Proton transport in ice at 30-140 K: Effects of porosity.

We examined the role of porosity, a crucial characteristic of amorphous solid water (ASW), on electrostatic charging and discharging of ASW films with 500 eV He(+) and Xe(+) ions, by measuring the surface potentials with a Kelvin probe. When a charged ASW film is heated, its surface potential decreases sharply, at temperatures that depend on the maximum temperature the film was once subject to. This sharp decrease of the surface potential is not due to a large thermally induced increase of the dielectric constant ε as proposed in other studies, since measurements of ε yielded a value of ∼3 below ∼100 K. Rather, the potential drop can be explained by the transport of the surface charge to the substrate, which depends on film porosity. We propose that the charge migrates along the walls of the pores within the ASW film, facilitated by the thermally induced reorientation of the incompletely coordinated molecules on the pore walls.