Immobility of protons in ice from 30 to 190 K

The anomalously fast motion of hydronium ions (H 3 O + ) in water is often attributed to the Grotthuss mechanism 1 , 2 , whereby protons tunnel from one water molecule to the next. This tunnelling is relevant to proton motion through water in restricted geometries, such as in ‘proton wires’ in proteins 3 and in stratospheric ice particles 4 . Transport of hydronium ions in ice is thought to be closely related to its transport in water 1 , 2 . But whereas claims have been made that such tunnelling can persist even at 0 K in ice 5 , 6 , 7 , counter-claims suggest that the activation energy for hydronium motion in ice is non-zero 8 , 9 , 10 . Here we use ‘soft-landing’ 11 , 12 , 13 of hydronium ions on the surface of ice to show that the ions do not seem to move at all at temperatures below 190 K. This implies not only that hydronium motion is an activated process, but also that it does not occur at anything like the rate expected from the Grotthuss mechanism. We also observe the motion of an important kind of defect in ice's hydrogen-bonded structure (the D defect). Extrapolation of our measurements to 0 K indicates that the defect is still mobile at this temperature, in an electric field of 1.6 × 10 8 V m −1 .