Metastable Beam Research Articles

Vacuum-ultraviolet light source for enhanced production of metastable krypton and xenon beams

Vacuum-ultraviolet light source for enhanced production of metastable krypton and xenon beams

A magnetically enhanced RF discharge source for metastable krypton production.

We describe a high intensity metastable Kr source based on a helical resonator RF discharge. By adding an external B-field to the discharge source, the metastable Kr flux is enhanced. The effect of geometric configuration and magnetic field strength has been studied and optimized experimentally. Compared to the helical resonator discharge source without an external B-field, the new source showed an enhancement factor of 4-5 in producing metastable Kr beams. This improvement has a direct impact on the radio-krypton dating applications as it can increase the atom count rate, resulting in a higher analytical precision.

Architected material analogs for shape memory alloys

Architected material analogs for shape memory alloys

Probing atomic and nuclear properties with precision spectroscopy of fine and hyperfine structures in the Li+7 ion

Precision spectroscopy of ${\mathrm{Li}}^{+}$ is a promising testing ground for bound-state quantum electrodynamics (QED) and for measurements of nuclear properties such as the Zemach radius. We investigate the hyperfine and fine-structure splittings of the $2\phantom{\rule{0.16em}{0ex}}^{3}S_{1}$ and $2{\phantom{\rule{0.16em}{0ex}}}^{3}{P}_{J}$ states of $^{7}\mathrm{Li}^{+}$ using saturated fluorescence spectroscopy based on a $\ensuremath{\sim}460\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ metastable ion beam. We measure in particular the $2\phantom{\rule{0.16em}{0ex}}^{3}S_{1}--2\phantom{\rule{0.16em}{0ex}}^{3}P_{J}$ transitions in $^{7}\mathrm{Li}^{+}$. With a triple nested loop scanning method, the long-term drift and systematic uncertainties are reduced or eliminated, resulting in a total uncertainty of less than 100 kHz. Our results are in good agreement with QED calculations. For the hyperfine splittings of $2\phantom{\rule{0.16em}{0ex}}^{3}S_{1}$, our measured values have a similar accuracy to previous measurements and theoretical calculations. For the $2{\phantom{\rule{0.16em}{0ex}}}^{3}{P}_{J}$ fine and hyperfine splittings, our measured results are one order of magnitude more accurate than those of previous measurements and have a similar accuracy to the theoretical values. The measurements lay the foundation for future work on the ${\mathrm{Li}}^{+}$ isotopes and their theoretical interpretation in terms of nuclear charge radii and the Zemach radii.

Production of C4+ (2s2p 3,1P) hollow states in collisions of 6-18 MeV C4+ (1s2, 1s2s 3S) mixed-state beams with gas targets

Normalized electron yields of the formation of 2s2p 3,1P states from the metastable states 1s2s 3,1S and from the ground state 1s21S were obtained in 6-18MeV C4+ collisions with H2, He, Ne and Ar gas targets. The method of zero-degree Auger projectile spectroscopy was used to detect electrons emitted in the Auger decay C4+ (2s2p 3,1P) → C5+ (1s) + e-with high resolution. These states are of particular importance in the detailed study of fundamental excitation mechanisms, i.e. electron-nucleus excitation, electron-electron excitation and electron-electron excitation with spin exchange. Currently, the role of the above mechanisms in the production of the 2s2p 3,1P states is investigated utilizing variable 1s2s 3S metastable fraction beams as a function of collision energy and target species. Our latest results are presented.

Mixed-State Ionic Beams: An Effective Tool for Collision Dynamics Investigations

The use of mixed-state ionic beams in collision dynamics investigations is examined. Using high resolution Auger projectile spectroscopy involving He-like ( 1 s 2 1 S , 1 s 2 s 3 , 1 S ) mixed-state beams, the spectrum contributions of the 1 s 2 s 3 S metastable beam component is effectively separated and clearly identified. This is performed with a technique that exploits two independent spectrum measurements under the same collision conditions, but with ions having quite different metastable fractions, judiciously selected by varying the ion beam charge-stripping conditions. Details of the technique are presented together with characteristic examples. In collisions of 4 MeV B 3 + with H 2 targets, the Auger electron spectrum of the separated 1 s 2 s 3 S boron beam component allows for a detailed analysis of the formation of the 1 s 2 s ( 3 S ) n l 2 L states by direct n l transfer. In addition, the production of hollow 2 s 2 p 1 , 3 P doubly- and 2 s 2 p 2 2 D triply-excited states, by direct excitation and transfer-excitation processes, respectively, can also be independently studied. In similar mixed-state beam collisions of 15 MeV C 4 + with H 2 , He, Ne and Ar targets, the contributions of the 1 s 2 , 1 s 2 s 3 , 1 S beam components to the formation of the 2 s 2 p 3 , 1 P states by double-excitation, 1 s → 2 p excitation and transfer-loss processes can be clearly identified, facilitating comparisons with theoretical calculations.

