Phase transitions in vapor-deposited water under the influence of high surface electric fields

Field ionization of vapor-deposited water on a platinum field emitter tip was studied over a temperature range of 103-150 K. Water adlayers 50-3600 Å thick were grown under field-free conditions by exposure of a cryogenically cooled emitter tip to water vapor in ultrahigh vacuum. Field ionization was probed by ramped field desorption, 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 ionization onset as a function of temperature and water thickness is presented and discussed. Experiments were performed for equal deposition and desorption temperatures over the range of 105 to 150 K and also for a mixed temperature case (145 K deposition, 110 K desorption) to investigate the nature of the deposited layer on ionization properties. The onset field for ionization increased linearly with adlayer thickness, but with different slopes for thin and thick layers of water. The thin layers were more sensitive to the deposition conditions of the adlayer than were the thick layers, with the difference attributed to different ionization locations in the two cases: at the exposed water-vacuum interface for thin layers and at the buried tip-water interface for thick layers. In the limit of zero adlayer thickness the onset field decreased from 0.5 to 0.2 V Å ^-1 as temperature increased from 105 to 150 K. An activation barrier of 0.7 eV (16 kcal mol ^-1) for ionization of amorphous and crystalline water was estimated from the temperature dependence of the onset field. This is in excellent agreement with the 0.74 eV (17 kcal mol ^-1) required to produce a pair of ions from a pair of solvated water molecules.