Eur. Phys. J. Spec. Top. (2021) 230: 4011-4023
https://doi.org/10.1140/epjs/s11734-021-00280-0

Regular Article

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

¹ Chemical Sciences Division, Lawrence Berkeley National Laboratory, 94720, Berkeley, CA, USA
² Department of Chemistry, University of California, 94720, Berkeley, CA, USA
³ Department of Chemistry, University of Southern California, 90089, Los Angeles, CA, USA
⁴ Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue B109, 60439, Lemont, IL, USA
⁵ SLAC National Accelerator Laboratory, 2575 Sand Hill Road, 94025, Menlo Park, CA, USA
⁶ Institute of Optics and Atomic Physics, Technical University of Berlin, Hardenbergstraße 36, 10623, Berlin, Germany
⁷ Department of Physics and Astronomy, University of Southern California, 90089, Los Angeles, CA, USA
⁸ Department of Physics, California Lutheran University, 91360, Thousand Oaks, CA, USA
⁹ Department of Physics, University of California, 94720, Berkeley, CA, USA
¹⁰ Mork Family Department of Chemical Engineering and Materials Science, USC, 90089, Los Angeles, CA, USA
¹¹ LFKP, ETH Zurich, John-von-Neumann-Weg 9, 8093, Zurich, Switzerland
¹² Paul Scherrer Institut (PSI), Forschungsstrasse 111, 5232, Villigen, Switzerland
¹³ LUXS Laboratory for Ultrafast X-ray Sciences, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne (EPFL), 1015, Lausanne, Switzerland
¹⁴ Present address: Ball Aerospace, Boulder, CO, USA
¹⁵ Present address: Paul Scherrer Institute, Forschungsstrasse 111, 5232, Villigen-PSI, Switzerland
¹⁶ School of Information Systems, Binus University, Jl. K.H. Syahdan No. 9, 11480, Palmerah, Jakarta Barat, Indonesia
¹⁷ Present address: Department of Chemistry, Aarhus University, Langelandsgade 140, 8000, Aarhus C, Denmark
¹⁸ State University of New York (SUNY) Adirondack, 12804, Queensbury, NY, USA
¹⁹ Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, 1304 W. Green St., 61801, Urbana, IL, USA

^y vilesov@usc.edu
^z ogessner@lbl.gov

Received: 14 May 2021
Accepted: 6 September 2021
Published online: 11 October 2021

Abstract

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, $E_{\text{kin}}$, of ${\text{He}}_j^{+}$ fragments (j = 1–4). Measurements span ${\text{He}}_N$ droplet sizes between $N\sim {10}^7$ and $\sim {10}^{10}$ (radii $R_0$ = 78–578 nm), and droplet charges between $\sim 9\times {10}^{-5}$ and $\sim 3\times {10}^{-3}$ 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 $E_{\text{kin}}$ 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 $E_{\text{kin}}$ 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 $E_{\text{kin}}$ and droplet ionization conditions that can only be disentangled through the use of coincident single-pulse TOF and scattering data.