Ignition of a helium nanoplasma by x-ray multiple ionization of a heavy rare-gas core

Abstract

The dynamics of an x-ray-ionized two-component core-shell nanosystem is probed using doped helium (He) nanodroplets. First, a soft x-ray pump pulse selectively inner-shell ionizes the core cluster formed of heavier rare-gas atoms, causing electron migration from the He shell to the highlycharged core. This ignites a He nanoplasma which is then driven by an intense near-infrared probe pulse. The ultrafast charge redistribution, evidenced by the rise of He + and He 2+ ion yields from the nanoplasma within 70 fs, leads to strong damping of the core cluster expansion. Thus, He droplets act as efficient tampers that reduce the radiation damage of embedded nanostructures, a property that could be exploited for improving coherent diffraction images.

Long-lasting XUV activation of helium nanodroplets for avalanche ionization

“…The simulation shows that EII by far (>95%) dominates the avalanche ionization channel, as it was also the case in all previous studies of pure and doped He droplets [11,13,24]. The photoelectrons act as seed electrons for the EII avalanche.…”

“…Based on these observations, we use a fit model of the nanoplasma pump-probe dynamics given essentially by the sum of two terms; one describes a transient enhancement of the ignition rate near t = 0 due to the resonant coupling of the NIR probe pulse to the XUV-activated He nanodroplets, S short (t) = A Θ(t) exp(−t/τ ). Similar dynamics has previously been observed in NIR and soft x-ray-activated He nanodroplets [13,41]. The second term accounts for long-lasting activation by the XUV pump pulse which we assume to be constant for t ≫ 0, S long (t) = B Θ(t).…”

“…To get more detailed insights into both the short-delay pump-probe dynamics and the long-term ionization process, we performed classical molecular dynamics (MD) simulations. The general features of the MD simulation method for the interaction of a cluster with the electric and magnetic field of a linearly polarized NIR Gaussian laser pulse were described previously [10,11,13,24,45]. In short, all atoms and nanoplasma electrons are treated classically, starting with a cluster of neutral atoms.…”

“…The plasma avalanche ionization is usually initiated by strong-field tunnel ionization (TI) of a few atoms in the nanocluster; the created quasi-free electrons are driven by the laser field unleashing an avalanche of secondary electrons mainly by electron-impact ionization (EII) of the surrounding atoms. Apart from TI, seed electrons have been created by XUV and x-ray photoionization [12][13][14]. In the expansion phase of the nanoplasma, correlated electronic decay processes involving quasifree electrons and highly excited atoms and ions can occur [15][16][17][18].…”

Direct inner-shell photoionization of Xe atoms embedded in helium nanodroplets

“…This paves the way to x-ray photoelectron spectroscopy (XPS) of unconventional atomic clusters and molecular complexes, which can form in He nanodroplets owing to their unique quantum fluid properties [53][54][55][56]. Furthermore, the selective multiple ionization of dopants in He nanodroplets is an efficient mechanism for igniting a nanoplasma [57]. Probing the dynamics of nanoplasmas by ultrashort XUV and x-ray pulses has attracted considerable attention in the free-electron laser science community [39,[58][59][60].…”