Origin of the weak maximum in ignition probability around 2×10^3 atoms in MD simulations

Ascertain the physical mechanism responsible for the weak maximum in the NIR-induced nanoplasma ignition probability near a helium nanodroplet size of approximately 2×10^3 atoms observed in the classical molecular dynamics simulations at a peak NIR intensity of 2×10^14 W cm^-2 for trajectories with n* = 4 and 6 excited He* atoms, and determine whether this feature arises from electron path-length effects, spatial clustering of He* seeds, or other size-dependent factors.

Background

The appendix presents classical molecular dynamics simulations assessing how many He* excitations are needed to ignite an NIR-driven ionization avalanche in helium nanodroplets. The ignition probability is evaluated versus droplet size under specified intensities and numbers of He* seeds. For I = 2×10^14 W cm^-2 and n* = 4 or 6, the ignition probability shows a steep initial rise with a weak maximum around ~2×10^3 atoms, followed by a slight decrease for larger droplets.

While speculative explanations involving competing trends (e.g., increased electron path length in larger droplets versus reduced likelihood of closely spaced He* seeds) are mentioned, the simulations explicitly note that the cause of the observed weak maximum remains unknown, motivating a definitive identification of the underlying mechanism.