Dissociation of water molecules with the scanning tunnelling microscope

We have bombarded amorphous water clusters adsorbed on Cu(1 1 1) with electrons from the tip of a low-temperature scanning tunnelling microscope. Dissociation of the water molecules is induced by reaching an anti-bonding orbital at UD=(1.6±0.5) V. For voltages below UD and above UO≈3 V our measurements indicate crystallisation of the molecular clusters and oxidation of the underlying substrate, respectively.