Imaging atom-probe analysis of a vitreous ice interface

The imaging atom-probe (IAP) mass spectrometer has been used to examine the interface between a metal surface and aqueous solutions of potassium chloride. The interface is preserved for IAP analysis by rapidly freezing a thin layer of the aqueous solution on the apex of a field-emitter tip. Cooling rates in excess of 10 ⁶ K/s can be achieved by plunging the liquid-coated tip into liquid propane in an inert argon atmosphere. A novel anode assembly allows the frozen interface to be transferred into the IAP without contamination and maintained at 80 K until field-desorption exposes the interface for analysis. The desorption process occurs at a field strength of ~ 10 V/nm, and removes the entire layer of ice from the tip apex as though it were an insulating film. Spatially resolved images of H ₂O ⁺ and K ⁺ (the dominant ions resulting from the desorption process) show no order on a nanometer scale. Desorption images suggest that the frozen layer exists on the tip surface in the vitreous state. No evidence for hexagonal epitaxy of the liquid layer prior to the freezing event has been observed on tungsten or gold surfaces.