before replacement, with 15 h of data collection on pure water being
typical before the chip required replacement. Pure solvents give the
most stable jet start/stop and running conditions. However, highly
concentrated aqueous solutions were also successfully run in the
LUED chamber. Ionic solutes at 100 mM and 500 mM concentration
ran for a maximum of 16 h without interruption (typical run time
8 h), despite some buildup of salt on the catcher cone and jet nozzle.
Sample chamber running pressures with solutes were as low as for
pure water jets, which ranged from 8 105 to 2 104 Torr.
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FIG. 5. Panel (a) shows the simulated scattering signal of water over the 1.5–4 Å1 range for temperatures ranging between 250 and 400 K. Panel (b) shows the evolution of
the first peak position in the scattering signal of water as a function of temperature. Panel (c) shows the first peak of water scattering data acquired at varying distances from
the chip. Panel (d) shows the estimated water temperature as a function of distance to the chip.
The gas-accelerated sheet jets, while designed for water jets, can
also deliver other solvents. Ethanol jets were demonstrated in vacuum
with a sample chamber pressure of 2 104 Torr. Due to its higher
vapor pressure, ethanol requires the collection bottle, as described in
Sec. IV C, to be held at 20 C. Sample-specific testing is necessary to
determine whether solute deposition on the nozzle, due to rapid evaporation, will destabilize the jet in vacuum.
The chip holder can accept different types of microfluidic chips
and is not limited to gas-accelerated nozzles. Converging-type42 nozzles, which create liquid sheets without gas flow, were also used in
the LUED chamber. These nozzles create larger sheets with width
and length dependent on the liquid flow rate. Typical sizes of 2 mm
in length and 0.5 mm in width are possible at sample flow rates of
2.4 ml/min, with thickness varying from 1.5 lm to 600 nm. The
larger sheet size allows the use of higher laser power than was
Struct. Dyn. 7, 024301 (2020); doi: 10.1063/1.5144518
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tolerated by the gas-accelerated chips, as the interaction region is further from the nozzle. These converging nozzles will be detailed in an
upcoming publication.
D. Noise levels
Noise levels for a representative time-resolved LUED experiment
are shown in Fig. 6. A pure water jet was pumped with 3 lm light, and
images were taken at several time delays. Noise is estimated from the
azimuthally integrated difference scattering by subtracting a smoothed
average difference signal, normalizing by the total laser-off scattering,
and calculating the rms of the remaining noise. The noise estimate was
applied to difference scattering measured at a fixed time delay for each
of 30 scans. Each scan comprises five images, each integrated for 5 s
(1800 shots per image).
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FIG. 6. Noise levels of difference scattering from pure water pumped with 3 lm
light at 2 ps time delay. Inset: rms noise after 7 min of integration as a function of s.
Main figure: noise in range s ¼ 0–0.75 Å1 (red), s ¼ 0.75–6 Å1 (blue), and
s ¼ 6–10 Å1 (green) as a function of integration time.
Noise below 0.2% rms in the 0.75–6 Å1 range is achieved after
7 min of integration time per pump-probe delay. However, noise in
the high-s region (> 6 Å1) is consistently higher and exhibits a jump
after 7 min. This could be related to fluctuations in the thickness of the
jet, which affects the ratio between low- and high-s scattering (as discussed in Sec. II C). The noise at the lowest s values, below 0.75 Å1, is
also higher, despite this being the area with the most counts. Small
fluctuations in beam pointing, possibly due to charging effects, could
contribute to the noise in this region.
For experiments on pure liquids, difference signals in the percent
range are possible, making these types of experiments feasible with
4–5 min of integration per time point. However, scattering from solution samples is dominated by the solvent, generally with thousands of
solvent molecules per solute molecule of interest. As a consequence,
the difference signals for solutes are typically very small, well below
0.1% compared to total scattering.43–45
III. OUTLOOK
The LUED instrument has extended the UED technique to liquidphase samples. An ultrathin liquid sheet jet, running in vacuum in the low
104 Torr regime, was demonstrated in water, concentrated (hundreds of
mM) aqueous solutions of ionic solutes and ethanol. Static scattering from
liquid water achieved structural sensitivity over a range of 10 Å with a resolution of 0.6 Å, and structural features associated with hydrogen bonding
were observed. The instrument achieved a time resolution of 209 fs
FWHM in pump-probe experiments at 30 pump incidence.
