Comparison of Electron and Ion Emission from Xenon Cluster-Induced

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A: Kinetics, Dynamics, Photochemistry, and Excited States

Comparison of Electron and Ion Emission from Xenon Cluster-Induced Ignition of Helium Nanodroplets Michael Kelbg, Andreas Heidenreich, Lev Kazak, Michael Zabel, Bennet Sebastian Krebs, Karl-Heinz Meiwes-Broer, and Josef Tiggesbaeumker J. Phys. Chem. A, Just Accepted Manuscript • Publication Date (Web): 21 Sep 2018 Downloaded from http://pubs.acs.org on September 21, 2018

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Comparison of Electron and Ion Emission from Xenon Cluster-Induced Ignition of Helium Nanodroplets Michael Kelbg,† Andreas Heidenreich,‡,¶ Lev Kazak,† Michael Zabel,† Bennet Krebs,† Karl-Heinz Meiwes-Broer,†,§ and Josef Tiggesbäumker∗,†,§ Institute of Physics, University of Rostock, Rostock, Germany, and Kimika Fakultatea, Euskal Herriko Unibertsitatea (UPV/EHU) and Donostia International Physics Center (DIPC), P.K. 1072, Donostia, Spain E-mail: [email protected]

∗

To whom correspondence should be addressed Institute of Physics, University of Rostock, Rostock, Germany ‡ Kimika Fakultatea, Euskal Herriko Unibertsitatea (UPV/EHU) and Donostia International Physics Center (DIPC), P.K. 1072, Donostia, Spain ¶ IKERBASQUE, Basque Foundation for Science, Bilbao, Spain § Department Life, Light and Matter, University of Rostock, Rostock, Germany †

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Abstract The charging dynamics of helium droplets driven by embedded xenon cluster ignition in strong laser fields is studied by comparing the abundances of helium and highly charged Xe ions to the electron signal. Femtosecond pump-probe experiments show that near the optimal delay for highly charged xenon the electron yield increases, especially at low energies. The electron signature can be traced back to the ionization of the helium environment by Xe seed electrons. Accompanying molecular dynamic simulations suggest a two-step ionization scenario in the Xe-He core shell system. In contrast to xenon, the experimental signal of the helium ions, as well as low-energy electron emission show a deviating delay dependence, indicating differences in the temporal and spacial development of the charge state distribution of Xe core and He surrounding. From the pump-probe dependence of the electron emission, effective temperatures can be extracted, indicating the nanoplasma decay.

Introduction Advances in laser technology make it possible to conduct optical experiments in the strong field regime. Above 1013 Wcm−2 the response of finite matter to intense laser pulses is found to be strongly nonlinear and thus exploits an interesting regime appealing to fundamental studies, as well as applications. 1 For example, hollow atoms have been identified in early studies 2 The transient nanoplasma formed by the exposure of free clusters is exceptional, as it provides special conditions like bulk-density matter at a low target density. The finite size of the particles and the molecular beam conditions essentially allow to record the full information regarding the interaction products. Experiments have analyzed x-rays, 3,4 high harmonics, 5 energetic 6 and highly charged 7 ions, and fast electron generation. 8 Exposure to tailored pulses show that a modified light field allows for the most effective generation of extreme conditions in nano-sized matter. 9,10 Triggered by experimental findings, there has been extensive theoretical progress over 2


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the past two decades. Experiments and simulations alike have proven that the nanoparticle response in intense laser fields differs in many respects from the atom and the bulk. 11,12 An important factor in many-body physics features the collective response of the nanoplasma, which strongly contributes to the charging dynamics. 13,14 Mie-resonance enhanced absorption and ionization take place at the critical radius given by the plasmon frequency. 15,16 Only recently, plasmonic control of electron acceleration has been achieved on attosecond timescales in metal clusters. 17 Energy transfer into the particle and the related outer ionization at the laser-cluster resonance leads to the build-up of a typically keV-deep mean field potential confining the inner-ionized electrons. As a consequence of the simultaneously occurring substantial increase in the plasma temperature, electron-ion recombination by three-body collisions is largely suppressed. Further, in the adiabatic expansion most of the delocalized electrons are elevated and occupy energy states near the vacuum level. 18,19 The special conditions in the expanding plasma lead to phenomena like the formation of energetic neutrals, 20 correlated electronic decay (CED), 21 interatomic Coulomb decay (ICD), 22,23 and three-body recombination (TBR), 24 which has to be taken into account when analyzing ion charge states and abundances as well as electron energies. 20,25 Helium droplets as a nanomatrix container for clusters provide an interesting testground for strong field laser-matter interactions. 26,27 For example, a helium droplet is transparent for photons up to 20 eV. Upon doping with impurities like xenon, the optical intensity threshold to trigger nanoplasma formation is strongly lowered from 1 × 1015 Wcm−2 to 7 × 1013 Wcm−2 , 28 which allows to experimentally access ignition scenarios. Molecular dynamic (MD) simulations reveal that doping with only a few Xe atoms is sufficient to trigger the laser-induced Coulomb explosion of droplets. 29 The seeding efficiency of dopants has been analyzed in detail and criteria have been determined to characterize the ignition process. 30,31 The strong field activated pump-probe nanoplasma dynamics of the Xe cluster core in helium droplets has been studied applying the molecular dynamics method. 32,33 In contrast to

