Formation probability of metastable molecular hydrogen anions HmDn− (m, n = 0-3 and m + n = 2, 3) in sputtering

- Formation of molecular hydrogen anions (H2−, D2−, HD−, H3−, HD2−, and D3−).
- Intensity ratios of molecular ions lower than expected statistical isotopic pattern.
- Strong depletion of the heavier D isotope in the sputtered ion flux by (47.0 ± 3.1)%.

The formation of molecular hydrogen anions (H2−, D2−, HD−, H3−, HD2−, and D3−) was investigated. These anion species were produced by sputtering of TiD2 and HfH2 targets with Cs+ ions and were identified by secondary-ion mass spectrometry (SIMS), yielding the corresponding abundance distributions. The intensity ratios of both the diatomic (HD−/D2−) and triatomic (HD2−/D3−) molecular ions are (distinctly) lower than what would be expected from the respective statistical isotopic pattern. This observation indicates that the different lifetimes of these metastable anions are decisive for their formation probability in sputtering and their survival in the mass spectrometer. In addition to molecular ions, the isotopic fractionation of the atomic hydrogen ions, D−/H−, was determined. A strong depletion of the heavier D isotope in the sputtered ion flux by (47.0 ± 3.1)% was found. This pronounced isotope fractionation could be due to a velocity-dependence of the ionization probability, amplified by the large mass difference of the hydrogen isotopes.

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