Researchers at RIKEN have made a groundbreaking discovery in the field of chemistry by isolating and observing an elusive water structure involving two water molecules, which had been predicted but never observed before. The finding, published in The Journal of Physical Chemistry Letters, could have far-reaching implications for fields such as astrochemistry and the corrosion of metals.

When an energetic particle or photon knocks out an electron from a water molecule, it creates a positive ion (cation; H2O+) and an electron. This ionization of water can trigger a cascade of other reactions with nearby molecules, playing important roles in biological processes, radiation chemistry, and corrosion at interfaces between water and metals.

Calculations have predicted that following the ionization of a water molecule, two isomers of a positively charged ion of a water dimer will form rapidly. One isomer (H3O+·OH) has been observed and is formed when a proton is transferred from one water molecule to another. However, the other isomer, with a half-bond (or hemibonded) structure (H2O·OH2)+, has never been isolated or confirmed by spectroscopic measurements, despite calculations suggesting that it has a higher energy than the proton-transfer dimer.

Now, Susumu Kuma of the RIKEN Atomic, Molecular and Optical Physics Laboratory and his co-workers have successfully isolated both water dimer ions by trapping them in tiny droplets of cold helium. Using infrared spectroscopy, they were able to determine the structures of the isomers, marking a significant advancement in the field.

The researchers achieved this feat by creating an ultracold environment, causing the water molecules in the helium droplets to cool rapidly as helium atoms evaporated from the droplet’s surface. This process led to the formation of the metastable hemibonded isomer due to its very fast stabilization within the cold droplets.

By probing the co-existence of the two isomers using computational and spectroscopic methods, Kuma and his team found that the spectroscopic signatures of the molecular ions were almost identical to those of bare ions, without helium surrounding them. This discovery opens up new possibilities for direct comparisons in measurements on water ions.