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Explosion dynamics of sucrose nanospheres
monitored by time of flight spectrometry and
coherent diffractive imaging at the split-anddelay beam line of the FLASH soft X-ray laser
Asawari D. Rath,1,7 Nicusor Timneanu,1,8 Filipe R. N. C. Maia,1 Johan Bielecki,1 Holger
Fleckenstein,2 Bianca Iwan,1 Martin Svenda,1 Dirk Hasse,1 Gunilla Carlsson,1 Daniel
Westphal,1 Kerstin Mühlig,1 Max Hantke,1 Tomas Ekeberg,1 M. Marvin Seibert,1
Alessandro Zani,1 Mengning Liang,2 Francesco Stellato,2 Richard Kirian,2 Richard
Bean,2 Anton Barty,2 Lorenzo Galli,2,5 Karol Nass,2 Miriam Barthelmess,2 Andrew
Aquila,3 Sven Toleikis,4 Rolf Treusch,4 Sebastian Roling,6 Michael Wöstmann,6 Helmut
Zacharias,6 Henry N. Chapman,2,5 Saša Bajt,4 Daniel DePonte,2 Janos Hajdu,1,3
and Jakob Andreasson1,9,*
1
Department of Cell and Molecular Biology, Uppsala University, Husargatan 3, SE-751 24 Uppsala, Sweden
2
Center for Free-electron Laser Science, DESY, Notkestrasse 85, 22607 Hamburg, Germany
3
European XFEL, Albert-Einstein-Ring 19, 22761 Hamburg, Germany
4
DESY, Notkestrasse 85, 22607 Hamburg, Germany
5
University of Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany
6
Physikalisches Institut, Westfälische Wilhelms-Universität, Wilhelm Klemm Str 10, 48149 Münster, Germany
7
Present affiliation: Bhabha Atomic Research Center, Mumbai, 400085, India
8
Present affiliation: Dept. of Physics and Astronomy, Uppsala University, Uppsala, Sweden
9
Institute of Physics ASCR, v.v.i. (FZU), ELI-Beamlines Project, 182 21 Prague, Czech Republic
*
[email protected]
Abstract: We use a Mach-Zehnder type autocorrelator to split and delay
XUV pulses from the FLASH soft X-ray laser for triggering and
subsequently probing the explosion of aerosolised sugar balls. FLASH was
running at 182 eV photon energy with pulses of 70 fs duration. The delay
between the pump-probe pulses was varied between zero and 5 ps, and the
pulses were focused to reach peak intensities above 1016 W/cm2 with an offaxis parabola. The direct pulse triggered the explosion of single aerosolised
sucrose nano-particles, while the delayed pulse probed the exploding
structure. The ejected ions were measured by ion time of flight
spectrometry, and the particle sizes were measured by coherent diffractive
imaging. The results show that sucrose particles of 560-1000 nm diameter
retain their size for about 500 fs following the first exposure. Significant
sample expansion happens between 500 fs and 1 ps. We present simulations
to support these observations.
©2014 Optical Society of America
OCIS codes: (140.7240) UV, EUV, and X-ray lasers; (170.1650) Coherence imaging;
(300.6350) Spectroscopy, ionization.
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#221500 - $15.00 USD Received 22 Aug 2014; revised 14 Oct 2014; accepted 14 Oct 2014; published 12 Nov 2014
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1. Introduction
An understanding of the interaction of high-intensity X-ray pulses with matter is of
fundamental importance for all applications where radiation induced processes are significant,
including the creation and probing of matter under extreme conditions or the structure
determination of bio-molecules. In this paper, we study the ionization and explosion
dynamics of organic samples injected into ultra-high intensity XUV pulse of femtosecond
duration from the FLASH soft X-ray laser. The interactions include elastic and inelastic
scattering, photoionisation, Auger processes, shake-up and shake-off excitations, inverse
bremsstrahlung heating, collisional ionization, and recombination. At high photon intensities,
these processes will lead either to a Coulomb explosion or a hydrodynamic expansion of the
sample [1,2]. The actual outcome depends on pulse length, wavelength, pulse intensity, and
the size, composition and density of the sample. A pure Coulomb explosion is expected in
very small samples in extremely intense pulses at very short wavelengths, while larger
samples are generally more likely to undergo hydrodynamic expansion.
