Mechanism for the Efficient Homogeneous Nucleation of Ice in a Weakly Ionized, Ultracold Plasma

Abstract

It is proposed that the rapid observed homogeneous nucleation of ice dust in a cold, weakly-ionized plasma depends on the formation of negative hydroxyl ions by fast electrons impacting water molecules. These OH⁻ ions attract neutral water molecules because of the high dipole moment of the water molecules and so hydrates of the form (OH)⁻(H₂O)ₙ are formed. The hydrates continuously grow in the cold environment to become macroscopic ice grains. These ice grains are negatively charged as a result of electron impact and so continue to attract water molecules. Because the hydroxyl ions are negative, unlike positive ions they do not suffer recombination loss from collision with plasma electrons. Recombination with positive ions is minimal because positive ions are few in number (weak ionization) and slow-moving as result of being in thermal equilibrium with the cold background gas.