The feasibility of using laser photodetachment as a means for isobar suppression in accelerator mass spectrometry has been investigated for the special case of HfF5-/WF5-. A method for absolute photodetachrnent cross section measurements was applied and the cross sections of tungsten pentafluoride and hafnium pentafluoride negative ions were measured. The measurements indicate that the photodetachment cross section for WF5- is at least 100 times larger than for HfF5- at the wavelength of the fourth harmonic of the Nd:YAG laser at 266 nm. The absolute cross section for WF5- at this photon energy was found to be (2.8 +/- 0.3) x 10(-18) cm(2), while an upper limit of 2 x 10(-2) cm(2) was obtained for the HfF5- cross section. The measured cross sections indicate that an optical filtering scheme for isobar suppression in accelerator mass spectrometry for the case of Hf-182 should be feasible. (C) 2013 Elsevier B.V. All rights reserved.

To unravel the mechanisms of ion formation in the Middleton type cesium sputter negative ion source, we have acquired and analyzed the spectrum of the light emitted from the source region during sputtering of three different cathode materials; carbon, aluminum and copper. All cathodes were analyzed under the same source settings but with different total current output. The cathode region was observed from a distance of 3m and the emitted light was analyzed by a fiber coupled spectrometer with a wavelength range from 350 to 1100nm. The spectra showed clear differences depending on the cathode material. The emitted light from the carbon cathode consisted almost entirely of lines from neutral cesium and no lines from ionized cesium. Both metallic cathodes additionally displayed several lines originating from positively ionized cesium. A weak, broad feature between 470 and 510nm was also seen in the carbon spectrum. This feature was not present in spectra from the two metallic cathodes. These results may suggest different mechanisms of negative ion formation in the ion source depending on the cathode material. As a side effect of the measurement it proved relatively easy to determine the source temperature, in the range 1020–1215K depending on source settings, to an accuracy of ±30K from the black body radiation.

We describe a microfluidic device for studying the orientational dynamics of microrods. The device enables us to experimentally investigate the tumbling of microrods immersed in the shear flow in a microfluidic channel with a depth of 400 mu m and a width of 2.5 mm. The orientational dynamics was recorded using a 20X microscopic objective and a CCD camera. The microrods were produced by shearing microdroplets of photocurable epoxy resin. We show different examples of empirically observed tumbling. On the one hand we find that short stretches of the experimentally determined time series are well described by fits to solutions of Jeffery's approximate equation of motion [Jeffery, Proc. R. Soc. London. 102 (1922), 161-179]. On the other hand we find that the empirically observed trajectories drift between different solutions of Jeffery's equation. We discuss possible causes of this orbit drift.

Ion beam purity is of crucial importance to many basic and applied studies in nuclear science. Selective photodetachment has been proposed to suppress unwanted species in negative ion beams while preserving the intensity of the species of interest. A highly efficient technique based on photodetachment in a gas-filled radio frequency quadrupole ion cooler has been demonstrated. In off-line experiments with stable ions, up to 104 times suppression of the isobar contaminants in a number of interesting radioactive negative ion beams has been demonstrated. For selected species, this technique promises new experimental possibilities in studies on exotic nuclei, accelerator mass spectrometry, and fundamental properties of negative atomic and molecular ions.

An apparatus for photodetachment studies on atomic and molecular negative ions of medium up to heavy mass (M ≃ 500) has been designed and constructed. Laser and ion beams are merged in the apparatus in a collinear geometry and atoms, neutral molecules and negative ions are detected in the forward direction. The ion optical design and the components used to optimize the mass resolution and the transmission through the extended field-free interaction region are described. A 90° sector field magnet with 50 cm bending radius in combination with two slits is used for mass dispersion providing a resolution of M∕ΔM≅800 for molecular ions and M∕ΔM≅400 for atomic ions. The difference in mass resolution for atomic and molecular ions is attributed to different energy distributions of the sputtered ions. With 1 mm slits, transmission from the source through the interaction region to the final ion detector was determined to be about 0.14%.

The efficiency of selective suppression of negative ions by photodetachment in a gas-filled radio frequency quadrupole ion cooler was investigated with a new detection method. A neodymium doped yttrium aluminum garnet laser beam at 1064 nm was used to remove Co− ions in the radio frequency quadrupole cooler and the remaining ions were then probed by photodetachment and neutral particle detection. More than 99.99% suppression of the Co− ions was observed. Under identical conditions, only 20% of a Ni− beam was suppressed. The results demonstrate that this isobar suppression technique can lead to nearly complete elimination of certain isobaric contaminants in negative ion beams, opening up new experimental possibilities in nuclear and atomic research and accelerator mass spectrometry.

