The Interaction of Molecules with Field Emitter Surfaces Studied by Field Ion Appearance Spectroscopy

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Abstract
On rhodium and tungsten field emitters the integral ion energy distribution is measured. The high energy onset of distribution curves is correlated to critical energy deficits with respect to the emitter potential. For gas phase field ionization of noble gases, H2, NH3, H2O the critical energy deficit is related to the ionization potential I of the neutral species without observable temperature or field dependence. Energy deficits of H+ from H2 and NH3, measured at Rh confirm the model of associated pair field ionization. – H3+ is generated by a protonation reaction with a critical energy deficit of 3.0 eV below that of H2. For NO on tungsten a strong layer formation is observed and investigated in respect of formation rates. Energy deficits of protonated noble gases (XeH+, KrH+, ArH+) from a NH3/NH4+ layer are the same as for NH3+. Protonation of noble gases also by water at tungsten and rhodium has no correlation to the heat of formation of XeH+, KrH+, and ArH+. Protonation occurs during a fast (< 10−12 s) proton stripping process. A field dependence of critical energy deficits for NH4+ car be explained by ion image attraction. Protonated water ions (H3O+) yield different critical energy deficits at different metal emitters (Rh and W), due to stronger interaction of water with tungsten. For the hydrazine parent ion N2H4+ at rhodium the increase of the critical energy deficit with increasing field strength is explained by field desorption of a field stabilized adsorption state, with a binding energy of 1.4 eV of N2H4 to the Rh field emitter. The field dependency of critical energy deficits of N2H5+ is similar to NH4+, taking in addition the dipole of the ion into account.

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