Sputtered molecular fluoride anions: HfF_n⁻ and WF_n⁻

Abstract

The emission of negatively charged HfF n − (n ≤ 5) and WF n − (n ≤ 7) molecular ions sputtered from a mixed Hf-W-PbF 2 fluoride sample by a 13 keV Cs + ion beam was studied. The emitted ions were detected in a high-sensitivity double-focusing secondary ion mass spectrometer. In the HfF n − and WF n − series, HfF 5 − and WF 6 − are the anions formed most abundantly, with their ratio amounting to HfF 5 − /WF 6 − ∼6, whereas HfF 5 − /WF 5 − ∼18. In particular, the rather high yield of the HfF 5 − anion could be essential for the sensitive detection of live 182 Hf, a radionuclide (half-live ∼9 million years) that might originate from stellar events in the vicinity of the Earth. Generally, these high ion yields appear to correspond with a high electron affinity of the respective molecule. In addition, their formation may be affected by the atomic composition at the sputtering site. Because their constituents originate from (initially) separated sources (the individual Hf, W and PbF 2 particles), the production of the HfF n − and WF n − molecular anions is envisaged to require an extensive mixing between these different reservoirs during sputtering. Furthermore, the energy distributions of these molecular anion species indicate that they are released from the surface via a collision cascade that also leads to the occurrence of fragmentation processes in the emission event.

On‐line HfF5−/WF5− separation in an O2‐filled radiofrequency quadrupole gas cell

“…The O 2 results obtained in this ISAMS work provide an on‐line isobar separation tool for the AMS measurement of 182 HfF 5 − , additional to the natural depletion of WF 5 − observed when the fluorinating agent PbF 2 is not used . However, the reaction mechanisms (the differences among O 2 , NO 2 , N 2 O, and possibly O 3 ), and the rate of reactions as a function of ion energy, must be studied further.…”

“…[7,10,17] However, the reaction mechanisms (the differences among O 2 , NO 2 , N 2 O, and possibly O 3 ), and the rate of reactions as a function of ion energy, must be studied further. The design of the isobar separator system also needs to be improved towards these challenging goals so that it will be able to: (1) transport and retard full beams of rare analyte anions, with normal extraction energies from Cs + sputter ion sources and with no arbitrary phase space limitation; (2) accept all retarded anions and deliver them with a small average energy and narrow energy range; and (3) finely control the anion energy throughout the gas cell so that the average ion energy could be maintained, within a narrow range, just above the threshold for the intended reactions.…”

“…It illustrates the performance of the optical matching in DC retardation, the capture of the retarded m/z 275 anions by the receiving RFQ at vacuum, and the prepared ion energy probability distribution (crossing the marked plane) prior to the onset of gas collisions. Despite the fact that the original beam is assumed to start with a single energy (close to the measured energy spectrum), an energy spread is produced inside the RFQ. Gerlich showed that this energy spread is fundamental to the radial control of ions by the RFQ as the gradient of the RF field drives ions back and forth laterally, which introduces variations in their instant kinetic energies, even for initially mono‐energetic ions.…”

RFQ Reaction Cells for AMS: Developments and Applications

“…Secondly, even for a constant ion source acceleration voltage, there is considerable variation in the energy (and hence axial momentum) of the emerging ions and the distribution of this energy shows quite different patterns for atomic in comparison with molecular anions. An example of this distribution for 12 C -ions and their multimers is given in Figure 3 of ref [24] and for F -, HF 5 -and WF 6 -, in Figure 3 of ref [25]. In both these cases, the atomic anions have a large amplitude high energy tail (~0.001 of the peak maximum at 100 eV above the central ion energy), but little low energy tail; the molecular anions show almost no high energy tail, but a smaller but significant low energy tail (~10-4 of the peak maximum at 100 eV below the central ion energy).…”

Generation of gas‐phase zirconium fluoroanions by electrospray of an ionic liquid

“…In an additional application, Hf and W fluoroanions were generated from the PbF 2 matrix. Since HfF 5 − and WF 5 − were both formed, there was still a problem with overlap of isobaric isotopic ions, which cause interference problems for the measurement of extreme isotope ratios by AMS . However, it was found that the tungsten species could be attenuated by reaction with O 2 or NO 2 , enabling measurement of the trace isotopes of Hf.…”