A requisite for high-resolution coherent diffractive imaging (CDI) is that the sample
structure remains sufficiently intact during the laser pulse. Simulations suggest that CDI can
be successfully used for single large molecules of biological origin if radiation damage from
the high X-ray dose can be outrun by ultra-short exposures [1]. This diffraction-beforedestruction approach has been verified in subsequent experiments [3–5]. However, efforts
towards experimental verification of the time-scales of the damage in various samples are
scanty [6–9]. In particular, characterization of the time-scale of the damage onset in noncrystalline biological particles at intensities above 1015 W/cm2 is lacking.
We present here experimental results and simulations on the ionization dynamics of
aerosolized sucrose balls (C12O11H22) produced from an aqueous sugar solution and exposed
#221500 - $15.00 USD Received 22 Aug 2014; revised 14 Oct 2014; accepted 14 Oct 2014; published 12 Nov 2014
(C) 2014 OSA
17 November 2014 | Vol. 22, No. 23 | DOI:10.1364/OE.22.028914 | OPTICS EXPRESS 28916
to FEL pulses of intensities ≥1016 W/cm2 using simultaneous Split-and-Delay ion
spectroscopy and CDI. The chemical composition of sucrose is close to biological matter and
the aerosol particle size (560-1000 nm with ~109 sucrose molecules in the sample) is
characteristic for the size of small cells or large viruses. Our main objective here is to
investigate the time scale of the expansion for such a sample during an actual single particle
CDI experiment. We use ion time-of-flight (ion-TOF) measurements to analyze changes in
carbon-charge states formed in the pump-probe exposures of sucrose nano-balls as a function
of delay between the pulses. We measure sample explosion at 0, 500 fs, 1 ps, 2 ps, and 5 ps
delays (with sub-femtosecond reproducibility) with the pump and delay pulses having the
same intensity. This wide scan of the time delays was chosen to match the time scales
previously observed in the expansion of micronsize samples [6,7]. Based on the relative
amount of detected high (4+) and low (1+) charge states of carbon we determine a
recombination parameter that suggests that the sample stays relatively intact up to 500 fs after
the exposure, and that significant sample expansion happens between 500 fs and 1 ps.
Furthermore we present simulations of the interaction under the experimental conditions that
describe the plasma dynamics and support our interpretations.
2. The experiment
The experiments were performed at beamline BL2 of the FLASH free electron laser facility
[10]. A schematic of the experimental set-up is shown in Fig. 1. The FEL was tuned to 6.8 nm
(182.3 eV) and pulse length of about 70 femtoseconds. The FEL beam was split equally into
pump and probe beams using an X-ray autocorrelator and delay line [11] and then recombined. The time jitter in the delay between pump and probe was ~280 as [12]. The
average pulse energy before splitting was measured using a gas monitor detector to be ~40 μJ
with individual pulse energies showing a standard deviation of 11.9 μJ. An Off-Axis
Parabolic mirror with a focal length of 268.94 mm and reflectivity ~15-20% around 6.8 nm
was used to focus the combined FEL beam to sub-micron size resulting in peak intensities
approaching 1017 W/cm2 at the focus. The position and size of the focus was determined on
solid targets, using an in-line microscope [13]. The spatial overlap between the pump and
probe pulses in the focal plane was ensured by observing the single shot twin craters they
formed in a silicon surface. This procedure was carried out for all delay settings, and the split
and delay parameters were stored.
Fig. 1. Scheme of the experimental setup. The autocorrelator divides the incoming FEL pulse
equally between a pump and (delayed) probe pulse. Both pulses are focused by a single off
axis parabola mirror to overlap spatially with each other and the sample beam in the focal
plane. During the interaction, resulting ions are detected by an ion-TOFMS and a diffraction
pattern is recorded on a pair of XCAM detectors simultaneously.
