Molecular anions sputtered from fluorides

Abstract

The emission of negatively charged ions from different fluoride samples (LiF, CaF 2, LaF 3 and HfF 4) induced by sputtering with a 14.5-keV Cs + ion beam was studied. Sputtered ions were detected in a high-sensitivity double-focusing mass spectrometer. In particular, the possible existence of small doubly charged negative molecular ions was investigated. But whereas singly charged species of the general type MF n − (where M represents a metal atom) were detected with high abundances, stable dianions were observed in an unambiguous way only for one molecule: HfF 6 2−. The flight time through the mass spectrometer of ∼35 μs establishes a lower limit with respect to the intrinsic lifetime of this doubly charged ion. For singly charged anions abundance distributions and, in selected cases, emission-energy spectra were recorded. For two ion species (Ca − and HfF 5 −) isotopic fractionation effects caused by the (velocity dependent) ionization process were determined: for both ions, the lighter isotopes exhibit a higher ionization probability. The magnitude of this fractionation per unit mass difference is 2.32%/amu for Ca − and 0.88%/amu for HfF 5 −.