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Figure 1: Schematic view of the experimental setup. The pick-up technique is applied He = 36, 000) with an average of 53 xenon to dope the molecular beam of droplets (Navg atoms, which form clusters inside the droplets. Pump-probe ionization is accomplished with ultrashort pulses from an Ti:Sapphire laser system and a Mach-Zehnder interferometer to supply two pulses scanning the dynamics of the target with an optical delay τ . The electron signals are measured with a field-free time-of-flight spectrometer and the ion signals by mass spectrometry. experiments, the simulations allow for a labeling of the particles. Hence, electrons stemming from helium can be distinguished from xenon. Fast helium electrons were obtained at earlier pump-probe times when compared to emission from Xe. This suggests that the charging of core-shell systems in strong laser fields may lead to striking signatures that will help to experimentally resolve complex issues such as the spatial and temporal development of inner ionization. In the present work we investigate the dynamics in the strong-field interaction of helium nanodroplets doped with xenon clusters by contrasting the signals of highly charged ions with the yield of electrons in a pump-probe experiment. Molecular dynamics simulations serve to analyze the seed-induced internal dynamics, whereas experimental data also includes the long term development of the evolving nanoplasma.

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Experimental Setup The method to generate a molecular beam of doped helium nanodroplets has been described elsewhere. 34 Fig. 1 shows the current experimental setup. Briefly, after expansion of precooled helium gas (20 bar, 10 K) through a 5 µm diameter nozzle, droplets with a mean radius He of 7 nm (Navg = 36 000) are formed. The molecular beam passes a 20 cm long gas cell contain-

ing xenon at a partial pressure of 8 × 10−5 mbar. Sequential pick-up of atoms by the droplets Xe = 53. leads to cluster formation (XeN Hedroplet ), 35 giving a mean dopant cluster size of Navg

The pick-up reduces the average number of He atoms by about 30% due to evaporation. Details of modeling the pick-up process are given in reference. 36 After differential pumping the droplets enter the interaction region where the laser pulses perpendicularly overlap with the molecular beam. An ultrashort optical laser system provides single short pulses with a duration of 50 fs (full width half maximum, FWHM) at a repetition rate of 1 kHz. For the pump-probe studies pulses with a temporal delay τ are generated. A 30 cm lens is used to focus the laser beam. For the current study an intensity ratio of about 2:1 between pump and probe laser beams is chosen with ILpeak = 7 × 1013 Wcm−2 and 4 × 1013 Wcm−2 , respectively. The charge states of He and Xe (Heq , Xeq ) from the Coulomb explosion are analyzed by ion mass spectrometry, whereas the energy of the electrons (Ee ) is determined with a field-free time-of-flight electron spectrometer.

Molecular dynamics simulations The molecular dynamics method to describe the interaction of a cluster with linearly polarized optical Gaussian laser pulses has been outlined in. 30,37 Starting with a particle consisting of neutral atoms, electrons enter the simulation when the criteria for tunnel ionization (TI), barrier suppression ionization (BSI) or electron impact ionization (EII) are met. This is checked at every time step, taking the local electric field at each atom as the sum of the laser electric field and the contributions of all particles in the cluster. Instantaneous TI 5