#221500 - $15.00 USD Received 22 Aug 2014; revised 14 Oct 2014; accepted 14 Oct 2014; published 12 Nov 2014
(C) 2014 OSA
17 November 2014 | Vol. 22, No. 23 | DOI:10.1364/OE.22.028914 | OPTICS EXPRESS 28917
Sugar nano-balls were created by first making aerosols from a sucrose solution (25% by
weight/volume) using a gas dynamic virtual nozzle [14] at atmospheric pressure, and then
guiding the micron to sub-micron sized droplets into an aerodynamic lens via a differentially
pumped relaxation chamber [5,15]. The pressure at the exit of the aerodynamic lens dropped
to 10−6 mbar and the transit time through the injector was 5-10 ms. As the droplets lose water
and the surface tension increases, the droplets turn into sugar balls with nearly perfect
spherical symmetry. The adiabatically cooled nano-balls leave the lens in a narrow beam of
about 10-20 μm diameter with velocities between 10 and 100 m/s. The size of the resulting
nano-balls was 560-1000 nm as measured from their diffraction patterns.
Ions ejected from the sample during explosion were detected, using an ion-time of flight
mass spectrometer (ion-TOFMS) mounted perpendicular to the particle and FEL beams. The
ion extraction region of the ion-TOFMS is customized to work efficiently with the sample
delivery [16]. In the present work the extracting and accelerating electric fields were set at
7.14 KV/cm and 8.333 KV/cm respectively and ions were detected by a triple plate (Z-gap)
MCP at the end of a field-free drift region 745 mm long. Concurrent to this, far-field
diffraction patterns of individual sugar balls were recorded on a pair of XCAM detector chips
placed approximately 140 mm from the interaction point in the forward direction. The direct
beam passed through a gap between the two detector halves.
3. Simulations
Large objects, like the sugar balls used in this study, are expected to undergo a hydrodynamic
expansion, following exposure. We simulate the interaction between the FEL pulse and the
sample using an atomic kinetics and radiation transfer code [17]. This plasma code uses a
non-local thermodynamic equilibrium (non-LTE) formalism to describe dynamics of the
atomic populations and photon distribution on ultra-short time scales and at extreme
intensities. The atoms are modeled using a screened hydrogenic model and the free electrons
are assumed to follow a Maxwellian energy distribution. The interaction between the
electrons and atoms/ions occurs mainly through thermalization, collisional ionization and
recombination processes. As the sample turns quickly into plasma at solid density, the
pressure ionization also known as continuum lowering is taken into account and modeled.
The simulations also include hydrodynamic expansion driven by the electron pressure, which
is modeled using Lagrangian formalism.
The above code has been earlier used to predict the effects of radiation damage in imaging
experiments on micron-sized biological objects [18,19], protein nanocrystals [20] and to
predict changes in opacity in solids at high intensities [21]. Recently, it has been successfully
used on data from FLASH to simulate saturated absorption in metal hydrides and the
acceleration of ions from bulk metallic samples at 13.5 nm wavelength and more than 1017
W/cm2 power density on the sample [22], as well as at the LCLS (0.62 nm, 1017 W/cm2)
where we have simulated ion diffusion and loss of the Bragg diffraction from protein
nanocrystals [9].
The simulations allow us to follow the expansion and recombination in the sample, during
and after the interaction with laser pulses with sub-femtosecond time steps. The split FEL
pulse was modeled as two consecutive Gaussian pulses (50 fs FWHM, 182 eV photon
energy) with equal intensities, with delays set at 0, 0.5, 1, 2 and 5 ps. Sample expansion was
followed in the simulations for 50 ps. The sample was modeled in 1 dimension, with
independently treated layers (typically 8 to 11) along the incident laser direction. We modeled
the sample composition and density starting from a dilution of sucrose (ρ = 1.587 g/cm3) in
water with 25% wt/vol. Considering evaporation of droplets into vacuum [23] we estimate
that a 1 μm droplet would shrink to a nanoball of diameter ~700 nm through evaporation of
water molecules and the sample density would be about 1.3 g/cm3 in the interaction zone. In
our simulations, this corresponds to about 24 water molecules for each sucrose molecule in
the droplet. The attenuation length under these conditions is 500 nm. Consequently, 80-90%
#221500 - $15.00 USD Received 22 Aug 2014; revised 14 Oct 2014; accepted 14 Oct 2014; published 12 Nov 2014
(C) 2014 OSA
17 November 2014 | Vol. 22, No. 23 | DOI:10.1364/OE.22.028914 | OPTICS EXPRESS 28918
of the incident radiation is expected to be absorbed in the sample. The damage dynamics
observed in the present study should be more severe than what can be expected at shorter
wavelengths and serve as an upper limit for high-resolution single particle CDI experiments.