Time-resolved experiments targeting changes in the overall structure of pure liquids are already achievable with current signal-to-noise
ratios, which allow measurement of difference scattering on the percent level within several hours of data collection. However, challenges
remain in studying molecules in solution, due to the large background
caused by solvent molecules. Typically, difference signals for structural
changes of 0.2 Å in solutes at 20–70 mM concentration are below
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0.1%.43 Several future improvements described below will facilitate
these lower signal experiments.
Improvements to signal-to-noise are possible through several system upgrades. Increasing the electron flux by running at a higher repetition rate will decrease required averaging times, with a repetition rate
of 1 kHz planned for the SLAC MeV UED facility. Furthermore, a
planned upgrade to a direct electron detector,46 capable of singleelectron detection, will also improve signal-to-noise. The direct
detector will enable an electron-counting detection scheme that can
potentially eliminate camera readout noise. In addition, the current
system requires averaging of many shots into a single image, and
saturation prevents imaging of the direct beam at the same time as the
scattering pattern. The direct detector will allow imaging of the beam
concurrently with the scattering pattern on a shot-by-shot basis. The
noise observed in the lowest s values with the current detector points
to a strong sensitivity to small changes in beam pointing, which can
wash out difference signals when many shots are averaged together.
With the single-shot detection scheme, beam pointing and proper
normalization can be accounted for in each image. In addition, jet stability can be improved by replacing pressure control of the accelerating
gas with a mass flow controller, which could eliminate the slow change
in jet thickness observed in the electron transmission.
The first experimental run has produced time-resolved experiments on pure water in several excitation regimes as well as observation of dissociation of I
3 in solution, which will be reported in
upcoming publications. The improvements detailed above will
extend the potential of the LUED technique from highly concentrated systems (such as neat solvent) to more dilute chemical samples. Future photochemical experiments have the potential to exploit
the Coulomb potential sensitivity of electrons to act as a complementary method to x rays, especially to observe reactions involving proton transfer.
IV. METHODS
A. Integration with the SLAC MeV UED beamline
The LUED sample chamber was installed 0.75 m downstream
of the photocathode RF gun in the SLAC MeV UED beamline, schematically depicted in Fig. 7 and described in further detail elsewhere.47
The 3.7 MeV electron beam produced by the S-band 1.6-cell photocathode RF gun is delivered to the interaction point, 1.15 m downstream, with a spot size of 88 37 lm2 FWHM at an average bunch
charge of 2 fC. Higher bunch charges (up to 100 fC) are available at the
expense of temporal resolution, spot size, and reciprocal-space resolution. The system operates at a repetition rate up to 360 Hz; in this
work, the repetition rate was 180 Hz for data shown in Figs. 2, 4, and 5,
and 360 Hz for data shown in Figs. 3 and 6. A series of differential
pumping stages with elongated chokes decouple the vacuum in the RF
gun (1010 Torr) from that of the sample chamber (104 Torr).
Located 3.2 m downstream of the interaction region, the SLAC MeV
UED detector consists of an in-vacuum back-illuminated phosphor
screen and high-reflectivity mirror, oriented 90 and 45 with respect
to the beam path, respectively. A schematic representation of the detector geometry is shown as an inset in Fig. 7. A hole through the center
of the phosphor screen (3 mm dia.) and mirror assembly (4.5 mm dia.)
allows the unscattered electrons to pass through the detector, improving the detector dynamic range and preventing saturation. Photons
generated by the incidence of a scattered electron on the phosphor
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FIG. 7. 3D CAD model of the SLAC MeV beamline and LUED sample chamber. Typical operating pressure of various differentially pumped sections is presented in Torr above
the beamline. The inset on the top right corner illustrates the geometry of the detector.