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probabilities are calculated by the Ammosov-Delaine-Krainov formula 38 and the EII cross sections by the Lotz formula 39 taking into account the local atomic Coulomb barrier in the cluster. 8 Ion-ion and electron-ion interactions are described by Coulomb and smoothed Coulomb potentials, respectively. The contributions by neutral atoms are disregarded except from the Pauli repulsion with respect to electrons. 40 Electron-ion three-body recombination is included. The simulations are carried out for He2171 and a Xe13 dopant cluster (both fcc structure) as a prototype sample of the experimental target. The initial interatomic distances are 3.6 ˚ A for He-He, 41 4.33 ˚ A for Xe-Xe (bulk) and 4.15 ˚ A for He-Xe. 42 Xe13 is placed in the center of the droplet, cutting out all He atoms within the He-Xe distance. The duration of both pump and probe pulses are τL = 70 fs (FWHM) at a photon energy of 1.44 eV. Simulations are carried out for peak maxima ILpeak = 5 × 1013 Wcm−2 for the pump pulse and ILpeak = 4 × 1013 Wcm−2 for the probe pulse, hence focal averaging is not incorporated. 43 The temporal length of a trajectory varies depending on the chosen delay to allow the system equal development after the probe pulse. These range from 700 fs for an optical delay of τ = 100 fs up to 1400 fs for τ = 900 fs. The results are averaged over sets of 10 trajectories, each with different initial conditions (slight distortions of the initial cluster geometry and different seeds of the random number generator for TI).

Results and discussion Simulations MD simulations provide insight into the femtosecond dynamics within the XeN HeM coreshell system, which are in general not directly accessible experimentally. In Fig. 2 the impact of pump-probe ionization for two different delays (τ = 100 fs, left; 300 fs, right) on the resulting average values of Xeq , Heq and Ee are shown. Although TBR is included in the simulations, only the bare ion charges are presented. The intensity of the activating pulse allows to initially ionize only the dopant xenon atoms by TI. Freed electrons accelerated 6


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in the combined electric field of laser and developing nanoplasma cause a cascade of EII events in the entire system. The evolution of Xeqavg and Eeavg (b, d) hints at an avalanchelike ionization of the droplet which manifests in a steep rise of Heqavg . The simulations reveal that the degree of helium ionization during the pump pulse strongly depends on the exact time the avalanche process is triggered (see individual trajectories in the supporting information). The delayed pulse couples to the previously formed plasma and electrons are strongly accelerated, accompanied by a steep increase in the average electron energy. The strength of the coupling depends on the match between laser and plasmon frequency. During the probe pulse with τ = 300 fs (Fig. 2, right) averaged electron energies of about Eeavg = 50 eV and Xeqavg = 7.5 are observed. Heqavg reaches a value of 1.5. For τ = 100 fs (Fig. 2, left) values of Eeavg = 80 eV are obtained, accompanied by an increase of Xeqavg and Heqavg to 9.8 and 1.9, respectively. Hence, helium is ionized completely. The sharp rise obtained in Eeavg at the shortest delay indicates resonance conditions. Therefore, the system has reached the critical radius, resulting in an enhanced absorption of laser energy. Ionization at longer delays becomes less efficient due to the increasing mismatch of laserand plasmon-frequency driven by the plasma-expansion and recombination. This is also reflected in the reduction of the average electron energy. More importantly, the system has expanded beyond the critical radius and does not reach the resonance condition through an increase in the degree of inner ionization. In addition, Xeqavg continuously decreases with longer delay times (see Fig. 3, top) indicating off-resonance conditions. For τ = 100 fs every individual trajectory leads to a full ionization of the helium droplet, whereas for τ ≥ 300 fs ignition takes place during either pump or probe pulse or in some cases the ignition is absent (for selected trajectories, see supporting information). The time dependence obtained in the selected observables leads to the conclusion that details of the ultrafast dynamics in the Xe-He core-shell systems can be traced by recording and comparing the ion signals and Ee in a pump-probe experiment.

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Highly charged xenon ions In order to experimentally study the charging dynamics with respect to the ion products, the resulting mass spectra are recorded at different delays and analyzed by concentrating on the signals of Heq and Xeq . Fig. 3 shows the average Xe charge state Xeqavg achieved in the experiments and the simulations (top) as well as the experimental yields Y q of Xeq as a function of τ (bottom). (Note, that in Fig. 2 bare ion charges are given to monitor the ionization process). Comparing simulated and measured average Xe charge states (top) a qualitative agreement can be found, except for τ = 100 fs. In the experiment Xe+/2+ is mainly produced from Xe atoms present in the interaction region which reduces the average charge state presented, in particular when both pulses overlap. Possible refinements to the calculations in order to achieve a more satisfying quantitative agreement would include simulations on larger doped droplets and an improved description of the ionization dynamics. For example, it is well-known, that the Lotz formula underestimates the electron-impact ionization cross-sections. For the bottom representation we normalized the count rate of q each Xeq to the corresponding maximum Ymax . A clear pump-probe effect is obtained for all

