Real‐Time Probing of Hot Electron Diffusion in Superfluid Helium Nanodroplets by Laser‐Assisted Electron Scattering

Tracking electron motion in nanostructures in real time remains a central challenge in nanotechnology and ultrafast science. Here, we demonstrate that hot electron propagation can be probed with femtosecond resolution using laser‐assisted electron scattering (LAES), a light–matter interaction process in which electrons exchange energy with strong laser fields during collisions with atoms. We employ superfluid helium nanodroplets of 5–10 nm diameter doped with single atoms and irradiate them with pairs of few‐cycle near‐infrared laser pulses. The first pulse releases a quasi‐free electron from the dopant via strong‐field ionization, while a time‐delayed second pulse probes its motion through the He environment via LAES. The yield of accelerated electrons directly reflects the propagation of hot electrons prior to localization into a bubble state. Our experimental findings are consistent with Monte Carlo simulations, in which the electron motion is characterized by a diffusive random walk with a diffusion coefficient of (1.7 ± 0.2) cm ² s ⁻¹ . The correspondingly high propagation length of several tens of nanometers sets an upper limit on electron transport in materials when inelastic scattering is suppressed.