AVS 47th International Symposium
    Surface Science Tuesday Sessions
       Session SS3-TuA

Paper SS3-TuA5
Water Ion Cluster Formation in High Electric Fields at and Near a Pt Emitter Tip

Tuesday, October 3, 2000, 3:20 pm, Room 210

Session: Water/Surface Interactions
Presenter: C.J. Rothfuss, University of Washington
Authors: C.J. Rothfuss, University of Washington
V.K. Medvedev, University of Washington
E.M. Stuve, University of Washington
Correspondent: Click to Email
Water ion cluster formation in high electric fields has been characterized on a field cleaned Pt emitter tip over temperatures ranging from 110K to 300K. The ion clusters were mass resolved using either an ExB filter or time of flight mass spectroscopy, or imaged directly using Field Ion Microscopy (FIM). Experimental results explain the qualitative trends observed in previous studies of this nature. For H@super +@(H@sub 2@O)@sub n@ cluster formation, the onset potential was found to be dependent upon the value of n and is lower for large masses. This dependency follows from a consideration of the thermodynamics of dissociative ion formation and proton solvation by nearby water molecules. However, the formation of large clusters is entropically unfavorable at high temperatures for low concentrations of water molecules on the surface and dynamically unfavorable at low temperatures. As a result high local concentrations and surface mobility are needed for creation of large n clusters. This local concentration has been experimentally imaged on the surface using FIM, under conditions favoring large n cluster formation, but is absent in the direct field ionization of water (H@sub 2@O@super +@) in space near the surface. With increasing field, each successive (n-->n-1) cluster becomes energetically accessible and becomes the dominant species due to its kinetic advantage. Above the condensation point for water on Pt (~165K), the limiting factor for determining the maximum n in field adsorbed cluster formation is the surface residence time of the water. As temperature increases, the residence time decreases and the propensity for forming large n clusters diminishes. Activation energies for ion cluster desorption were found to be 0.85 eV, 0.76 eV and 0.55 eV for n=3, 4 and 5, respectively. Below the condensation point, surface diffusion limits the supply function for large cluster formation, so lower temperatures result in decreased ion signal for large n clusters.