AVS 46th International Symposium
    Applied Surface Science Division Friday Sessions
       Session AS-FrM

Paper AS-FrM11
The Use of Field Ionization Methods to Probe the Influence of High Interfacial Electric Fields on Electrochemical Phenomena

Friday, October 29, 1999, 11:40 am, Room 6A

Session: New or Improved Surface Related Analytical Techniques
Presenter: V.K. Medvedev, University of Washington
Authors: V.K. Medvedev, University of Washington
D.L. Scovell, University of Washington
C.J. Rothfuss, University of Washington
E.M. Stuve, University of Washington
Correspondent: Click to Email
One characteristic of the electrode/electrolyte interface is the presence of high electric fields, typically on the order of 1 V/Å. With sharp field emitter tips, sufficiently high electric fields can be generated at the tip surface by application of a few kV bias potential. We have developed a field ionization microscopy and mass spectroscopy system for studying the influence of high electric fields on ionization of water. Water is adsorbed on a platinum field emitter adius 350 Å under both field-free and applied field conditions. Water adlayers ranging in thickness from 0 to 5000 Å have been examined at temperatures ranging from 30-300 K at pressures below 10@super -6@ torr. Water ionization was detected by time-of-flight and Wien filter (ExB) mass spectroscopies and imaged on a phosphor screen. Experimental and numerical results to date show that: (1) fields of only 0.2-0.5 V/Å can increase the ionic content of the water layer by several orders of magnitude; (2) the ions formed are hydrated by as many as 10 water molecules; (3) the onset of water ionization on a clean Pt tip increases linearly with temperature over the range of 170-300 K; and (4) ions formed at the tip/water interface must diffuse through the water layer prior to detection. The distribution (n) of (H@sub 2@O)@sub n@H@super +@ clusters is a strong function of ionization conditions and provides information on the nature of surface diffusion as a function of temperature. The diffusional barrier for ion transport through the water layer appears to be a function of applied electric field and the nature (amorphous vs. crystalline) of the water layer.