ADSORPTION OF H2O ON PT FIELD EMITTER - SURFACE-DIFFUSION AND FIELD-INDUCED EFFECTS

Adsorption of an H2O layer onto a Pt field emitter tip under the influence of an electric field and the surface diffusion of water were studied by using the field electron microscopy method. The presence of a negative field (field electron microscopy mode of operation), examined in the range of 33-44 MV/cm, significantly reduced the water coverage on the emitter at temperatures above 120 K. The reduction could also occur upon drawing a high density field emission current when the emitter was kept at 78 It. Surface diffusion of water, which was observed in the temperature range 120-132 K, corresponded to the ''unrolling the carpet'' mechanism and started from a water multilayer to a surface region water- and/or hydrogen-submonolayer covered. This was accompanied by the transition process from the state of the current- and field-induced redistribution of H2O to the state of thermal equilibrium. The activation energy of the diffusion was found to be 19 and 25 kJ/mol depending on the crystallographic direction. A positive electric field of 44 MV/cm, which was applied at temperatures of the substrate in excess of 121 It, decreased the field emission of the system and raised the desorption temperature of the layer over 720 It. It is assumed that the negative electric field causes reorientation of H2O molecules at the surface of platinum tip. The positive as well as negative electric fields promote the field desorption of water, which is in accordance with the results reported before.