Sputtered molecular fluoride anions MFn- (M = B, Si, P, K, Ti, In, W, Pb)

The emission of negatively charged ions from different fluoride samples (Li2SiF6, KBF4, KPF6, K2TiF6, PbF2, and W–PbF2) 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. A distinct flux of molecular anions of the type MFn- (n being the number of F atoms and M = B, Si, P, K, Ti, In, W, or Pb) was observed and their abundance distributions (the relative intensities as a function of n) were determined. They exhibit a considerable variation, depending on M and n. For some of those molecules, the distributions exhibit a correlation with the valence v of the respective M, in a way such that anions with n = v + 1 show (very) high intensities, whereas others have much lower yields or could not be identified unambiguously. Typically, those high yields correspond to a high electron affinity of the respective molecules. From these data, it is concluded that the electron affinities of those molecular anions have a decisive influence on their emission probability. This conjecture is also corroborated by the observed exponential scaling of the WFn- anion intensities with the corresponding electron affinities. In addition, the formation and stability of the molecule may be affected by the atomic composition at the sputtering site and their possible fragmentation due to energetic excitations in the sputtering event.