screen are coupled out of the beamline and collected by a 50 mm f/1.2
lens onto an Andor iXon Ultra 888 electron-multiplying charge-coupled device (EMCCD). The SLAC MeV UED beamline is driven by a
Ti:sapphire laser system, producing 65-fs 800 nm laser pulses at a repetition rate of 360 Hz. A small portion of this laser light (400 lJ) is frequency tripled to 266 nm and used in the generation of photoelectrons
at the RF gun, with the remaining (>12 mJ) made available to the optical excitation of samples at wavelengths ranging from 240 to 2400 nm
accessible via harmonic generation or optical parametric amplification.
The delivery of optical pump pulses to the interaction point is discussed in Sec. IV B 2.
B. Sample chamber
The LUED sample chamber was constructed to house a variant
of the ultrathin free-flowing liquid sheet sample delivery system developed by Koralek et al. and described elsewhere.15 In brief terms, a submicrometer free-flowing liquid sheet is formed by the flattening and
shaping of a cylindrical liquid jet by two converging gas jets. The liquid
and gas jets are delivered to vacuum using a three-channel borosilicate
microfluidic chip (Micronit). The resulting liquid sheet is collected a
few millimeters below the chip. A more detailed description of this
sample delivery system can be found in Sec. IV C. The LUED sample
chamber design fulfills five major requirements: maintain three orders
of magnitude pressure differential between the sample chamber and
the in-coupling mirror, allow near-collinear pump and probe of the
target sample, allow remote alignment of the chip and catcher assembly, accommodate an interferometric sheet thickness measurement
tool, and allow for quick and unrestricted access to the interaction
region. Design features addressing these requirements are presented in
Secs. IV B 1–IV B 5.
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1. Vacuum system
The gas load associated with using a free-flowing liquid sheet can be
divided into two main contributions: the continuous flow of gas used to
flatten the otherwise cylindrical liquid jet into a thin sheet and the gas
load associated with the evaporation of liquid sample. The magnitude of
the latter is strongly dependent on the vapor pressure of the sample. Our
liquid sheets are shaped using helium, the elastic scattering cross sections
of which is small enough so as not to contribute significantly to the diffraction signal. In the LUED chamber, the helium and sample gas loads
are managed using two vertically mounted 1300 Ls1 turbo molecular
pumps. Additionally, a high surface area cold trap cryo-cooled to 70 K
can also be used to help manage the gas load associated with sample evaporation. Under typical flow-rates of 0.25 ml/min of liquid sample and
100 sccm of helium, chamber pressures of 104 Torr can be maintained
for more than 24 h. Pressures here reported were achieved without the
use of the cold-trap and thus represent the upper limit of our operating
conditions. A differential pumping stage fitted with two 30 Ls1 turbo
molecular pumps and a protruding capillary maintains up to three orders
of magnitude pressure differential between the sample chamber and the
MeV beamline. This differential pumping stage, henceforth referred to as
the incoupling cube, is also responsible for housing and preventing the
chemical contamination of optics used in the incoupling of quasicollinear pump pulses. The two differential pumping turbo molecular
pumps in the incoupling cube are backed by an 80 Ls1 turbo molecular
pumping station, thus ensuring the compression ratios necessary for the
pumping of helium. In the case of pressure spikes, adequate vacuum isolation between the MeV beamline and the sample chamber is ensured by
two gate valves installed on either side of the chamber and interlocked to
the beamline vacuum gauges. Typical pressures for the chamber, incoupling cube, MeV beamline, and RF gun are presented in Fig. 7.