4. Results and discussion
Sucrose nanoparticles are injected into the vacuum chamber together with a low-pressure
sheath gas consisting of He and air. The FEL pulses interact with particles and also with the
sheath gases. The residual gas peaks are present in the mass spectra in both “hits” and
“misses”. A hit produces a diffraction pattern on the CCD detectors, and the ions released
through the explosion are registered by the ion-TOF. We used the integrated proton yield to
select hits from misses and verified hits by their corresponding diffraction patterns. In order
to minimize the effects of drifts in the background signal the ion spectra from ± 10 misses
adjacent to a hit are averaged and serve as the background spectrum for that hit. Simultaneous
recording of diffraction patterns together with the ion-TOF spectra allows us to correlate the
size of individual aerosols and the signatures of the explosions on a shot-to-shot basis. Figure
2(a) shows a representative diffraction pattern recorded from a hit on a single sucrose
nanosphere with 1 ps delay between pump and probe. The symmetric rings in the pattern
show that this particle is approximately spherical and fringe separations indicate a diameter of
692 nm. The diffraction patterns recorded during the experiments show that the typical
sucrose aerosols exposed to the pump-probe beams were spherical with diameters in the range
560-1000 nm and that single nanosphere hits occur mixed with multiple hits (data include
both). The large size of the samples combined with a spread in the size distribution does not
allow us to directly detect any visible changes in the diffraction patterns that could indicate
expansion. Figure 2(b) shows the ion-TOF mass spectrum (red) of the single hit recorded
simultaneous with the diffraction pattern in Fig. 2(a). The corresponding background
spectrum (black curve in Fig. 2(b)) is also shown with its baseline shifted downwards for
clarity.
Fig. 2. Simultaneous measurement by CDI and ion-TOF spectrometry. (a) Diffraction pattern
of a single particle as recorded on a pair of XCAM detectors separated by a gap to allow the
FEL beam pass in between. The ring structure indicates that the sample particle is spherical
with a diameter of 690 nm. The pattern is the sum of scattering of the pump and delayed probe
pulses from the sample. (b) The ion-TOF signal recorded simultaneously for the same hit. Red
line represents the single hit spectrum with a time delay of 1ps. Black line is the averaged
background signal originating from the residual gas (shifted vertically for clarity). Note the
increase in proton signal (M/Q = 1) and appearance of carbon ions at M/Q = 12 (C+) and 3
(C4+) in the hit spectrum.
Figure 2(b) shows that hits are characterized mainly by a very strong proton peak and an
appearance of peaks from H2+, C+ and C4+. The present measurements extend up to M/Q = 90
but no fragments with M/Q higher than 40 were observed. A further observation from
comparing the signal from ionization of the background gas and the hits is that the ion signal
#221500 - $15.00 USD Received 22 Aug 2014; revised 14 Oct 2014; accepted 14 Oct 2014; published 12 Nov 2014
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17 November 2014 | Vol. 22, No. 23 | DOI:10.1364/OE.22.028914 | OPTICS EXPRESS 28919
from the background gas outweighs the signal from the target particle. This is despite the fact
that the sample is expected to be entirely ionized (at least one ionization per atom) under the
given interaction parameters. This observation shows that the ions formed in the interaction
largely recombine before they can be extracted by the ion-TOFMS and that an understanding
of the recombination process is critical to the interpretation of the ion-TOFMS data. Along
these lines it can be argued that ions detected by the ion-TOFMS originate primarily from the
outer layer of the sample that is directly exposed to the incoming beam. Owing to the
anisotropic nature of sample expansion (the surface hit by the pulse will expand most
rapidly), ions from this part of the sample will be most likely to escape the effects of
subsequent recombination.