The electron affinity of tungsten has been measured using laser photodetachment threshold spectroscopy in a collinear geometry. The electron affinity was determined to 6583.6(6) cm-1 by observing the onset of the process when W- ions in the \(5d^56s^2\) 6S5/2 ground state are photodetached producing neutral W atoms in the \(5d^46s^2\) 5D0 ground state. The measured value is in agreement with previous measurements and improves the accuracy by almost two orders of magnitude. Further, a photodetachment signal below the ground state photodetachment threshold was found, which indicates the existence of a bound excited state in W-.

We have experimentally investigated the structure of the Pt- ion using laser photodetachment threshold spectroscopy. The experiment was conducted using a collinear laser-ion beam apparatus, in which the residual atoms created in the photodetachment process were detected. A p-wave threshold was observed in the photodetachment spectrum at an energy of 6851(13) cm(-1). We conclude that this onset originates from a photodetachment transition in which the initial state of Pt- is the previously unobserved 5d(10)6s S-2(1/2) state and the final state of Pt is the 5d(9)6s D-3(3) ground state. The excitation energy of the S-2(1/2) state is determined to be 10 289(13) cm(-1). This value can be compared with a multiconfiguration Dirac-Fock calculation performed by Thogersen [Phys. Rev. Lett. 76, 2870 (1996)], which yielded an excitation energy of 11 301 cm(-1). Our data show no indication of the presence of any other state of Pt-. We conclude that the structure of the Pt- ion is now fully known.

An improved measurement of the limit of isobar suppression by photodetachment has been made. A gas-filled rf quadrupole ion guide was used to slow down negative ions for the purpose of acquiring long interaction times with the applied laser light. A Nd:YAG laser beam (1064nm) was used to selectively deplete a beam of Co- ions inside the rf ion guide and the remaining ions was probed using photodetachment by another Nd:YAG laser and a neutral detector. The neutral detection system allowed for a detection limit more than one order of magnitude better than the previous measurements. With this improved detection method, we show that the suppression of Co- ions was at least four orders of magnitude.

We have experimentally investigated the structure of the ${\mathrm{Pt}}^{\ensuremath{-}}$ ion using laser photodetachment threshold spectroscopy. The experiment was conducted using a collinear laser-ion beam apparatus, in which the residual atoms created in the photodetachment process were detected. A $p$-wave threshold was observed in the photodetachment spectrum at an energy of $6851(13)\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. We conclude that this onset originates from a photodetachment transition in which the initial state of ${\mathrm{Pt}}^{\ensuremath{-}}$ is the previously unobserved $5{d}^{10}6s\phantom{\rule{0.2em}{0ex}}^{2}S_{1∕2}$ state and the final state of Pt is the $5{d}^{9}6s\phantom{\rule{0.2em}{0ex}}^{3}D_{3}$ ground state. The excitation energy of the $^{2}S_{1∕2}$ state is determined to be $10\phantom{\rule{0.2em}{0ex}}289(13)\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. This value can be compared with a multiconfiguration Dirac-Fock calculation performed by Th\o{}gersen et al. [Phys. Rev. Lett. 76, 2870 (1996)], which yielded an excitation energy of $11\phantom{\rule{0.2em}{0ex}}301\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. Our data show no indication of the presence of any other state of ${\mathrm{Pt}}^{\ensuremath{-}}$. We conclude that the structure of the ${\mathrm{Pt}}^{\ensuremath{-}}$ ion is now fully known.

