Charging and Ion Ejection Dynamics of Large Helium Nanodroplets Exposed to Intense Femtosecond Soft X-Ray Pulses*

Ion ejection from helium nanodroplets exposed to intense femtosecond soft x-ray pulses is studied via single-pulse ion time-of-flight (TOF) spectroscopy in coincidence with small-angle x-ray scattering. Scattering images encode droplet size and absolute photon flux incident on each droplet. Ion TOF spectra report on maximum ion kinetic energies (KE) of He⁺_n=1-4 fragments. Measurements span He_N droplet sizes from N ≈ 10⁷ to 10¹⁰ and droplet charges from ~9×10^-5 to ~4×10^-3 e/atom, spanning Coulomb explosion conditions to substantial frustration of outer ionization. The combination of absolute x-ray intensities, cluster sizes, and ion KE on an event-by-event basis reveals the correlations between ionization conditions and ejection dynamics of the fragments. The observed maximum He⁺ KE is compared to estimates of the maximum based on the cluster Coulomb potential resulting from unscreened ions and considering the impact of ion-atom collisions for ions emerging from within the cluster. The maximum He⁺ KE is found to be governed by Coulomb repulsion from unscreened cations across all expansion regimes. Findings are consistent with the emergence of a charged spherical shell around a quasi-neutral plasma core with increasing frustration.