Angew. Chem. Int. Ed. http://doi.org/f3gw9r (2015)
Water underpins the vast majority of biological and atmospheric phenomena. However, while the properties of bulk water are reasonably well understood, clusters of water molecules, which are usually involved in the onset of such phenomena, are relatively unexplored because they are much harder to study. Michel Farizon and colleagues at the University of Lyon, the University of Grenoble and Leopold Franzens University have now developed a method to measure the speed distribution of water molecules as they evaporate from clusters as small as two to eight molecules.
The researchers carefully selected their clusters based on mass, and then shot an argon atom at them. The energy of the impact gets redistributed among the O–H oscillators of the water cluster, causing one of the molecules to evaporate. A detector then measures the speed and orientation of this leaving molecule and meaningful statistics are obtained by repeating the experiment multiple times.
Farizon and colleagues observe that the velocity distribution of evaporated water molecules is composed of a low- and a high-speed component. The low-speed molecules are expected from a complete redistribution of the impact energy among the vibrational modes of the cluster. The high-speed evaporation (about 30% of cases) occurs before all of the energy has reached a Maxwell–Boltzmann distribution. Such a non-ergodic chemical regime is fundamentally different from that of bulk water or even larger clusters.