Anionic H2O clusters as precursors to ice grain nucleation in cryogenic plasma

Ice grains nucleate rapidly when water vapor is injected into a weakly-ionized laboratory plasma with cryogenically cooled background gas. As homogenous nucleation of water ice is thermodynamically forbidden in neutral gas under experimental conditions, we attribute grain formation to ion-induced nucleation. Using time-resolved ion mass spectrometry, we detect the clustering of water molecules around the ions in the plasma. As the lifetime of cations in the plasma is short, positively-charged clusters never grow larger than a few monomers. In contrast, as the plasma becomes electronegative upon water injection, long-lived anions such as OH- and O2- accumulate in the plasma's positive potential well and are hydrated to much larger cluster sizes, forming precursors to the formation of stable ice. We present direct evidence of cluster formation, compare their formation rates and stability to theoretical models, and present experimental evidence for the growth of anionic clusters into ice grains. The results provide a controlled laboratory analogue for ion‑mediated ice formation across natural environments such as the Earth's mesosphere, protoplanetary disks, and molecular clouds.