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 A was studied as a function of temperature over the range of 80-145 K and water layer thickness from 100 to 3000 A. The water adlayer was grown under field-free conditions by exposure to water vapor in ultrahigh vacuum. Field ionization was probed by ramped field desorption RFD in which desorption of ionic species hydrated protons is measured while increasing the applied electric field linearly in time. The dependence of the field required for onset of ionization as a function temperature 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 VA with temperature increasing from 80 to 145 K. An activation barrier of 0.75 eV for ionization of water to produce hydrated protons and hydroxide ions was estimated from the temperature dependence of the onset field. The onset field increased with water layer thickness due to dielectric screening by water in qualitative agreement with the predictions of a previous model based on a spherical tipwater layer geometry and a variable permittivity of water. A breakpoint in the slope of onset field vs. thickness was interpreted as a transition in the ionization location from the water-vacuum interface to the tip-water interface. The relevance of these experiments in simulating electrodeelectrolyte interfaces is discussed.