Nucleation of adamantane clusters in weakly ionised plasmas: dispersion cohesion, surface energy, and charging

Abstract The formation of molecular clusters in low-temperature plasmas is governed by an intricate interplay of dispersion interactions, electrostatic charging, and surface energetics. While chemical growth pathways have been extensively studied, the role of non-chemical interactions in determining characteristic cluster sizes remains poorly understood. In this work, we develop a unified free-energy framework for molecular clusters embedded in weakly ionised Ar/H 2 plasmas. The approach combines first-principles calculations of the molecular polarisability of adamantane with a dielectric description of the plasma response, including collisional damping and plasma screening. Dispersion interactions are treated as a bulk-like cohesive contribution with a physically motivated microscopic cutoff, while electrostatic charging is described within floating-potential theory. We show that, for realistic discharge parameters, the total free energy does not generally exhibit an effective equilibrium radius. Instead, the free-energy landscape is characterised by a nucleation barrier with an effective critical radius separating subcritical clusters from downhill growth. By mapping this nucleation threshold scale as a function of pressure, electron temperature, and gas composition, we demonstrate that plasma conditions provide a systematic control over the nucleation scale. Our results suggest that experimentally observed steady cluster sizes in weakly ionised plasmas are governed by kinetic balance rather than free-energy minimisation, and that gas composition and plasma parameters can be used as effective control knobs for early-stage cluster formation.