The photon energy in our experiment is 182 eV, which is well below the K-absorption
edge of C, N and O, but enough to strip off outer shell electrons in C by single photon
absorption. In addition, outer shell electrons can also easily be lost through secondary
processes, such as collisional ionization. As a result, ionized carbon atoms can be expected to
reach the 4+ ionization and the behavior of the C4+ state population is expected to display
similarities with the proton population as a function of pulse intensity and/or delay. Following
the initial photoionization, further ionization competes with recombination processes and the
C4+ and C+ populations become indirectly linked to each other through the intermediate
charge states. In the ion-TOF spectra, the peaks corresponding to C+ (at M/Q = 12) and C4+
(at M/Q = 3) do not have any contribution from other ionic species from the sample or
background. This allows us to unambiguously employ the ion yields of these charge states for
an investigation of the plasma expansion by defining a recombination parameter K as the
ratio of I(C4+) to I(C+) where I(C4+) and I(C+) are the ion yields of C4+ and C+ ions
respectively.
Figure 3(a) shows background corrected ion yields (integrated ion-TOF signals) of the C4+
and C+ peaks for different delays with the recombination parameter K defined above. Figure
3(b) shows the ion populations of all charge states up to 4+ at different delays as obtained
from simulations (with the recombination parameter from the simulations in the inset). Figure
3(b) shows simulated carbon-ion yields for the surface layer of the sample at the end of
simulations (50 ps). At this point the plasma expansion is very low and the density reached
~3% of the initial value of sugar nano-ball. A comparison between Figs. 3(a) and 3(b) shows
that the relative ion yields of C4+ and C+ measured in the experiments are in qualitative
agreement with the trend from these ions suggested by simulations. We note that a very weak
signal becomes detectable above the background at M/Q = 4 for delays longer than 1 ps.
However, due to the overall weakness and the uncertainty of the origin of this peak (it can be
ascribed to either C3+ from the sample or O4+ that may originate both from the background
and the sample) it is not discussed in further detail here. More significantly, the substantial
C2+ signal predicted from simulations is surprisingly not observed in the experimental data.
We have previous indications of absence (or suppression) of C2+ peaks from experimental
observations with hard X-rays and biological samples at high intensities [16, 24]. This
indicates that although the present model captures the relative changes of the 4+ and 1+
charge states, it does not provide a complete picture of the recombination process.
#221500 - $15.00 USD Received 22 Aug 2014; revised 14 Oct 2014; accepted 14 Oct 2014; published 12 Nov 2014
(C) 2014 OSA
17 November 2014 | Vol. 22, No. 23 | DOI:10.1364/OE.22.028914 | OPTICS EXPRESS 28920
Fig. 3. Ion yields of carbon as a function of delay (a) Background corrected average carbon ion
yield for charge states 4+ and 1+, which can be distinctly detected for various delays between
pump and probe beams. Error bars represent the respective standard deviations. The inset
shows the recombination parameter defined as the ratio of ion yields K = I(C4+)/ I(C+) for
delays between 0 to 5 ps. (b) Ion population of charge states C4+, C3+, C2+ and C+ from
simulations of sample expansion for five delays (0 to 5 ps) at a laser intensity of 1016 W/cm2
after 50 ps simulation time. Inset shows as a function of delay the recombination parameter K
derived from the simulated C4+ and C+ ion yields.
Figure 4(a) shows time of flight spectra for the hydrogen peak averaged over all hits
occurring at the same delay (0-5 ps). The shift in maxima of the peaks towards lower times of
flight suggests a slight increase in the kinetic energy of the protons as a function of delay. For
a delay of 0.5 ps, not much increase is observed while for rest of the delays, it is in the range
of ~10-20 eV which is in line with the simulations on ion temperatures presented below.
Figure 4(b) shows the integrated proton yield in the hits after background correction for these
delays. As expected the behavior of the proton yield with delay is qualitatively similar to that
of C4+. We also observe that both display a maximum for 2 ps delay with a slight decrease in
signal for the 5 ps delay (see Figs. 3(a) and 4(b)). The observed increase in the kinetic
energies of H+ ions (Fig. 4) and C+ ions (not shown here) supports the idea that the plasma is
getting hotter when the second pulse is delayed.
