Charge recombination in soft x-ray laser produced nanoplasmas

The ionization and charge separation processes of nanoplasmas created by resonant excitation of atomic clusters in intense soft x-ray pulses have been investigated. Through irradiation with femtosecond pulses from the FLASH free electron laser (FEL) at λ = 13.7 nm and power densities exceeding 1014 W cm−2 the clusters are highly ionized with transient atomic charge states up to 9+. Variation of the cluster composition from pristine to doped and core–shell systems allows tracking of the spatial origin and charge states of the fragments yielding insight into the nanoplasma dynamics. The data give evidence for efficient charge redistribution processes leading to a Coulomb explosion of the cluster outer part and recombination of the nanoplasma core. The experiments show qualitatively different processes for (soft) x-ray produced nanoplasmas from the optical (IR) strong-field regime where the clusters disintegrate completely in a Coulomb explosion. (Some figures in this article are in colour only in the electronic version) Understanding the interaction of light with matter has been a central theme of physics over the past century, starting with the concept of the photon and the inception of quantum theory. The invention of the laser and the continuing advance in laser technologies has made it possible to explore regimes of nonlinear light–matter interaction leading to novel laserbased concepts for particle accelerators, plasma formation and nuclear fusion [1]. Currently, we are witnessing the advent of intense lasers in the x-ray regime. One of the most exciting prospects of research with x-ray lasers is direct imaging of nonperiodic nanoscale objects, such as biomolecules, nanocrystals, living cells and viruses [2]. Even though it is crucial for the success of the imaging experiments, understanding the interaction of intense x-ray pulses with atomic systems and the underlying dynamics is still in its infancy. To date, virtually all studies about the ionization as well as nuclear dynamics of nanometer-sized structures in intense (soft) x-ray pulses are of theoretical nature [3–6] and no experimental data are available. For the experimental investigations of matter in intense light pulses atomic clusters are ideal because their size can be tuned from the molecular to the bulk-like regime and there is no energy dissipation into surrounding media. The ionization dynamics of clusters in intense laser pulses depend considerably on the radiation wavelength. In the infrared spectral regime the cluster is ionized by the optical field and the resulting transient nanoplasma is efficiently heated by the external laser field via inverse bremsstrahlung (IBS) and collective effects, leading to a Coulomb explosion of the cluster [7]. Because the ponderomotive energy scales with ω−2(ω-laser frequency) and thus, the direct effect of the laser field on the electron movement is small, it was a big surprise when experiments in the vacuum-ultraviolet spectral regime at 100 nm and intensities up to 1013 W cm−2 reported unexpectedly high-energy absorption and complete Coulomb 0953-4075/08/181001+05$30.00 1 © 2008 IOP Publishing Ltd Printed in the UK J. Phys. B: At. Mol. Opt. Phys. 41 (2008) 181001 Fast Track Communication explosion of clusters [8]. The efficient energy absorption of the clusters is theoretically explained with more realistic potentials for IBS [9], barrier suppression in the ionized cluster [10] and enhanced heating through many-body collisions in a transient strongly coupled nanoplasma [11]. While theoretically IBS is predicted to be the dominant absorption mechanism down to 62 nm [12] recent photoemission experiments at λ = 32 nm and similar intensities find no evidence for it [13]. Instead, energy deposition is best described by photoabsorption and photoemission which becomes frustrated in the Coulomb field of the charging cluster, suppressing the formation of a nanoplasma for intensities up to 1013 W cm−2 [13]. In this communication we present first experimental data about intense laser–cluster interaction with resonant excitation in the soft x-ray regime. Pristine Xe, Ar and heterogeneous Xe–Ar clusters are irradiated at λ = 13.7 nm and power densities exceeding 1014 W cm−2 leading to nanoplasma formation with transient charge states up to 9+. The ionization dynamics including charge transfer and charge separation processes are followed by a controlled variation of the cluster composition from pristine over low doping level to core–shell systems. Tracking the specific m/q ratios in time-of-flight (tof) spectra yields insight into the spatial origin and charge states of the fragments. Low doping levels in the bulk and at the surface allow the creation of extreme charge states at selected sites in the cluster for investigation of charge transfer processes. By varying the surface layer thickness of core– shell systems the Cluster explosion and charge recombination processes, i.e., evolution of the nanoplasma are probed. The experiments evidence