Xeq up to q=19 in a range up to 1 500 fs. Signals from higher charge states up to q=25 are detected, but are either too weak to be analyzed with sufficient statistics or overlap with contributions from the residual gas and helium ions. For conditions where both pulses partially overlap, the spectrum shows an enhanced yield of only Xe+ (Fig. 3, left bottom corner), which clarifies that the focusing conditions are close to the atomic BSI intensity threshold of q 1×1014 Wcm−2 . 28 For each Xeq one can extract a charge-dependent optimal delay τopt , which

gives the most effective pump-probe conditions for high-yield ion production. For example, q 7+ the yields of Xe7+ maximizes for τopt = 500 fs. For the highest charge states τopt approaches

a value of 200 fs. This value characterizes the best conditions for reaching the highest possible Xe charge states for the chosen laser parameters and target composition. A similar q development of τopt was obtained in other experiments for larger Ag and Xe clusters. 44,45

Furthermore, previous MD simulations

32

on Xe309 He10 000 at intensities of 2.5 × 1014 Wcm−2 8


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have predicted that the highest Xe charge states are recorded at the detector when choosing q an optical delay of τopt = 200 fs. From the theoretical analysis it became clear that the value q of τopt represents a compromise between recombination and resonant energy absorption. The

results suggest that the dynamics of cluster charging in strong laser fields can be divided into an initial preionization triggered nanoplasma expansion and a subsequent plasmon-enhanced charging into high-q states in a core-shell system. Further support arises from a recent optimal control study on silver clusters in helium droplets, where a dual pulse excitation scheme was also obtained as an optimal pulse envelope solution. 10

Helium ions In contrast to Xeq , the He-ions show a different delay dependence. At the given laser intensity of ILpeak = 7 × 1013 Wcm−2 helium atoms cannot be ionized directly. However, electrons from the photoactive xenon core are accelerated by the superimposing fields of laser and dopand ions, 8,30,46 which leads to an oscillating electron motion with a spatial extent exceeding the Xe cluster dimension. Consequently, fast electrons penetrate into the droplet environment which allows for EII of helium. Additionally, the electric field of the ionized dopant atoms lowers the Coulomb barrier of surrounding He atoms. 31 Hence, Xe seed electrons trigger the helium charging in an avalanche-like process. 32 The efficiency of such an avalanche is strongly dependent on the dynamics of the ionization process and therefore a pump-probe effect can be obtained, see Fig. 4 (top). The MD simulations on Xe13 He2171 give the highest He2+ yield at the shortest optical delay (τ =100 fs) and a monotonous decrease with increasing delay time. In the experiment conducted on larger droplets the yield of doubly charged helium increases He to a maximum at around τopt = 700 fs and then decreases again. A similar dependence on

the optical delay is found for He+ as well as for the total yield of Xeq , q = 2 − 19. The value He of τopt reflects the time it takes for the initially overdense plasma to expand and reach an

eigenfrequency which is resonant with the frequency of the probe pulse, in agreement with recent findings. 33 9


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Electrons The pump-probe dependence of the Xeq (see Fig. 3) illustrates that the chosen laser intensity is not sufficient to produce high charge states without resonance effects. The generation of high Xe charge states (e.g., Xe25+ ) in the nanoplasma environment requires electron energies of several hundred eV. 47 This suggests a local Xeq -induced deep mean field potential which confines the fast electrons. The size of the Xe cluster with respect to the surrounding helium droplet suggests that the electron emission is dominated by the overwhelming number of helium atoms. As shown by simulations, initially fast electrons in the confining potential finally find themselves near the vacuum level as a result of the adiabatic expansion of the plasma cloud. 18 This leads to a thermal electron emission pattern as obtained in the experiment. The corresponding electron spectra are characterized by energies below Ee < 4 eV. Moreover, the majority of electrons show up with Ee < 0.5 eV. Fig. 4 (bottom) features a sequence of 19 pump-probe electron spectra. Similar to Heq and the total Xeq yield the electron signal enhances around τeopt = 700 fs. Interestingly, the additional peak at Ekin = 1.2 eV e shows up only near τopt and not for longer delays > 2 ps (see Fig. 5). We observe a small shift e . Accompanying experiments reveal that to higher electron energies when approaching τopt

