Ab initio Simulations of EMI-BF4 Neutral-Surface Interactions in Electrospray Thrusters

TL;DR

The paper addresses SSE-induced complications in ground-testing electrospray thrusters by revealing, via energy-resolved QM/MM ab initio MD, how neutral EMI-BF4 collisions with Au extractor surfaces progress through a three-stage energy sequence: ionic dissociation ($10$–$20$ eV), neutralization ($30$–$40$ eV), and covalent fragmentation ($>50$ eV). Using CP2K with GAPW, a carefully defined QM region, and a rigid Au slab, the study quantifies fragment spectra, charge states via RESP analysis, energy partitioning, and deflection-angle distributions, linking molecular outcomes to measurable SSE diagnostics. Key findings include a heavy-fragment forward-scattering bias, broad scattering of light fragments, a metastable peak near $50$ eV with abundant neutrals, and the persistence of charged secondaries under neutral bombardment, explaining experimental observations of SSE near thresholds. The work informs debiasing strategies for TOF-SIMS and RGA measurements, underscores the need for neutral diagnostics, and suggests future model enhancements (polarizable metals, longer timescales, varied surfaces) to improve predictive capability for device design and lifetime optimization.

Abstract

Electrospray thrusters promise compact, high specific impulse propulsion for small spacecraft, yet ground characterization remains confounded by secondary species emission and incomplete diagnostics of neutral products. To address these limitations, we perform energy-resolved mixed quantum/classical (QM/MM) ab initio molecular dynamics (MD) of neutral 1-ethyl-3-methylimidazolium tetrafluoroborate, EMI-BF4, colliding with Au extractor surfaces with impact energies from 10 to 100 eV to resolve fragment species spectra, charge states, kinetic energy partitioning, and scattering geometry. The simulations reveal a three-stage sequence with impact energy: the low energy regime, 10 to 20 eV, which favors ionic dissociation, intermediate energy regime, between 30 to 40 eV, opens a neutralization window, and high energy regime, greater than 50 eV, drives covalent fragmentation into many light products with mixed charge states. Fractional energy distributions show a transition from few-body, energy-concentrated outcomes in the low energy regime to many-body, energy-dispersed outcomes in the high energy regime. Deflection angle distributions exhibit a strong mass-to-angle anti-correlation such that heavier fragments favor small deflection, whereas lighter fragments populate larger deflection angles. The fraction of transient metastables peaks near 50 eV, coinciding with abundant neutral fragment production. Importantly, neutral bombardment still produces charged secondaries at the target even when the upstream ion plume is fully suppressed by a decelerating electrode. These findings provide a basis for de-biasing facility measurements by pairing tandem time-of-flight secondary ion mass spectrometry and residual gas analyzer with suppression-bias corrections to inform the design of electrospray thrusters that reduce interception and contamination on extractor surfaces.

Figures (10)

• Figure 1: Simulation domain for extractor surface collisions from incident EMI-BF4 neutrals.
• Figure 2: Molecular geometry of the DFT optimized incident EMI-BF4 projectile from the right, top, and front views.
• Figure 3: Equilibration temperature for the incident EMI-BF4 neutral during the canonical ensemble.
• Figure 4: Maximum relative deviation of the total energy in microcanonical trajectories across impact kinetic energies.
• Figure 5: Lennard-Jones pair potentials for short-range interactions between Au and QM atoms.