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 w as studied over a temperature range of 103-150 K. Water adlayers 50-3600 An gstrom thick were grown under field-free conditions by exposure of a cryoge nically cooled emitter tip to water vapor in ultrahigh vacuum. Field ioniza tion was probed by ramped field desorption, in which desorption of ionic sp ecies (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 di scussed. Experiments were performed for equal deposition and desorption tem peratures over the range of 105 to 150 K and also for a mixed temperature c ase (145 K deposition, 110 K desorption) to investigate the nature of the d eposited layer on ionization properties. The onset field for ionization inc reased linearly with adlayer thickness, but with different slopes for thin and thick layers of water. The thin layers were more sensitive to the depos ition conditions of the adlayer than were the thick layers, with the differ ence 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 onse t field decreased from 0.5 to 0.2 V Angstrom(-1) as temperature increased f rom 105 to 150 K. An activation barrier of 0.7 eV (16 kcal mol(-1)) for ion ization of amorphous and crystalline water was estimated from the temperatu re 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. (C) 2000 Published by Elsevier Science B.V. All r ights reserved.