Field ionization of thin water layers adsorbed onto a platinum field emitter was investigated by numerical simulation and analysis of experimental data. The numerical simulation, which includes a field-dependent relative permitivity, was developed to predict the field distribution around a water-covered field emitter tip. The model predicts that the dominant field occurs at the water-vacuum interface in thin layers. In the experiments, water adlayers were grown under field-free conditions by exposure of a cryogenically cooled emitter tip to water vapor in ultrahign vacuum. Field ionization was probed by ramped field desorption (RFD) in which desorption of ionic speces (hydrated protons) is measured while increasing the applied electric field linearly in time. The onset field of ionization decreased from 0.3 to 0.2 V/A as temperature increased from 130 to 150 K. An activation barrier of 0.7 eV (16 kcal/mol) for ionization of water to produce hydrated protons and hydroxide ions was estimated from the temperature dependence of the onset field. The experimental trends agree with the predicted trends for thin water layers.
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