the peak can also be obtained from pure He droplets and therefore represents a signature of the helium plasma. The time dependence suggests that the feature also originates from the laser-induced increase of the electron temperature. In particular at the critical density an enhanced emission of low energy electrons is expected, 18 see Fig. 5. In addition, electrons from the hot nanoplasma occupy low-binding energy levels in the expansion. 19 Correlated decay may lead to electron emission at certain energies, 21 e.g., at 1.2 eV. A closer look at single pulse as well as off-resonance (e.g. τ =20 ps) excitation, see Fig. 5 supports our consideration. In order to work out details of the emission process of the low energy electrons, effective temperatures (Teeff ) are extracted from the spectra shown in Figs. 4 and 5, assuming a Boltzmann type of energy distribution in the energy window (Ekin < 0.5 eV). Supporting angular resolved measurements reveal an isotropic emission characteristic which underpins 10


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the assumption that the signal originates from thermal electrons. The results of the fit procedures are presented in Fig. 6. Compared to single pulse excitation, Teeff enhances by e . A pump-probe effect can be obtained up to 1.8 ps. The increase in a factor 6 at τopt

Teeff at short delays images the development of the entire system towards effective heating conditions. On the descending slope and up to 1.8 ps the probe pulse still enables to heat up the system which is manifested in higher Teeff compared to single pulse or pump-probe excitations at a longer delay. The impact of the probe pulse on the electron distribution in the confined system is evident from Fig. 2. With respect to the Coulomb explosion, the pumpprobe dependence at longer delays reflects the transition from a coupled plasma system into separated particles. Therefore, slow electrons from doped helium nanodroplets may serve as a sensitive sensor for the metal-to-nonmetal transition 48 in an expanding nanoplasma. Hence, in contrast to x-rays, 49 which give insufficient data about the expansion dynamics, electron emission provides details not only about field-enhanced acceleration, 50 but also about the long-term plasma conditions.

Core-shell response The effect of the dopant size on the seed efficiency has been discussed. 51 Our MD calculations conducted on a small Xe-He core-shell system also demonstrate the impact of seeding on the droplet charging (see Fig. 2). For example, for τ =100 fs first the Xe charge state increases followed by a rapid, strong ionization of the helium shell. Further, Eeavg sharply rises with the He proves probe pulse impact and reaches significantly higher values. The development of τopt

that the dynamics of Xe and He are interwoven, i.e. any response of the xenon cluster affects q the charging of the helium environment, such as τopt (Fig. 3). The tight development of Heq

and Ee suggests that both observables probably image the same process (see Fig. 4, top). The electron signal includes the emission from XeN , however, the amount of dopant atoms is too small to have a marked influence on the electron signal. Finally, the comparison to the Xeq yields gives clear indications about the complex interplay of the core-shell constituents. In 11


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particular, the delay dependence of the observables allows to probe core and shell ionization q dynamics. The development of the highly charged xenon ion yields (τopt = 200 fs) represents

the evolution of the xenon-core, whereas the helium-shell dynamics is characterized by Heq He e = τopt =700 fs). In future experiments, e.g., a correlation and the electron emission (τopt

analysis may further clarify the nature of these entangled processes.

Conclusion The response of xenon-doped helium nanodroplets has been investigated in a joint experimental and theoretical effort with the aim to reveal the complex dynamics in a core shell nanoplasma and its impact on the particle emission. The results reveal a clear difference in the charging dynamics of the dopant xenon cluster and the surrounding helium matrix. Our findings hint at a transient system with distinct spacial peculiarities. The He ion signals and the low energy electron emission is largely driven by the xenon-induced ignition of the droplet. Additional evidence stems from the efficient probe pulse induced heating which is reflected in the effective electron temperature.

Acknowledgement Main parts of the helium droplet machine were provided by J.P. Toennies and his group at the MPI Göttingen. The Deutsche Forschungsgemeinschaft (TI 210/ 8-1, SFB 652) is gratefully acknowledged for financial support. A.H. is grateful for financial support from the Spanish Ministerio de Economia y Competividad (ref. no. CTQ2015-67660-P). Computational and manpower support provided by IZO-SGI SG Iker of UPV/EHU and European funding (EDRF and ESF) is gratefully acknowledged.

