ISOTOPE, MOLECULAR AND SURFACE EFFECTS ON HYPERTHERMAL SURFACE-INDUCED DISSOCIATIVE IONIZATION

Organic molecules acquired with hyperthermal (1-20eV) kinetic energy u ndergo efficient surface ionization. This hyperthermal surface ionizat ion (HSI) may produce both positive and negative fragment ions. The di ssociative ionization of alkyl halides results in the production of ne gative 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 length and the bromi ne isotope effect on the obtained HSI mass spectra were studied in sev eral alkyl halide molecules. While the negative ion formation yield is found to be independent of the size of the alkyl radical, the positiv e ion formation yield strongly increases with the size of the CnH2n 1 radical for n = 1-4 and then a quasi saturation is observed. The obs erved radical fragmentation increases with the incident molecular kine tic energy and was affected by the molecular structure and the surface temperature and cleanliness. A considerable (up to 24%) heavy bromine isotope increased ionization is observed at intermediate molecular ki netic 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 fragmentat ion 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 r adical, 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 t he surface, and then scatter away as lower mass ions with ion yield wh ich depends on their surface reneutralization probabilities. Thus, the observed fragmentation pattern is governed by surface chemical and el ectron transfer processes and not by gas phase unimolecular ion dissoc iation, as found with large polyatomic molecules.