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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 off-axis 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
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-before-destruction 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 non-crystalline 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 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 re-combined. 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.
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% 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 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.
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.
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.
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.
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 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 X-ray 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”.
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