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References (1) Pukhov, A. Strong Field Interaction of Laser Radiation. Rep. Prog. Phys. 2003, 66, 47–101. (2) McPherson, A.; Thompson, B.; Borisov, A.; Boyer, K.; Rhodes, C. MultiphotonInduced X-Ray Emission at 4-5 keV From Xe Atoms with Multiple Core Vacancies. Nature 1994, 370, 631–633. (3) Parra, E.; McNaught, S.; Fan, J.; Milchberg, H. Pump-Probe Studies of EUV and XRay Emission Dynamics of Laser-Irradiated Noble Gas Droplets. Appl. Phys. A 2003, 77, 317. (4) Issac, R.; Vieux, G.; Ersfeld, B.; Brunetti, E.; Jamison, S.; Gallacher, J.; Clark, D.; Jaroszynski, D. Ultra Hard X-Rays from Krypton Clusters Heated by intense laser fields. Phys. Plas. 2004, 11, 3491–3496. (5) Vozzi, C.; Nisoli, M.; Caumes, J.-P.; Sansone, G.; Stagira, S.; De-Silvestri, S.; Vecchiocattivi, M.; Bassi, D.; Pascolini, M.; Poletto, L. et al. Cluster Effects in High-Order Harmonics Generated by Ultrashort Light Pulses. Appl. Phys. Lett. 2005, 86, 111121. (6) Lezius, M.; Dobosz, S.; Normand, D.; Schmidt, M. Explosion Dynamics of Rare Gas Clusters in Strong Laser Fields. Phys. Rev. Lett. 1998, 80, 261–4. (7) Snyder, E. M.; Buzza, S. A.; Castleman Jr., A. W. Intense Field-Matter Interactions: Multiple Ionization of Clusters. Phys. Rev Lett. 1996, 77, 3347–50. (8) Fennel, T.; Döppner, T.; Passig, J.; Schaal, C.; Tiggesbäumker, J.; Meiwes-Broer, K.-H. Plasmon-Enhanced Electron Acceleration in Intense Laser Metal-Cluster Interactions. Phys. Rev. Lett. 2007, 98, 143401. (9) Zamith, S.; Martchenko, T.; Ni, Y.; Aseyev, S.; Muller, H.; Vrakking, M. Control of

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the Production of Highly Charged Ions in Femtosecond-Laser Cluster Fragmentation. Phys. Rev. A 2004, 70, 11201. (10) Truong, N. X.; Hilse, P.; Göde, S.; Przystawik, A.; Döppner, T.; Fennel, T.; Bornath, T.; Tiggesbäumker, J.; Schlanges, M.; Gerber, G. et al. Optimal Control of the Strong-Field Ionization of Silver Clusters in Helium Droplets. Phys. Rev. A 2010, 81, 013201. (11) Fennel, T.; Meiwes-Broer, K.-H.; Tiggesbäumker, J.; Reinhard, P.-G.; Dinh, P. M.; Suraud, E. Laser-Driven Nonlinear Cluster Dynamics. Rev. Mod. Phys. 2010, 82, 1793– 1842. (12) Saalmann, U.; Siedschlag, C.; Rost, J. M. Mechanisms of Cluster Ionization in Strong Laser Pulses. J. Phys. B 2006, 39, R39. (13) Fennel, T.; Meiwes-Broer, K.-H.; Bertsch, G. Ionization Dynamics of Simple Metal Clusters in Intense Laser Fields by the Thomas-Fermi-Vlasov Method. Eur. Phys. J. D 2004, 29, 367–378. (14) Saalmann, U.; Rost, J.-M. Ionization of Clusters in Intense Laser Pulses Through Collective Electron Dynamics. Phys. Rev. Lett. 2003, 91, 223401. (15) Ditmire, T.; Donnelly, T.; Rubenchik, A. M.; Falcone, R. W.; Perry, M. D. Interaction of Intense Laser Pulses With Atomic Clusters. Phys. Rev. A 1996, 53, 3379–402. (16) Köller, L.; Schumacher, M.; Köhn, J.; Teuber, S.; Tiggesbäumker, J.; Meiwes-Broer, K.H. Plasmon-Enhanced Multi-Ionization of Small Metal Clusters in Strong Femtosecond Laser Fields. Phys. Rev. Lett. 1999, 82, 3783–6. (17) Passig, J.; Zherebtsov, S.; Irsig, R.; Arbeiter, M.; Peltz, C.; Göde, S.; Skruszewicz, S.; Meiwes-Broer, K.-H.; Tiggesbäumker, J.; Kling, M. et al. Nanoplasmonic Electron Acceleration by Attosecond-Controlled Forward Rescattering in Silver Clusters. Nature Commun. 2017, 8, 1181. 14


