Appearance Potentials of Field Ions

Abstract

The appearance potentials (AP) of various ions, which are produced by field ionization at platinum tip emitters have been measured by a retarding potential energy analyzer coupled to the output of a single-focusing magnetic mass spectrometer. Adsorption layers of water and organic molecules on the emitter surface were found to have no significant influence on the AP measurements. The APs of the H₃O⁺ and (H₂O)₂H⁺ ions in the water spectrum and the CH₃OH₂ ⁺ and (CH₃OH)₂H⁺ ions in the methanol spectrum suggest that the emitter surface is involved in the protonation reactions. The APs of the other ions in the water and methanol spectra have also been studied. The APs of the molecular ions of the paraffins allow conclusions to be drawn about the energy deposition function for field ionization. The fragment ions of n-heptane were found to have surprisingly low APs, up to 2.4-eV less than the molecular ion. The AP of the CH₃ ⁺ ion in the methane spectrum indicates that it arises from a surface reaction.