Periodic Nature of Metal-Noble Gas Adduct Ions in Glow Discharge Mass Spectrometry

The observation that ZnAr⁺ ion currents in a glow discharge can measure as high as 30% of those of Zn⁺ prompted a systematic study of metal-noble gas diatomic species. Twenty-four elements in combination with neon, argon, and krypton were included. Periodicity of behavior was observed from one row to the next with all three noble gases; periodicity was also observed as the identity of the noble gas was changed. The diatomic noble gas adduct ions of zinc, cadmium, and mercury (group 12) each displayed a concentration relative to the corresponding metal ion that was well above that of other elements in their respective rows. Investigation of the cause of this phenomenon eliminated glow discharge pressure and power conditions. Binding energies of the various species were qualitatively consistent with the observation of relative abundances of metal-noble gas diatomic ions as they varied with the identity of the noble gas, but did not explain why ZnX⁺, CdX⁺, and HgX⁺ form in what seem to be anomalously high abundance. Variations in the sputtering rates of the transition metals (Zn > Cu > Ni > Fe) are consistent with the observation that ZnX⁺ > CuX⁺ > NiX⁺ > FeX⁺, the resulting increase in collision frequency (with increasing sputtering rate) is believed to account for the relative abundances of these adduct ions in the discharge.