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(18) Arbeiter, M.; Peltz, C.; Fennel, T. Electron-Relocalization Dynamics in Xenon Clusters in Intense Soft-X-Ray Fields. Phys. Rev. A 2014, 89, 043428. (19) Komar, D.; Kazak, L.; Almassarani, M.; Meiwes-Broer, K.-H.; Tiggesbäumker, J. Highly Charged Rydberg Ions from the Coulomb Explosion of Clusters. Phys. Rev. Lett. 2018, 120, 133207. (20) Rajeev, R.; Trivikram, T. M.; Rishad, K.; Narayanan, V.; Krishnakumar, E.; Krishnamurthy, M. A Compact Laser-Driven Plasma Accelerator for Megaelectronvolt-Energy Neutral Atoms. Nature Phys. 2013, 9, 185. (21) Sch¨ utte, B.; Arbeiter, M.; Fennel, T.; Jabbari, G.; Kuleff, A.; Vrakking, M. J. J.; Rouzée, A. Observation of Correlated Electronic Decay in Expanding Clusters Triggered by Near-Infrared Fields. Nature Comm. 2015, 6, 8596. (22) Santra, R.; Greene, C. Xenon Clusters in Intense VUV Laser Fields. Phys. Rev. Lett. 2003, 91, 233401. (23) Marburger, S.; Kugeler, O.; Hergenhahn, U.; Möller, T. Experimental Evidence for Interatomic Coulombic Decay in Ne Clusters. Phys. Rev. Lett. 2003, 90, 203401. (24) Kagan, Y.; Vartanyants, I.; Shlyapnikov, G. Kinetics of Decay of Metastable Gas Phase of Polarized Atomic Hydrogen at Low Temperatures. Zh. Eksp. Teor. Fiz 1981, 81, 1113–1140. (25) Fennel, T.; Ramunno, L.; Brabec, T. Highly Charged Ions from Laser-Cluster Interactions: Local-Field-Enhanced Impact Ionization and Frustrated Electron-Ion Recombination. Phys. Rev. Lett. 2007, 99, 233401. (26) Döppner, T.; Fennel, T.; Diederich, T.; Tiggesbäumker, J.; Meiwes-Broer, K.-H. Controlling the Coulomb Explosion of Silver Clusters by Femtosecond Dual-Pulse Laser Excitation. Phys. Rev. Lett. 2005, 94, 013401. 15


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(27) Tiggesbäumker, J.; Stienkemeier, F. Formation and Properties of Metal Clusters Isolated in Helium Droplets. Phys. Chem. Chem. Phys. 2007, 9, 4748 – 4770. (28) Augst, S.; Strickland, D.; Meyerhofer, D. D.; Chin, S. L.; Eberly, J. H. Tunneling Ionization of Noble Gases in a High-Intensity Laser Field. Phys. Rev. Lett. 1989, 63, 2212–5. (29) Mikaberidze, A.; Saalmann, U.; Rost, J. M. Energy Absorption of Xenon Clusters in Helium Nanodroplets Under Strong Laser Pulses. Phys. Rev. A 2008, 77, 041201. (30) Heidenreich, A.; Gr¨ uner, B.; Rometsch, M.; Krishnan, S. R.; Stienkemeier, F.; Mudrich, M. Efficiency of Dopant-Induced Ignition of Helium Nanoplasmas. New J. Phys. 2016, 18, 073046. (31) Heidenreich, A.; Schomas, D.; Mudrich, M. Dopant-Induced Ignition of Helium Nanoplasmas - A Mechanistic Study. J. Phys. B 2017, 50, 244001. (32) Peltz, C.; Fennel, T. Resonant Charging of Xe Clusters in Helium Nanodroplets Under Intense Laser Fields. Eur. Phs. J. D 2011, 63, 281. (33) Krishnan, S. R.; Peltz, C.; Fechner, L.; Sharma, V.; Kremer, M.; Fischer, B.; Camus, N.; Pfeifer, T.; Jha, J.; Krishnamurthy, M. et al. Evolution of Dopant-Induced Helium Nanoplasmas. New J. Phys. 2012, 14, 075016. (34) Bartelt, A.; Close, J.; Federmann, F.; Quaas, N.; Toennies, J.-P. Cold Metal Clusters: Helium Droplets as a Nanoscale Cryostat. Phys. Rev. Lett. 1996, 77, 3525–3528. (35) Tiggesbäumker, J.; Stienkemeier, F. Formation and Properties of Metal Clusters Isolated in Helium Droplets. Phys. Chem. Chem. Phys. 2007, 9, 4748 – 4770. (36) B¨ unermann, O.; Stienkemeier, F. Modeling the Formation of Alkali Clusters Attached to Helium Nanodroplets and the Abundance of High-Spin States. Eur. Phys. J. D 2011, 61, 645–655. 16