qualitatively different dynamics in xray produced plasmas compared with the optical strong-field regime. The data show efficient charge redistribution within the cluster leading to a Coulomb explosion of the outer layers and recombination of the x-ray excited nanoplasma core which is in contradiction to the standard Coulomb explosion model assuming complete disintegration of the cluster. The current findings are of central importance for evolving strategies to delay the Coulomb explosion of bio-molecules with a tamper [6] for future imaging with x-ray lasers [2, 3, 14]. The experiments are performed at the FLASH free electron laser in Hamburg [15]. The FEL beam is focussed with multilayer optics on a beam spot of d ≈ 15 μm. Prior to focussing the far wings of the beam are cut with apertures. The average pulse energies are 12 μJ and the pulse durations are 10 fs [15]. All data are acquired in singleshot mode. With the current experimental conditions power densities exceeding 1014 W cm−2 are obtained as evidenced by the highest observable charge states of atomic Xe [16]. The photon energy is set to 90.5 eV (λ = 13.7 nm) in the middle of the broad Xe 4d giant resonance ( E = 50 eV). The photon absorption cross section for the cluster materials is 24.7 Mbarn for Xe compared with 1.4 Mbarn for Ar, translating to about 17 and 1 photons per photoionization cross section σ , respectively. The charged cluster fragments are detected with a tof spectrometer with an entrance aperture of 500 × 1000 μm perpendicular to the beam which is shorter than the Rayleigh length of the focus. In order to avoid secondary ionization events in the tof spectrometer the cluster beam is heavily diluted with a large source–interaction zone distance, double skimmer set-up, resulting in sample densities of less than 50 clusters in the focal volume under the spectrometer aperture. The clusters are prepared by adiabatic expansion of rare gases through a pulsed 100 μm conical nozzle with an half-opening angle of 15◦ and their sizes are determined with scaling laws for pristine clusters [17, 18]. For doping of the clusters two different techniques are used. Surface doping is achieved through the pick-up technique, where the Ar cluster jet crosses a Xe beam of atoms from a 600 μm nozzle about 10 mm away and a backing pressure of 1.5 mbar [19]. The Ar host clusters are prepared with a stagnation pressure of 28 bar and a nozzle temperature of 300 K. The Ar clusters doped in the interior and the Xe/Ar core–shell systems are prepared by co-expansion of premixed gases. At low concentration of ≈0.01% Xe in Ar isolated Xe atoms are embedded in the Ar matrix [19]. At high concentrations exceeding 5–10% of Xe in Ar, pure Xe clusters are formed because then Ar acts merely as a seeding gas which is completely evaporated off the growing cluster. In a narrow range between 1–5% Xe in Ar a strong enrichment of the Xe in the cluster is observed [20–22]. The Xe nucleates in the core of the Ar cluster but not all Ar is evaporated [21, 22]. This formation of the core–shell structure is due to phase separation resulting from the different cohesive energies and melting points of Xe and Ar. Thus, the enrichment factor of Xe in the Ar cluster, or in other words, the thickness of the Ar surface layer, depends sensitively on the Xe concentration of the gas premix. For the current experiments a gas premix of 2% Xe in Ar is expanded at 7 bars and nozzle temperatures of 300 K. These parameters have been chosen because they yield the same gas-independent cluster condensation parameter ∗ [17, 18] as in previous detailed photoelectron spectroscopy investigations about such systems [21]. The corresponding cluster size according to the scaling laws for pristine clusters [17, 18] is 〈N〉 ≈ 400 with the typical size distribution N ≈ N full-width at half-maximum. Additionally, a larger core–shell system with 〈N〉 ≈ 4000 is prepared by expanding the same gas premix at 17 bars. For the current experimental conditions a Xe enrichment factor of about 10 has been measured in different previous investigations [21–23]. Therefore the core–shell structures can be roughly estimated to consist of a Xe core with approximately 1–2 and 3–4 Ar shells on top, respectively. In figure 1 the tof data converted to m/q spectra of atomic Xe as well as pristine Xe and Ar clusters exposed to the FEL pulse are shown. With the given power densities, Xe atoms (the top panel in figure 1) are multiply ionized with charge states up to 9+ by sequential singleand multi-photon ionization [16]. In the atomic spectra the Xe isotope distribution is well resolved. Compared to previous measurements [16] a complete depletion of Xe and reduced intensity of the other low-charge states is observed which can be attributed to the cut low-power density far wings of the beam as described above. The spectrum for Xe clusters is significantly different (the middle panel in figure 1). Here, the singly ionized monomers are the dominant signal and also larger Xen fragments are present. In the inset the m/q range of the Xe clusters up to