Isotope, molecular and surface effects on hyperthermal surface induced dissociated ionization

Abstract

Organic molecules acquired with hyperthermal (1–20 eV) kinetic energy undergo efficient surface ionization. This hyperthermal surface ionization (HSI) may produce both positive and negative ions. The dissociative ionization of alkyl halides results in the production of negative ions of the functional group having high electron affinity, and positive ions of the alkyl radical, which can further dissociate into smaller fragments. The effect of the alkyl chain lenght and the bromine isotope effect on the obtained HSI mass spectra were studied in several alkyl halide molecules. While the negative ion formation yield is found to be independent of the size of the alkyl radical, the positive ion formation yield strongly increases with the size of the C n H 2n + i radical for n = 1–4 and then a quasi saturation is observed. The observed radical fragmentation increases with the incident molecular kinetic energy and was affected by the molecular structure and the surface temperature and cleanliness. A considerable (up to 24%) heavy brominem isotope increased ionization is observed at intermediate molecular kinetic energies. Piperidine HSI on a rhenium filament exhibits a single (M - 1) ion while its HSI from an oxidized rhenium filament having a much higher work function is characterized by a much richer fragmentation pattern. The dissociative ionization mechanism is rationalized in terms of a surface—molecule electron transfer followed by an immediate dissociation into a negative halogen ion and an alkyl radical. This radical, which usually has a low ionization potential, can transfer an electron to the surface and scatter as a positive ion. At high kinetic energies, the radicals or positive ions can further dissociate near the surface, and then scatter away as lower mass ions with ion yield which depends on their surface reneutralization probabilities. Thus, the observed fragmentation pattern is governed by surface chemicl and electron transfer processe and not by gas phase unimolecular ion dissociation, as found with large polyatomic molecules.