Determination of the state production ratios , and from fast () mixed-state He-like ion beams in collisions with H2 targets

New results are presented on the ratio concerning the production cross sections of Li-like quartet and doublet P states formed in energetic ion–atom collisions by single electron transfer to the metastable component of the He-like ion beam. Spin statistics predict a value of Rm = 2 independent of the collision system in disagreement with most reported measurements of . A new experimental approach is presented for the evaluation of Rm having some practical advantages over earlier approaches. It also allows for the determination of the separate contributions of ground- and metastable-state beam components to the measured spectra. Applying our technique to zero-degree Auger projectile spectra from 4.5 MeV (Benis et al 2002 Phys. Rev. A 65 064701) and 25.3 MeV (Zamkov et al 2002 Phys. Rev. A 65 062706) mixed state He-like ion collisions with H2 targets, we report new values of for boron and for fluorine. In addition, the ratios of and populations from either the metastable and/or ground state beam component, also relevant to this analysis, are evaluated and compared to previously reported results for carbon collisions on helium (Strohschein et al 2008 Phys. Rev. A 77 022706) including a critical comparison to theory.

Evidence for the non-statistical population of the 1s2s2p4P metastable state by electron capture in 4 MeV collisions of B3+(1s2s 3S) with H2 targets

We have revisited previously published data involving collisions of mixed 4MeV B3+(1s21S,1s2s3S) with H2 targets (Benis et al., 2002) in search of evidence for the non-statistical production of the 1s2s2p4P long-lived metastable state by single electron capture. Using our recently published method for the accurate determination of the effective solid angle of Auger decaying metastable projectile states in combination with knowledge of the 1s2s3S metastable beam fraction allowed us to determine the ratio R=4P/2P of 1s2s2p quartet to doublet production cross sections both formed by electron capture. Our present determination of R=2.8±0.5, clearly departs from the expected value of R=2 based on statistical spin recoupling arguments and thus provides evidence for the active presence of additional population mechanisms in a new collision system.

Probing interactions between Rydberg atoms with large electric dipole moments in amplitude-modulated electric fields

Dipole-dipole interactions between helium atoms in Rydberg-Stark states with principal quantum number $n=53$ and approximately linear Stark energy shifts, resulting from induced electric dipole moments of approximately 7900 D, have been investigated experimentally. The experiments were performed in pulsed supersonic metastable helium beams, with particle number densities of up to $\sim10^9$ cm$^{-3}$. In the presence of amplitude-modulated, radio-frequency electric fields, changes in the spectral intensity distributions associated with the transitions to these states that are attributed to dipole-dipole interactions within the ensembles of excited atoms have been observed. The experimental results are in excellent agreement with calculations of the Rydberg energy level structure carried out using Floquet methods, and excitations shared by up to 4 atoms. The use of these Rydberg-Stark states as sensors for non-resonant broadband radio-frequency electrical noise is also discussed.

Characterization of a metastable neon beam extracted from a commercial RF ion source

We have used a commercial RF ion-source to extract a beam of metastable neon atoms. The source was easily incorporated into our existing system and was operative within a day of installation. The metastable velocity distribution, flux, flow, and efficiency were investigated for different RF powers and pressures, and an optimum was found at a flux density of 2 × 1012 atoms/s/sr. To obtain an accurate measurement of the amount of metastable atoms leaving the source, we insert a Faraday cup in the beam line and quench some of them using a weak 633 nm laser beam. In order to determine how much of the beam was quenched before reaching our detector, we devised a simple model for the quenching transition and investigated it for different laser powers. This detection method can be easily adapted to other noble gas atoms.

