Electric field effects in ionization of water-ice layers on platinum

Field ionization of water-ice adsorbed onto a platinum field emitter tip of radius 350 Angstrom was studied as a function of temperature over the rang e of temperature 80-145 K and of water layer thickness 100-3000 Angstrom. T he water adlayer was grown under field-free conditions by exposure to water vapor in ultrahigh vacuum. Field ionization was probed by ramped field des orption (RFD), in which desorption of ionic species (hydrated protons) is m easured while increasing the applied electric field linearly in time. The d ependence of the field required for onset of ionization as a function tempe rature and thickness is presented and discussed. In the limit of thin water layers, the onset field of ionization decreased from 0.6 to 0.3 V/Angstrom with temperature increasing from 80 to 145 K. An activation barrier of 0.7 5 eV for ionization of water to produce hydrated protons and hydroxide ions was estimated from the temperature dependence of the onset field. The onse t field increased with water layer thickness, and a break point in the slop e of onset field versus thickness was interpreted as a transition in the io nization location from the water-vacuum interface to the tip-water interfac e. The relevance of these experiments in simulating electrode-electrolyte i nterfaces is discussed.