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2. Incoupling optics
The LUED setup was designed to accommodate three optical
pumping geometries: quasi-collinear, 15 counter-propagating, and
30 co-propagating with respect to the electron beam path. In its
collinear configuration, the pump laser is incoupled through an
in-vacuum 90 holey-mirror positioned inside the incoupling cube
stage, and delivered to the interaction point via a long capillary. A
4 mm thick copper shower-stopper protects the mirror substrate
from potentially damaging stray MeV electrons. The position of the
capillary can be adjusted remotely in two degrees of freedom [see
Fig. 8(b)], which facilitates the overlap between pump and probe
beams and allows adequate clearances to be maintained between the
beams and the inner walls of the capillary. The length and internal
diameter of the capillary can be adjusted depending on the vacuum
requirements and vapor pressure of the sample, with typical dimensions ranging between 10 and 30 mm in length and 1.2–2.5 mm in
internal diameter. Geometric constraints imposed by the capillary,
as well as the damage threshold of the incoupling mirror, limit the
collinear optical pump energy and spot size to 200 lJ and 300 lm,
respectively. In experiments where higher pump fluences are
required, such as studies on warm dense matter48 or strong field ionization, two 4.5 in. conflat windows oriented at 15 and 30 with
respect to the interaction region are available. These optical pump
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geometries allow access to high pump fluences, albeit at the cost of
temporal resolution (see Sec. II B).
3. In-vacuum manipulation
The position of the borosilicate microfluidic chip with respect
to the heated-catcher is controlled by three vacuum stages in an xyz
configuration. Two cameras positioned 30 and 90 with respect to
the electron beam path provide real-time feedback on the positions
of the chip and catcher, allowing for remote operation and alignment. A PEEK (polyether ether ketone) interface that holds the chip
also mounts a copper charge-guard that protects the chip from
charging by stray MeV electrons. The charge-guard is lowered
towards path of the electron beam using a piezo-vacuum-stage. A
diagnostic paddle containing a YAG screen and several crystalline
samples is mounted onto the chip clamp with its sample plane
matching that of the liquid sheet. This paddle is used to optimize the
spatial overlap between the pump and probe beams, as well as provide a rough estimate of the time-zero position based on the profile
of the Debye Waller response of photo-excited crystalline samples.
The chip assembly and catcher are mounted on an xy-stage assembly, thus allowing the diagnostic paddle to be moved into the path of
the electron beam, while maintaining the alignment between the
catcher and chip. A CAD (computer-aided design) model illustrating
FIG. 8. (a) CAD model of the inside of the SLAC MeV LUED chamber. (b) CAD model of the incoupling mirror and capillary assembly. (c) Photography of the liquid sheet in
false color. (d) CAD model depiction of the interaction region geometry. The chamber walls are omitted for visualization purposes.
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the layout of vacuum stages and geometry of the interaction region is
shown in Fig. 8(a).
4. Interferometric sheet thickness measurement
The thickness of a free-flowing liquid sheet can be estimated
from the position and number of thin-film interference fringes
observed when a sheet is illuminated with a monochromatic source of
a known wavelength.49 The condition for constructive interference is
1
(9)
2nfilm d cos ðhÞ ¼ m k;
2
where nfilm and d are the refractive index and thickness of the sheet,
respectively, k is the wavelength of the monochromatic source in
the liquid medium, h is the angle of reflection in the interior of the
film, and m is an integer. Inside the LUED sample chamber, the
free-flowing liquid sheet was illuminated with 505 nm light from a
light-emitting diode (LED) device, mounted on a window upstream
of the interaction region and angled 30 from normal with respect to
the liquid sheet. The resulting thin-film interference fringes are
recorded using a camera mounted at a 60 with respect to the light
source. The result of these thickness measurements is presented in
Fig. 4.
5. Accessibility
The LUED chamber has a 24 16 in. hinged door allowing
nearly unrestricted access to the inside of the chamber. The large
access door facilitates the servicing and replacement of microfluidic
chips. A 12 in. gap underneath the chamber allows access to the sample collection bottle, valves, and cooling system associated with the
heated catcher system described in Sec. IV C.
C. Sample delivery
The microfluidic chip [R in Fig. 8(d)] is held in a PEEK interface (M), which connects to liquid (O) and gas (P) lines. The gas line
is fed 0–150 psi helium via a remote-controlled pressure regulator.
