Near-Threshold Photodetachment of WF₅ ^- Molecular Anions in a Cryogenic Storage Ring

The molecular anion WF₅ ^- constitutes an isobaric interference for the mass spectrometric detection of the HfF₅ ^- anion. The latter is expected to form an extremely sensitive way of detecting the radionuclide ¹⁸²Hf at natural levels using, for example, accelerator mass spectrometry.

Photoinduced detachment of WF₅ ^- molecular anions was studied in the Double ElectroStatic Ion Ring ExpEriment (DESIREE), with the ring cooled to cryogenic temperatures (~13 K). Photon energies from 2.5 to 4.5 eV were applied using an OPO laser system for excitation. This energy range covers the threshold for the removal of an electron from that anion species.

Photoinduced reactions were observed which indicate the existence of an energy threshold for WF₅ ^- anions above which the neutralization yield increases strongly. In that range, the data of the neutral signal could be fitted by a linear function of photon energy E, yielding a threshold energy E_th = 3.13 ± 0.01 eV. This result constitutes a substantial improvement both in the range and the resolution of the photon energy as compared to previous data and provides now a rather precise value for that threshold.

This knowledge is required for any attempts to suppress WF₅ ^- anions by photodetachment in the ultrasensitive detection of the very rare radioisotope ¹⁸²Hf using the ¹⁸²HfF₅ ^- analyte anion; for that, the ¹⁸²WF₅ ^- anion would be a stable isobaric mass interference for analyses by accelerator mass spectrometry. The radionuclide ¹⁸²Hf is considered of great interest in various astrophysical environments such as nucleosynthetic processes.