Field Induced Ion Molecule Reactions in Adsorbed Layers Studied by Pulsed Field Desorption

Abstract

The formation of ions under field ionization conditions is partly due to ion molecule reactions in adsorbed layers on the emitter surface. In order to study these reactions a pulsed field desorption (PFD) technique was applied, by which ions resulting from these reactions are detected with high relative intensity. The PFD-mass spectra of some small organic molecules are discussed with respect to the principal reaction mechanisms and types of ions formed. It was found that if reactions involving chemical interaction with the surface are left out of consideration only proton transfer reactions are the primary processes. Desorption products resulting from proton transfer reactions are, for example, (M+H)⁺, (2M+H)⁺, (3M+H)⁺, (M+CH₃)⁺ and (2M)⁺ ions in the case of methanol. The formation of complexes preceeding atom rearrangement reactions are suppressed by the high electric field and consequently could not be detected. Secondary reactions of the (M-H)^•-radical lead to the elimination of H^- from molecules and in certain cases to the formation of addition complexes. Field induced reactions were found to occur almost adiabatically, i.e. it is not necessary to supply the ionization potential of the proton donor in a proton transfer reaction. Products of reactions with ionization energies above the ionization potential of the molecules involved in the reaction are not observable. To a first approximation the electron transition probability for field reactions seems to be dependent on the energy of formation of the ions and not on the type of reaction.