Fig. 4. Hydrogen ion-TOF spectra (a) Proton peak averaged over all hits for the given delay
displayed as a function of time of flight for delays between 0 to 5 ps. The proton kinetic
energies in eV are displayed at the top axis with time of flight 1.049 μs corresponding to 0 eV
(b) Proton yield obtained for different delays. The black squares () represent yields averaged
over all hits for that delay while grey circles () show yields for individual hits (number of
single hits is 9, 7, 14, 9 and 15 for each delay). Error bars show the standard deviation.
#221500 - $15.00 USD Received 22 Aug 2014; revised 14 Oct 2014; accepted 14 Oct 2014; published 12 Nov 2014
(C) 2014 OSA
17 November 2014 | Vol. 22, No. 23 | DOI:10.1364/OE.22.028914 | OPTICS EXPRESS 28921
More detailed results from simulations on various plasma parameters are summarized in
Fig. 5 where we show the results from the outermost sample layer (of approximately 100 nm
thickness) that is directly hit by the FEL pulse and is assumed to expand hydrodynamically.
This is the part of the sample that is exposed the most to the laser and dynamics in this layer
should make up an upper limit for the sample expansion. Figure 5(a) shows the pulse
structure of pump (black) and probe with delays of 0, 0.5, 1, 2 and 5 ps (red, blue, green,
magenta and cyan, respectively). Figures 5(b)–5(e) follow same color representation. Figure
5(b) shows the C4+ ion population as a function of time. At the arrival of each pump/probe
pulse the number of C4+ ions increases due to ionization initiated by the exposures (from
direct ionization of the cold solid target by the pump and the subsequent ionization of the
plasma by the probe). At times longer than the largest delay (5 ps) the C4+ population
decreases for all delays owing to electron-ion recombination. However, for delays longer than
0.5 ps, a relatively large number of C4+ ions are present even a long time after the exposure.
This result is in line with our experimental observation of C4+ ion yields. Figure 5(c) shows
the behavior of ion temperature with time for the pump-probe delays used in the experiments.
The ion temperature starts increasing after the arrival of the pump/probe pulses owing to
electron-ion thermalization (with a typical time scale of 500 fs) and then decreases with time
as the plasma cools down in the subsequent hydrodynamic expansion. The simulations show
the formation of a hotter plasma with increasing delay, which is reflected in the experiments
as an increase in average kinetic energy (shorter flight time) of the protons as evident in Fig.
4(a). Hotter ions lead to faster hydrodynamic expansion resulting in ions arriving faster at the
detector. These simulations indicate proton energies in the range of 15 to 35 eV in line with
our experimental observations (Fig. 4). It may be noted that although our simulations suggest
that both the number of C4+ ions and the ion temperature (seen as the proton acceleration in
the experimental data) are simple indicators of the plasma expansion rate, it is clear that the
population dynamics between C4+ and C+ charge states is the more sensitive and reliable
indicator of expansion. Figure 5(d) shows the time evolution of the plasma density for
different delays. The change in density at 2 and 5 ps (to ~60% and ~30% of initial solid
density value) corresponds to expansion in particle diameter by ~20% and ~50%. Given that
the average sample size and the focus are about 0.7 and 1 micron respectively this sample
expansion could explain the decrease in signal seen for both C4+ and protons in Figs. 3(a) and
4(b) at the delay of 5 ps. Figure 5(e) shows the rate of change of density which is related to
the expansion rate. Deviations from the black curve show that the arrival of the delayed pulse
initiates an acceleration of the sample expansion and that this is most prominent for delays
between 0.5 and 2 ps. This indicates that the sample is still close to solid density when the
second pulse hits after these delays and that this gives the critical time scale for sample
recombination and expansion.
#221500 - $15.00 USD Received 22 Aug 2014; revised 14 Oct 2014; accepted 14 Oct 2014; published 12 Nov 2014
(C) 2014 OSA
17 November 2014 | Vol. 22, No. 23 | DOI:10.1364/OE.22.028914 | OPTICS EXPRESS 28922
Fig. 5. Simulations with a non-LTE plasma code of the interaction between the FEL pulse and
a sucrose nano-ball of diameter 700 nm. (a) The pump and probe pulses are assumed to be
Gaussian. The pump pulse is black, followed by a second identical pulse at various delays (0,
0.5, 1, 2 and 5 ps represented by red, green, blue, magenta and cyan colors respectively. Panels
(b)-(e) follow same color representation. Total laser intensity is 1016 W/cm2 distributed equally
between the two pulses. Evolution with time of the (b) population of C4+ ions, (c) ion
temperature, (d) sample density and (e) rate of change of density is shown for various delays.
