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Candidate for Laser Cooling of a Negative Ion: High-Resolution Photoelectron Imaging of
摘要： Laser cooling is a well-established technique for the creation of ensembles of ultracold neutral atoms or positive ions. This ability has opened many exciting new research fields over the past 40 years. However, no negatively charged ions have been directly laser cooled because a cycling transition is very rare in atomic anions. Efforts of more than a decade currently have La? as the most promising candidate. We report on experimental and theoretical studies supporting Th? as a new promising candidate for laser cooling. The measured and calculated electron affinities of Th are, respectively, 4901.35e48T cm?1 and 4832 cm?1, or 0.607 690(60) and 0.599 eV, almost a factor of 2 larger than the previous theoretical value of 0.368 eV. The ground state of Th? is determined to be 6d37s2 4Fe 5=2. The consequence of this there are several strong electric dipole transitions between the bound levels arising from configurations 6d37s2 and 6d27s27p in Th?. The potential laser-cooling transition is 2So 3=2 with a wavelength of 2.6 μm. The zero nuclear spin and hence lack of hyperfine structure in Th? reduces the potential complications in laser cooling as encountered in La?, making Th? a new and exciting candidate for laser cooling.
关键词： Laser cooling,photoelectron imaging,electron affinity,Th?,negative ion
更新于2025-09-12 10:27:22
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Design and preliminary operation of a laser absorption diagnostic for the SPIDER RF source
摘要： The ITER Heating Neutral Beam (HNB) injector is required to deliver 16.7 MW power into the plasma from a neutralised beam of H?/D?, produced by an RF source and accelerated up to 1 MeV. To enhance the H? production, the surface of the acceleration system grid facing the source (the plasma grid) will be coated with Cs because of its low work function. Cs will be routinely evaporated in the source by means of specific ovens. Monitoring the evaporation rate and the distribution of Cs inside the source is fundamental to get the desired performances on the ITER HNB. In order to proper design the source of the ITER HNB and to identify the best operation practices for it, the prototype RF negative ion source SPIDER has been developed and built in the Neutral Beam Test Facility at Consorzio RFX. A Laser Absorption Spectroscopy diagnostic will be installed in SPIDER for a quantitative estimation of Cs density. By using a wavelength tunable laser, the diagnostic will measure the absorption spectrum of the 852 nm line along 4 lines of sight, parallel to the plasma grid surface and close to it. From the absorption spectra the line-integrated density of Cs at ground state will be measured. The design of this diagnostic for SPIDER is presented, with details of the layout and of the key components. A preliminary installation of the diagnostic on the test stand for Cs ovens is also described, together with its first experimental results; the effect of ground state depopulation on collected measurements is discussed and partially corrected.
关键词： Laser absorption spectroscopy,Negative ion source,Heating neutral beam,Caesium
更新于2025-09-11 14:15:04
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Band Structure Effects on the Charge Exchange Processes in H <sup>+</sup> Colliding with a Cu(111) Surface
摘要： The low-energy ion scattering (LEIS) technique was used to experimentally determine the formation of positive and negative ions in the scattering of protons by a Cu(111) surface for a large scattering angle in the backscattering configuration and a wide range of incoming energies (2?8 keV). Two different collisional geometries were analyzed for a 135° fixed scattering angle: 45°/90° and 67.5°/67.5° incoming/exit angles measured with respect to the target surface. The total fraction of backscattered ions ranges from 10% to 25%, and a peculiarly high yield of negative ions, which always exceeds that of positive ions, was detected for the whole energy range analyzed. A strong dependence of the measured ion fractions with the geometrical conditions was experimentally found. On the theoretical side, a first-principles quantum-mechanical formalism that takes into account the three possible final charge states of the H+ in a correlated way and the fine details of the band structure of the Cu(111) surface was applied to describe the charge transfer processes involved in the experimental situation. The theoretical calculation leads to a nonmonotonous dependence with the incoming energy that properly describes the experimental results, especially the negative ion fraction in the specular collisional geometry. The oscillatory behavior predicted by the theory in the range of low energies is clear evidence of the charge exchange between localized states, that is, the situation related with the presence of the surface state immersed in the L-gap present in the Cu(111) surface. The positive ion fraction is discussed for the first time for this collisional system. The differences found between the measurements and the theory seem to indicate that the neutralization to excited states and also the formation of excited negative hydrogen ions are possible charge exchange channels in the dynamic process analyzed.
关键词： negative ion formation,quantum-mechanical formalism,low-energy ion scattering,Cu(111) surface,charge transfer
更新于2025-09-10 09:29:36
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ion in a quantum well with one expanding wall
摘要： The standard closed orbit theory is extended to investigate the photodetachment of a negative ion near a moving wall. The photodetachment cross section of the H? ion in a quantum well with one expanding wall is specifically put forward. In contrast to the photodetachment of the negative ion in a static quantum well, the returning kinetic energy of the detached electron is different from its initial value after collision with the moving wall, therefore an additional modulation factor appears in the oscillating cross section, which depends on the electron’s initial outgoing and returning momentum. Four different types of closed orbits are found for the detached electron in a quantum well with one expanding wall, and their connections with the oscillating cross section are analyzed quantitatively. The calculation results suggest that the photodetachment cross section of this system depends on the speed of the moving wall and the initial distances from the ion to the walls sensitively. The method used in this paper is universal and can be extended to study the photodetachment of the nonhydrogenic negative ion near a moving wall, whether the wall is moving with constant velocity or oscillatory. The results provide insight into the behavior of photodetachment dynamics of the negative ion in the presence of a moving boundary and may have potential interest for future experimental researches.
