Evaluation of relative electron ionization cross-sections for some oxides and oxyacid salts.

Rationale: The total or relative cross-sections for the ionization of polyatomic molecules by electron ionization are essential data in a wide range of applications. In addition to the total ionization cross-sections, relative ionization cross-sections are sometimes also required. Accurate values of electron ionization cross-sections of high-temperature vapor species are of importance in mass spectrometric investigations. So the need for experimental ionization cross-section data for high-temperature vapor species is vital.

Methods: Measurements were performed by high-temperature Knudsen effusion mass spectrometry with a MS-1301 mass spectrometer. Vaporization was carried out using molybdenum or tungsten effusion cells containing samples of pure Au and CeO . The vapor compositions over the CeO -Mo and CeO -W systems were determined by Knudsen effusion mass spectrometry. The method of ion currents comparison was used to measure partial pressures. The enthalpies of the reactions under study were calculated using the third law procedure.

Results: The standard formation enthalpies of cerium molybdates and tungstates were determined from the relative ionization cross-sections of molybdenum, tungstate, cerium, and cerium salts. Cross-sections were calculated by different methods: simple adding atomic cross-sections with correction factor, using known ratios of complex ions, adding oxide cross sections. The traditional approach to determination of ionization cross-sections exploiting the values of atomic cross-sections and the additivity rules as well as literature experimental data was used.

Conclusions: The enthalpies of formation of gaseous cerium molybdates and tungstates were evaluated by measurements of the reaction enthalpies for several independent reactions. The best agreement was obtained with the data from the analysis of experimentally determined relative ionization cross-sections, and using ionization cross-sections of polyatomic molecules recommended by Drowart et al. The widely used additivity method, in spite of the additional corrections introduced, gives the worst convergence of the values under consideration.