The solid black curve shows the effect of pump alone. Values are taken from the outer-most
layer of sample illuminated directly by the laser.
To explain the observed variation in ion yields of C+ and C4+ with delay, we propose a
model presented schematically in Fig. 6. At 0 delay the single intense pulse formed by the
pump and probe pulses arriving simultaneously, generates highly ionized plasma with a large
number of carbon ions in state 4+. Secondary collisional processes owing to plasma heating
could contribute to further ionization, while recombination processes reduce the extent of
ionization. During its hydrodynamic expansion, before the nano-plasma density reduces to a
threshold value where ion-TOF electric fields become effective, a significant degree of
recombination takes place resulting in low yields of C4+ and high yields of C+ detected by the
ion-TOF. For the case of short time delay (Fig. 6(b)) the higher ionization states created by
the pump in the target start de-populating with time due to recombination. The effect of
recombination, however, is wiped out by the arrival of the probe as it again pushes a large
number of carbon atoms/ions into higher ionization states up to 4 + state. Since the plasma hit
by the probe is already somewhat expanded following the pump pulse, the amount of
recombination following the second pulse is smaller compared to case (a) and as a result we
detect an increase in the C4+ ion yield while the C+ ion yield decreases. As the delay increases
further (Fig. 6(c)) the probe sees an increasingly dilute plasma compared to the short-delay
cases. As a result the ionization states reached in the interaction with the probe are retained to
an even greater extent. Consequently, as long as the probe exposes the same plasma hit by the
pump to a second burst of ionization, we expect a diminishing C+ signal and a more intense
C4+ peak with increasing delay.
#221500 - $15.00 USD Received 22 Aug 2014; revised 14 Oct 2014; accepted 14 Oct 2014; published 12 Nov 2014
(C) 2014 OSA
17 November 2014 | Vol. 22, No. 23 | DOI:10.1364/OE.22.028914 | OPTICS EXPRESS 28923
Fig. 6. Schematic of the ionization and recombination model to describe the observed behavior
of C4+ and C+ with (a) no delay, (b) 0.5 ps and (c) 2 ps delay between pump and probe pulses.
The top shows the sequence of major events during the interaction, photoionization and
subsequent secondary processes (here denoted PI) and recombination. At the bottom of each
figure the TOF spectra averaged over all hits for that delay are shown. The data are shown
without background correction, which explains the variations in background signal (mainly
He, N and O related peaks).
In this model the recombination parameter represents the amount of recombination taking
place after the second pulse has hit the sample which is directly linked to the sample density
at the time when the delayed pulse hits, and hence the expansion of the plasma. In the present
experiments we observe a distinct jump in this parameter between 0.5 and 1 ps (inset Fig.
4(a)). We interpret this as a sign that the sample (in particular the outermost layer that is hit
by the pulses) is relatively intact during the first 500 fs following the first pulse and that
sample expansion is significant between 0.5 and 1 ps, after its arrival.
Experiments and hydrodynamics simulations support our model based on a recombination
parameter to describe the anisotropic expansion of a bio-like sample in a soft X-ray single
particle CDI experiment. The work complements and extends earlier time resolved
experiments performed on targets deposited on a solid support [6,7], nanocrystalls [9] or
small noble gas clusters [8]. In particular the present experiments at 6.8 nm wavelength and
>1016 W/cm2 intensity on ~600 nm sugar balls in the form of aerosols without any support
show the same time-scale for the sample expansion as femtosecond time delay holography
experiments performed on supported 140 nm diameter polystyrene beads at 32 nm
wavelength and ~1014 W/cm2 peak intensity [6].
