Charging and ion ejection dynamics of large helium nanodroplets exposed to intense femtosecond soft X-ray pulses

Journal Article · Mon Oct 11 00:00:00 EDT 2021 · European Physical Journal. Special Topics

DOI:https://doi.org/10.1140/epjs/s11734-021-00280-0· OSTI ID:1872757

Saladrigas, Catherine A. ^[1]; Feinberg, Alexandra J. ^[2]; Ziemkiewicz, Michael P. ^[1]; Bacellar, Camila ^[1]; Bucher, Maximilian ^[3]; Bernando, Charles ^[2]; Carron, Sebastian ^[4]; Chatterley, Adam S. ^[1]; Decker, Franz-Josef ^[5]; Ferguson, Ken R. ^[5]; Gomez, Luis ^[2]; Gorkhover, Taisia ^[6]; Helvy, Nathan A. ^[1]; Jones, Curtis F. ^[7]; Kwok, Justin J. ^[2]; Lutman, Alberto ^[5]; Rupp, Daniela ^[8]; Tanyag, Rico P. ^[2]; Möller, Thomas ^[9]; Neumark, Daniel M. ^[1] ; Bostedt, Christoph ^[10]; Vilesov, Andrey F. ^[2]; Gessner, Oliver ^[11]

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1. Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States); Univ. of California, Berkeley, CA (United States)
2. Univ. of Southern California, Los Angeles, CA (United States)
3. Argonne National Lab. (ANL), Argonne, IL (United States); SLAC National Accelerator Lab., Menlo Park, CA (United States); Technical Univ. of Berlin (Germany)
4. California Lutheran Univ., Thousand Oaks, CA (United States)
5. SLAC National Accelerator Lab., Menlo Park, CA (United States)
6. SLAC National Accelerator Lab., Menlo Park, CA (United States); Technical Univ. of Berlin (Germany)
7. Univ. of Southern California, Los Angeles, CA (United States); State Univ. of New York (SUNY), Adirondak, NY (United States)
8. Technical Univ. of Berlin (Germany); Eidgenoessische Technische Hochschule (ETH), Zurich (Switzerland)
9. Technical Univ. of Berlin (Germany)
10. Argonne National Lab. (ANL), Argonne, IL (United States); SLAC National Accelerator Lab., Menlo Park, CA (United States); Paul Scherrer Inst. (PSI), Villigen (Switzerland); Ecole Polytechnique Federale Lausanne (Switzerland)
11. Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)

We report ion ejection from charged helium nanodroplets exposed to intense femtosecond soft X-ray pulses is studied by single-pulse ion time-of-flight (TOF) spectroscopy in coincidence with small-angle X-ray scattering. Scattering images encode the droplet size and absolute photon flux incident on each droplet, while ion TOF spectra are used to determine the maximum ion kinetic energy, E_kin, of He$$^{+}_{j}$$ fragments (j = 1–4). Measurements span He_N droplet sizes between N~10⁷ and ~10¹⁰ (radii R₀ = 78–578 nm), and droplet charges between ~9×10^-5 and ~3×10^-3 e/atom. Conditions encompass a wide range of ionization and expansion regimes, from departure of all photoelectrons from the droplet, leading to pure Coulomb explosion, to substantial electron trapping by the electrostatic potential of the charged droplet, indicating the onset of hydrodynamic expansion. The unique combination of absolute X-ray intensities, droplet sizes, and ion E_kin on an event-by-event basis reveals a detailed picture of the correlations between the ionization conditions and the ejection dynamics of the ionic fragments. The maximum E_kin of He⁺ is found to be governed by Coulomb repulsion from unscreened cations across all expansion regimes. The impact of ion-atom interactions resulting from the relatively low charge densities is increasingly relevant with less electron trapping. The findings are consistent with the emergence of a charged spherical shell around a quasineutral plasma core as the degree of ionization increases. The results demonstrate a complex relationship between measured ion E_kin and droplet ionization conditions that can only be disentangled through the use of coincident single-pulse TOF and scattering data.

• Research Organization: Argonne National Laboratory (ANL), Argonne, IL (United States); Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States); SLAC National Accelerator Laboratory (SLAC), Menlo Park, CA (United States)
• Sponsoring Organization: German Research Foundation (DFG); National Science Foundation (NSF); Swiss National Science Foundation; USDOE Office of Science (SC), Basic Energy Sciences (BES). Chemical Sciences, Geosciences & Biosciences Division
• Grant/Contract Number: AC02-05CH11231; AC02-06CH11357; AC02-76SF00515
• OSTI ID: 1872757
• Alternate ID(s): OSTI ID: 1846301
• Journal Information: European Physical Journal. Special Topics, Journal Name: European Physical Journal. Special Topics Journal Issue: 23 Vol. 230; ISSN 1951-6355
• Publisher: Springer
• Country of Publication: United States
• Language: English

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