The electronic structure of and

Spontaneous and photo-induced decay processes of WF5 - and HfF5 - molecular anions in a cryogenic storage ring

Spontaneous and photo-induced decay processes of HfF₅ ^- and WF₅ ^- molecular anions were investigated in the Double ElectroStatic Ion Ring ExpEriment (DESIREE). The observation of these reactions over long time scales (several tens of ms) was possible due to the cryogenic temperatures (13 K) and the extremely low residual gas pressure (∼10^-14 mbar) of DESIREE. For photo-induced reactions, laser wavelengths in the range 240 to 450 nm were employed. Both anion species were found to undergo spontaneous decay via electron detachment or fragmentation. After some ms, radiative cooling processes were observed to lower the probability for further decay through these processes. Photo-induced reactions indicate the existence of an energy threshold for WF₅ ^- anions at about 3.5 eV, above which the neutralization yield increases strongly. By contrast, HfF₅ ^- ions exhibit essentially no enhanced production of neutrals upon photon interaction, even for the highest photon energy used in this experiment (∼5.2 eV). This suppression will be highly beneficial for the efficient detection, in accelerator mass spectrometry, of the extremely rare isotope ¹⁸²Hf using the ¹⁸²HfF₅ ^- anion while effectively reducing the interfering stable isobar ¹⁸²W in the analyte ion ¹⁸²WF₅ ^-. The radionuclide ¹⁸²Hf is of great relevance in astrophysical environments as it constitutes a potential candidate to study the events of nucleosynthesis that may have taken place in the vicinity of the solar system several million years ago.

Stability Evaluation of Candidate Precursors for Chemical Vapor Deposition of Hafnium Diboride (HfB2)

Alternative candidate precursors to [Hf(BH₄)₄] for low-temperature chemical vapor deposition of hafnium diboride (HfB₂) films were identified using density functional theory simulations of molecules with the composition [Hf(BH₄)₂L₂], where L = -OH, -OMe, -O-t-Bu, -NH₂, -N=C=O, -N(Me)₂, and -N(CH₂)₅NH₂ (1-piperidin-2-amine referred to as Pip2A). Disassociation energies (E _D), potential energy surface (PES) scans, ionization potentials, and electron affinities were all calculated to identify the strength of the Hf-L bond and the potential reactivity of the candidate precursor. Ultimately, the low E _D (2.07 eV) of the BH₄ ligand removal from the Hf atom in [Hf(BH₄)₄] was partially attributed to an intermediate state where [Hf(BH₄)₃(H)] and BH₃ is formed. Of the candidate precursors investigated, three exhibited a similar mechanism, but only -Pip2A had a PES scan that indicated binding competitive with [Hf(BH₄)₄], making it a viable candidate for further study.

The quest for AMS of 182Hf – why poor gas gives pure beams

The long-lived radioisotope 182Hf (T1/2 = 8.9 Ma) is of high astrophysical interest as its potential abundance in environmental archives would provide insight into recent r-process nucleosynthesis in the vicinity of our solar system. Despite substantial efforts, it could not be measured at natural abundances with conventional AMS so far due to strong isobaric interference from stable 182W. Equally important is an increase in ion source efficiency for the anions of interest.

The new Ion Laser InterAction Mass Spectrometry (ILIAMS) technique at VERA tackles the problem of elemental selectivity in AMS with a novel approach. It achieves near-complete suppression of isobar contaminants via selective laser photodetachment of decelerated anion beams in a gas-filled radio-frequency quadrupole (RFQ) ion cooler. The technique exploits differences in electron affinities (EA) within elemental or molecular isobaric systems neutralizing anions with EAs smaller than the photon energy. Alternatively, these differences in EA can also facilitate anion separation via chemical reactions with the buffer gas.

We present first results with this approach on AMS-detection of 182Hf. With He +O2 mixtures as buffer gas in the RFQ, suppression of 182WF5− vs 180HfF 5− by >105 has been demonstrated. Mass analysis of the ejected anion beam identified the formation of oxyfluorides as an important reaction channel. The overall Hf-detection efficiency at VERA presently is 1.4% and the W-corrected blank value is 182Hf/180Hf = (3.4 ± 2.1)×10−14. In addition, a survey of different sample materials for highest negative ion yields of HfF 5− with Cs-sputtering has been conducted.

On-line HfF5(-)/WF5(-) separation in an O2-filled radiofrequency quadrupole gas cell

RATIONALE

An experimental Isobar Separator for Accelerator Mass Spectrometry (ISAMS) instrument has been used to demonstrate an on-line separation of HfF5(-) from its isobar WF5(-). This is necessary, in addition to sample preparation chemistry, for measuring (182)Hf at natural levels by Accelerator Mass Spectrometry (AMS).

METHODS

The device utilizes a radiofrequency quadrupole (RFQ) controlled gas cell, wherein anion-gas reactions at eV energies attenuate the interfering isobars of the analyte molecular anions, leaving HfF5(-) for AMS analysis. The RFQ also helps to control the multiple scattering resulting from the ion-gas collisions.

RESULTS

O2 gas was used in the HfF5(-)/WF5(-) separation and WF5(-) was attenuated by nearly 3 orders of magnitude while maintaining ~75% transmission of HfF5(-). It is expected that the transmission and attenuation can be increased by further research.

CONCLUSIONS

This result advances the possibility of detecting natural (182)Hf when AMS is supplemented with an isobar separator in the injection system.