关键词： moving wall,closed orbit theory,negative ion,quantum well,photodetachment
更新于2025-09-10 09:29:36
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Selective negative-ion formation from core-valence doubly excited states of the water molecule
摘要： This study focuses on the role of negative-ion formation in the decay of core-valence doubly excited water molecules. Combining negative- and positive-ion coincidence measurements with calculated energies of core-valence doubly excited states, we find that O? and H? production is enhanced selectively. In particular, we suggest that O? production is correlated to double occupancy of the antibonding 4a1 virtual orbital, while H? appears at electronic states with double occupancy in 2b2. We also show that H? and O? can be created as a result of electron recapture close to the O 1s ionization potential.
关键词： core-valence doubly excited states,electron recapture,negative-ion formation,water molecule,O 1s ionization potential
更新于2025-09-09 09:28:46
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Atmospheric Pressure MALDI-IMS Using p-Nitroaniline as the Matrix at High Spatial Resolution in the Positive and Negative Ion Modes
摘要： Matrix-assisted laser desorption/ionization imaging mass spectrometry (MALDI-IMS) can provide information on the spatial distribution and amount of numerous molecules on a single tissue section. A single sample that can be used in both positive and negative ion analyses is needed, since this can reduce losses of rare clinical samples. Although p-nitroaniline (NIT) has been shown to be applicable for use as a matrix in MALDI-IMS in the positive and negative ion modes, it is volatile and instable under vacuum conditions. In this study, we applied NIT as a volatile matrix for atmospheric pressure (AP) MALDI-IMS analysis, using a Mass Microscope, because NIT is considered to remain stable on the sample under AP conditions. Our objective was to optimize NIT usage by evaluating peak intensities acquired in the positive and negative ion modes. We evaluated signal intensities under 3 different conditions; dilution at different volumes, at different concentrations, and in two types of solvents. We also measured the matrix weight deposited on samples and determined the optimum condition with respect to matrix weight and signal intensity. The optimum condition was determined as follows; a solvent volume of 2 mL, an NIT concentration of 30 mg/mL, and 100% methanol as the solvent. Mass spectra obtained using these conditions provided comparable signals with 2,5-dihydroxybenzoic acid and 9-aminoacridine. Using tandem mass spectrometry, the most intense signals in both ion modes were identified as phosphatidylcholine and phosphatidylinositol. Moreover, the lipid distribution in the mouse cerebellum could be visualized at a high spatial resolution of 5 μm on a single section, in both the positive and negative ion modes. This study shows that optimization of NIT is determined by 3 parameters (solvent volume, NIT concentration, type of solvent). Using the optimum condition, it was possible to analyze the same tissue section by AP MALDI-IMS in both the positive and negative ion modes, at a sufficiently high sensitivity for tandem mass spectrometry and imaging at a spatial resolution of 5 μm.
关键词： AP MALDI,Positive and negative ion modes,p-Nitroaniline,Phospholipids,High spatial resolution,IMS
更新于2025-09-09 09:28:46
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Influence of magnetic filter and magnetic cage in negative ion production in helicon oxygen plasma
摘要： Negative ion rich oxygen plasma at low pressure is produced in a Helicon Plasma Source setup, which is primarily designed to perform electronegative gas plasma experiments including the studies of ion-ion plasma. The negative ion fraction and hence the negative ion density are obtained by using a two probe technique in which the electron current is obtained by using an RF compensated cylindrical Langmuir probe and positive ion saturation current is obtained by using an RF compensated planar probe. By measuring the negative ion fraction, both with and without a magnetic filter, the importance of the magnetic filter field in the production of negative ions in oxygen plasma is investigated. The maximum value of negative ion fraction a (n?/ne) is calculated to be approximately around 9 when the value of temperature ratio c (Te/T?) is taken as 10. The observed negative ion fraction and other plasma parameters are explained by considering the set of reactions that are involved in the production and loss of negative ions.
关键词： magnetic cage,magnetic filter,helicon plasma,oxygen plasma,negative ion
更新于2025-09-04 15:30:14
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Preliminary results of optical emission spectroscopy by line ratio method in the RF negative ion source at ASIPP
摘要： Optical emission spectroscopy (OES) using the trace rare gases of Ar and Xe have been carried out in a radio frequency (RF) driven negative ion source at Institute of Plasma Physics, Chinese Academy of Science (ASIPP), in order to determine the electron temperature and density of the hydrogen plasma. The line-ratio methods based on population models are applied to describe the radiation process of the excited state particles and establish their relations with the plasma parameters. The spectral lines from the argon and xenon excited state atoms with the wavelength of 750.4 nm and 828.0 nm are used to calculate the electron temperature based on the corona model. The argon ions emission lines with the wavelength of 480 nm and 488 nm are selected to calculate the electron density based on the collisional radiative model. OES has given the preliminary results of the electron temperature and density by varying the discharge gas pressure and RF power. According to the experimental results, the typical plasma parameters is Te≈2?4 eV and ne≈1×1017?8×1017 m-3 in front of plasma grid.
关键词： RF negative ion source,optical emission spectroscopy,neutral beam injection
更新于2025-09-04 15:30:14