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(46) Saalmann, U.; Rost, J. M. Rescattering for Extended Atomic Systems. Phys. Rev. Lett. 2008, 100, 133006. (47) Kaufman, V.; Sugar, J.; Rowan, W. L. Spectra of Copperlike and Zinclike Xenon: Xe XXV and Xe XXVI. J. Opt. Soc. Am. B 1988, 5, 1273–1274. (48) Diederich, T.; Döppner, T.; Braune, J.; Tiggesbäumker, J.; Meiwes-Broer, K.-H. Electron Delocalization in Magnesium Clusters Grown in Supercold Helium Droplets. Phys. Rev. Lett. 2001, 86, 4807–4810. (49) Irsig, R.; Shihab, M.; Kazak, L.; Bornath, T.; Tiggesbumker, J.; Redmer, R.; MeiwesBroer, K.-H. The Interaction of Intense Femtosecond Laser Pulses with Argon Microdroplets Studied Near the Soft X-Ray Emission Threshold. J. Phys. B 2018, 51, 024006. (50) Passig, J.; Irsig, R.; Truong, N. X.; Fennel, T.; Tiggesbäumker, J.; Meiwes-Broer, K. H. Nanoplasmonic Electron Acceleration in Silver Clusters Studied by Angular-Resolved Electron Spectroscopy. New J. Phys. 2012, 14, 085020. (51) Heidenreich, A.; Gr¨ uner, B.; Schomas, D.; Stienkemeier, F.; Krishnan, S. R.; Mudrich, M. Charging Dynamics of Dopants in Helium Nanoplasmas. J. Mod. Opt. 2017, 64, 1061–1077.

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Figure 2: MD simulations of Xe13 He2171 showing the development of certain observables in the confined system for selected pump-probe delays (left column: τ = 100 fs, right column: 300 fs.). Laser pulses with a duration of τL =70 fs and peak intensities of ILpeak = 5 × 1013 Wcm−2 and ILpeak = 4 × 1013 Wcm−2 for pump and probe, respectively, are applied. (a) laser pulse envelopes (b) average xenon charge state (Xeqavg ); (c) average helium charge state (Heqavg ); (d) average electron energies (Eavg e ) for selected optical delays τ .

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Optical delay t [fs] Figure 3: (color online) top: Comparison of experimental (, blue) and theoretical ( , green) average Xe charge states as function of the optical delay τ recorded in a strong-field experiment on XeN Hedroplet . bottom: As top, but focusing on the yields Y q of xenon ions, q = 1−19. The yields of the highly charged Xe ions are normalized line-by-line for each Xeq . The dotted line is used to guide the eye and highlights the development of the optimal delay q q τopt . For highest charge states τopt approaches a value of 200 fs. For Xe16+ the signal partially overlaps with residual gas which leads to an artificial enhancement of Y 16+ at shorter delays.

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Optical delay t [fs] Figure 4: (color online) top: He2+ signal, total Xeq (q = 2−19) and electron yield for different optical delays. bottom: Dependence of the low energy electron spectra on the pump-probe delay. Note, that the yields are presented on a log-scale. To improve the presentation the electron spectra have been smoothed. Independent on the chosen observable a maximum in the signals is obtained for τopt =700 fs.

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Electron energy [eV] Figure 5: (color online) Electron energy spectra recorded for selected optical delays. (τ = e e 700 fs (τopt ); 1500 fs (off τopt ); 20 ps, pump pulse only). The increase of the electron signal above 1 eV at τ = 20 ps compared to single pulse excitation can be attributed to probe-pulse induced field ionization of electrons in low binding energy levels. 19 This assumption is backed by the observation, that in contrast to the faster electrons, the low-energy emission remains unchanged under both conditions.

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Optical delay [fs] Figure 6: Impact of the delayed probe pulse on the effective electron temperatures Teeff , see text. The tail of the falling slope of Teeff approaches the temperature for single pulse only ionization (dashed line) and highlights the nanoplasma decay in the expansion process. The solid line is used to guide the eyes

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Comparison of Electron and Ion Emission from Xenon Cluster-Induced

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