The liquid line connects to a multi-position valve actuator to allow
switching between several inputs. The main (sample) input is fed by
a high-performance liquid chromatography (HPLC) pump, which
can deliver stable liquid flow at the typical flow rates of 0.1–0.4 ml/
min. Pure solvent is delivered through a second input from a pressurized steel bottle, with flow controlled by a second remote pressure
regulator. The gas and liquid are filtered through 5 lm frits to prevent chip clogging.
A heated catcher [Innovative Research Solutions, I in Fig. 8(d)] is
positioned close to the tip of the chip and captures the jet after less
than 1 mm of in-vacuum flow, to control the pressure in the sample
chamber and to allow reuse of the sample. The catcher consists of a
hollow copper–beryllium cone, heated to 100 C, with a 500 lm hole
to capture the jet. Below the cone is a flexible tube which leaves the
vacuum chamber through a feedthrough and ends in a collection bottle below the chamber. This bottle is kept under vacuum by a
chemical-resistant membrane pump. To prevent evaporation of the
captured sample, the collection bottle is submerged in a chilled bath
kept at a temperature where the vapor pressure of the solvent is below
10 mbar. The collection bottle can be valved off from the chamber and
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emptied without venting the sample chamber, allowing longer running
times between venting.
Freezing of the jet, often an issue in liquid-phase experiments
under vacuum, was mitigated by the heated catcher. In-vacuum
start and stop operation of the jet, without venting the sample
chamber was possible. The main failure modes of the jet were clogging, mitigated by filters, and laser damage, which set an upper
limit to the laser power available to gas-accelerated jet experiments. Incidence of laser pulses with fluence exceeding 1.4 J/cm2 at
800 nm onto jets 200–300 lm below the chip caused reproducible
and non-reversible failure of the jet.
AUTHOR’S CONTRIBUTIONS
J.P.F.N. and K.L. contributed equally to this work. J.P.F.N.
designed the LUED sample chamber and analyzed the static water
data; K.L. designed the sample delivery system and analyzed the
time-dependent diffraction data; J.P.F.N. and K.L. wrote the manuscript; M.L. performed time-resolved LUED measurements of water
pumped with 800 nm light; D.P.D. developed the gas-accelerated
and converging liquid jets and supervised their adaptation to UED;
E.B. and Y.L. acquired LUED data; M.C. supervised the analysis of
LUED data and advised the writing of the manuscript; C.C. participated in adapting converging liquid jets to UED; M.D. developed the
control system and data acquisition tools used in the LUED experiments; S.G. and R.S. advised the design of the LUED sample chamber and sample delivery system, respectively; M.M. performed
LUED experiments using converging nozzles; X.S. optimized the
MeV electron beam for LUED experiments; K.J., S.W., and C.Y.
supervised the commissioning of the LUED sample chamber;
T.J.A.W., J.Y., and A.A.C. performed LUED experiments, advised on
the design and implementation of the LUED chamber and sample
delivery, and advised the writing of the manuscript; X.J.W. supervised the project.
ACKNOWLEDGMENTS
The work reported was mainly supported by the DOE BES
SUF Division Accelerator and Detector R&D program, the LCLS
Facility, and SLAC under Contract Nos. DE-AC02-05-CH11231
and DE-AC02–76SF00515. A.A.C. and T.J.A.W. are supported by
the U.S. Department of Energy, Office of Science, Basic Energy
Sciences, Chemical Sciences, Geosciences, and Biosciences Division.
J.P.F.N. and M.C. are supported by the U.S. Department of Energy
(DOE) Office of Basic Energy Sciences, Chemical Sciences,
Geosciences, and Biosciences Division, AMOS program, under
Award No. DE-SC0014170. K.L. was supported by a Melvin and
Joan Lane Stanford Graduate Fellowship and a Stanford Physics
Department fellowship. The authors thank Dr. Kasper Kjaer for
providing the x-ray scattering data of liquid water and Gregory M.
Stewart for making Fig. 1.
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