There are however a number limitations in our study. The chosen delays give only a
course sampling of the sample expansion on the few 100 fs to ps time scale. Future studies
should use smaller delays between pump and probe to give the fs refinement required to study
what happens during the imaging pulse. Furthermore, the number of analyzed samples in
#221500 - $15.00 USD Received 22 Aug 2014; revised 14 Oct 2014; accepted 14 Oct 2014; published 12 Nov 2014
(C) 2014 OSA
17 November 2014 | Vol. 22, No. 23 | DOI:10.1364/OE.22.028914 | OPTICS EXPRESS 28924
single exposure is limited (Fig. 4(b) shows the number of samples), making our results less
statistically significant and more indicative of the processes we observe. The plasma model
captures the heating and changes in recombination as a function of delay without tweaking,
however the agreement is not quantitative. We also note the discrepancy between the model
prediction of C2+ and C3+ yields and our experimental observation. Recent experiments on C60
molecules interacting with femtosecond X-ray pulses [25] show the appearance of all charge
states in C and how this distribution is shifted towards higher charge states for longer X-ray
exposures, as expected from modeling.
The present work represents the first time-resolved study in the exact geometry of a single
particle CDI experiment with samples in the size regime most interesting for soft-X-ray
imaging and pulse parameters closing in on those required for high resolution single shot
CDI. (Sub-nm resolution has been predicted from single shot exposures of cell-like particles
in the water window [18]). The sample size and composition make the sucrose nanoball a
good model for biological cell and the plasma dynamics in both is expected to be similar. In
bioimaging experiments at the submicrometer scale in soft X-ray regime a high contrast is
expected [18,19], even though the samples are subjected to a high absorbed radiation dose
and nonhomogeneous heating [19]. These mechanisms will be less relevant in the limit of
smaller samples and high energy X-rays [1], where molecular processes and chemical
bonding can influence the sample fragmentation [25].
Although the present experiments were performed at intensities up to 3 orders of
magnitude higher than previous time resolved experiments on non-crystalline samples, and at
a shorter wavelength than the previous FLASH experiments, we still observe expansion
dynamics that becomes measurable 0.5–1 ps after exposure to a femtosecond FEL pulse. This
is substantially longer than the characteristic pulse length of femtosecond X-ray lasers used
for CDI application. Indeed, these results could open up a case for developing laser driven Xray sources for future CDI applications in the soft X-ray regime (recent simulations indicate
that a plasma laser could produce 5*1014 soft X-ray photons in 200 fs pulses) [26].
5. Conclusions
We performed time resolved fragmentation studies on sucrose nano-particles using X-ray
pump-probe ion Time-of-Flight mass-spectroscopy simultaneous to coherent diffractive
imaging. The results allow us to define a recombination parameter based on the observed
carbon-charge states from which we can deduce the expansion rate of the sample. Our
analysis indicates that a bio-like particle (sucrose nano-ball) remains largely intact for at least
500 fs following a high intensity X-ray exposure in a coherent diffractive imaging experiment
and that substantial sample expansion occurs between 0.5 and 1 ps after the exposure. The
experimental observations also indicate a systematic absence of C2+ ions that cannot be
explained in plasma model with atomic kinetics. Further experiments are needed to elucidate
the fragmentation dynamics in detail on sub-ps time scale.
Acknowledgments
This work was supported by the following agencies: The Swedish Research Council, the Knut
and Alice Wallenberg Foundation, the European Research Council, the Röntgen-Ångstrom
Cluster, the Ministry of Education, Youth and Sports of the Czech Republic (ELI-Beamlines
Registered No. CZ.1.05/1.1.00/02.0061) and the Academy of Sciences of the Czech Republic
(M100101210). The ion-TOF spectrometer was built and implemented through a grant from
Stiftelsen Olle Engkvist Byggmästare. Simulations were performed on resources provided by
the Swedish National Infrastructure for Computing at UPPMAX, project s00111-71 and
s00112-67. Portions of this research were carried out at the FLASH facility. We are grateful
to the scientific and technical staff of the FLASH for their outstanding facility and support.
SR, RM and HZ thank the BMBF for financial support of project 05KS4PMC/8 within the
priority research program FSP301 “FLASH”.
#221500 - $15.00 USD Received 22 Aug 2014; revised 14 Oct 2014; accepted 14 Oct 2014; published 12 Nov 2014
(C) 2014 OSA
17 November 2014 | Vol. 22, No. 23 | DOI:10.1364/OE.22.028914 | OPTICS EXPRESS 28925

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