We are investigating the possibility to use laser photodetachment of negative ions as an isobaric selective filter in accelerator mass spectrometry (AMS). If successful, this method can be used to obtain higher sensitivity realized through better selectivity by suppression of molecular and/or elemental isobaric interference in different investigations using ultra rare isotopes in the 10(-13) range and below. The radionuclide of particular interest in this work is Hf-182. Detection of this isotope in terrestrial samples would indicate reminiscences from a recent supernova explosion in the vicinity Of OUT solar system. A major problem when detecting the heavy trace element is its separation from the stable isobar W-182, which contributes a strongly interfering background. Currently, this background is suppressed by using suitable molecular ions in the injection stage of the accelerator but does not yet yield sufficient selectivity for detection in the required low 10(-13) range [1].We have performed a first study to investigate techniques for additional suppression of the interfering W-182 isobar using the laser photodetachment process. Test experiments have been carried out in a compact ion beam apparatus using pulsed tunable laser radiation to investigate the relative yield of photodetachment signals between the most promising negative fluoride ion for AMS use, i.e. the Hafnium penta-fluoride (HfF5), and WF5, as the major contaminant. HfF5 exhibits high production yield within the ion source and negligible photodetachment cross section at the energy of frequency tripled output from a Nd:YAG laser. In WF5 the cross section for photodetachment, at the same photon energy, is found to be more than one order of magnitude bigger. This opens up the possibility for removing interfering tungsten molecules in a mass selected ion beam using standard fixed frequency laser equipment.

A merged beams technique has been used to investigate collisions between electrons and $\text{Si}_{2}{}^{\ensuremath{-}}$ ions over a relative kinetic energy range of 0\char21{}210 eV. Absolute cross sections for pure electron detachment, detachment plus dissociation, and dissociation involving atomic and ionic products were measured. The dominant process over the energy range studied is pure electron detachment. A search for a resonance associated with a ${\text{Si}}_{2}$ dianion was made but none was observed.

We are investigating the possibility of using laser photodetachment of negative atomic and molecular ions as an additional isobaric selection filter in accelerator mass spectrometry. The aim of this study is to find a possibility to further improve the detection limit for long-lived heavy radionuclides at AMS facilities. We will focus on the astrophysical relevant radionuclide Hf-182, which is one of the isotopes measured with the 3 MV tandem AMS facility VERA (Vienna Environmental Research Accelerator) at the University of Vienna. Laser-induced isobar suppression is also of importance for radioactive-beam facilities.The present detection limit for measuring the isotope ratio Hf-182/Hf at VERA is 1 x 10(-11). The limiting factor is the strong background of the stable isobar W-182. Currently this background is suppressed using suitable molecular ions in the injection stage. Selective laser photodetachment of the negative ions at the injector call lead to an additional suppression of the interfering isobar. Test experiments have been carried out at the negative ion laser spectroscopy setup at Goteborg University. In a small ion beam apparatus pulsed tunable laser radiation is used to measure the photodetachment cross-section of different atomic and molecular negative ions. We will present studies of the photodetachment process for various tungsten and hafnium molecules with the aim to find a selective isobaric suppression scheme using laser photodetachment spectroscopy in combination with AMS. (c) 2008 Elsevier B.V. All rights reserved.

In this paper, we demonstrate how laser photodetachment mass spectrometry (LPMS) can be used to selectively detect 13C− ions in the presence of 12C− ions in a low energy ion beam. An isotopically enriched beam of carbon ions consisting of equal amounts of 13C− and 12C− ions was extracted from an ion source. The ions interacted with a laser beam in a collinear geometry over a distance of 70cm. Residual atoms produced in the photodetachment process were detected in a neutral particle detector placed downstream of the collinear interaction region. By making use of the Doppler effect we were able to selectively photodetach 13C− ions. The number of detected 13C atoms was 13 times larger than the number of detected 12C atoms. The population of the excited, weakly bound 2D excited state of the C− ion was depleted by the use of a second laser. This significantly reduced the background accompanying the signal arising from the photodetachment of the 4S ground state C− ion. Different applications of the LPMS method will be discussed in the paper.

The negative ion of cerium is investigated using tunable laser photodetachment threshold spectroscopy. The relative cross section for photodetachment from Ce- is measured over the photon energy range 0.61-0.75 eV using a crossed laser-beam-ion-beam technique. The spectrum of neutral atom production reveals a photodetachment threshold at 0.65 eV, which is interpreted as the threshold for the Ce- (4f5d(2)6s(2) H-4(7/2)) to Ce (4f5d6s(2) (1)G(4)) ground-state to ground-state transition yielding the electron affinity of Ce to be 0.65(3) eV. At least five narrow peaks are observed in the cross section over the range 0.62-0.70 eV due to negative ion resonances, and their energies and widths are measured. The results are compared to other recent experimental and theoretical studies of Ce-.

Received 9 November 2007DOI:https://doi.org/10.1103/PhysRevA.76.059903©2007 American Physical Society