Supercritical water provides distinctly different solvation
properties compared to what is known from liquid water.
Despite its prevalence deep in the Earth’s crust and its role
in chemosynthetic ecosystems in the vicinity of hydrothermal vents,
molecular insights into its solvation mechanisms
are still very scarce compared to what is known for liquid
water. Recently, neutral metal particles have been detected
in hydrothermal fluids and proposed to explain the transport of
gold species to ore deposits on Earth. Using ab initio
molecular dynamics, we elucidate the solvation properties of
small gold species at supercritical conditions. The neutral
metal clusters themselves contribute enormous THz intensity
not because of their intramolecular vibrations, but due
to their pronounced electronic polarization coupling to the
dynamical supercritical solvent, leading to a continuum
absorption up to about 1000 cm‐1 . On top, long‐lived
interactions between the gold clusters and solvation water leads
at these supercritical conditions to a sharp THz resonance
that happens to be close to the one due to H‐bonding in
liquid water at ambient conditions. The resulting distinct
resonances can be used to analyse the solvation properties
of neutral metal particles in supercritical aqueous solutions.