Note: Manipulation of supersonic atomic beams with static magnetic fields

The inhomogeneous magnetic field of a permanent-magnet planar Halbach array is used to either deflect or to specularly reflect a supersonic beam of neutral atoms. Metastable neon and helium beams are tested to experimentally evaluate the performance of this array in a range of configurations. Results are compared with numerical simulations and the device is presented as a high precision tool for the manipulation of neutral atom beams.

Alignment Selection of the Metastable CO(a 3Π1) Molecule and the Steric Effect in the Aligned CO(a 3Π1) + NO Collision

The aligned metastable CO(a (3)Π1) molecular beam was generated by an electronic excitation through the Cameron band (CO a (3)Π1 ← X (1)Σ(+)) transition. Beam characterization of the aligned molecular beam of CO(a (3)Π1) was carried out by (1 + 1) REMPI detection via the b (3)Σ(+) state. The REMPI signals showed the clear dependence on the polarization of the pump laser, and the experimental result was well reproduced by the theoretical simulation. This agreement confirms that aligned metastable CO(a (3)Π1) can be generated and controlled by rotating polarization of the pump laser. By using this technique, a single quantum state of CO(a (3)Π1) can be selected as a metastable molecular beam. The steric effect in the energy-transfer collision of CO(a (3)Π1) with NO forming the excited NO was carried out with this aligned CO(a (3)Π1) molecular beam. We find that the sideways orientation of CO(a (3)Π1) is more favorable in the formation of the excited NO(A (2)Σ(+), B (2)Π) than that for the axial collisions. The obtained steric effect was discussed with the aid of the spatial distribution of CO(a (3)Π1) molecular orbitals, and we find that specific rotational motion of CO(a (3)Π1) in each state may not be a dominant factor in this energy-transfer collision.

Lyman-α radiation of a probing metastable hydrogen beam to measure electric fields in diluted fluids and plasmas

The interaction between a metastable H(2s) atomic hydrogen beam and an external electric field leads to the emission of the Lyman-α line. It originates in the Stark mixing of the near-degenerate 2s1/2 and 2p1/2 levels separated by the Lamb shift. The quenched radiation proportional to the square of the electric field amplitude is recovered in vacuum by using such an atomic probe beam. For larger electric field, saturation is observed and related to the beam finite transit time. We also observe the strong enhancement of the signal when the field is oscillating at the Lamb shift frequency. This technique is applied in a plasma, offering an alternative way to measure weak electric fields by direct and non-intrusive means.

Magnetic properties of O2adsorbed on Cu(100): A spin-polarized metastable He beam study

Magnetic properties of O${}_{2}$ adsorbed on Cu(100) were investigated by monitoring the spin dependence in Penning ionization of metastable He(2${}^{3}$$S$) under external magnetic fields of 0--5 T. A clear spin polarization was found for the 3${\ensuremath{\sigma}}_{g}$ and 1${\ensuremath{\pi}}_{u}$ orbitals of physisorbed O${}_{2}$ under external fields, while the spin polarization disappeared when O${}_{2}$ was changed into the chemisorbed state at $>$50 K. The magnetic susceptibility at the surface of multilayer and monolayer of physisorbed O${}_{2}$ on Cu(100) was similar to that for the bulk liquid O${}_{2}$. Observed exchange splittings and spin polarization suggest that a physisorbed O${}_{2}$ molecule has a magnetic moment close to that for an isolated O${}_{2}$ molecule even at submonolayer coverages, while a density functional theory calculation predicts a much reduced magnetic moment for O${}_{2}$ directly adsorbed on Cu(100).

Selective detection of metastable helium atoms by elastic scattering collisions

A method for efficient and selective detection of $2{\phantom{\rule{0.16em}{0ex}}}^{1}\phantom{\rule{-0.16em}{0ex}}S$ and $2{\phantom{\rule{0.16em}{0ex}}}^{3}\phantom{\rule{-0.16em}{0ex}}S$ metastable He atoms is presented. Elastic collisions with Ar atoms in a diffuse gas spatially separate these $2S$ atoms from the copropagating UV photons that are inherently present in metastable beams. The deflected $2S$ atoms are detected on a large-solid-angle Faraday cup. A He discharge lamp can selectively quench $2{\phantom{\rule{0.16em}{0ex}}}^{1}\phantom{\rule{-0.16em}{0ex}}S$ atoms, and optical pumping together with a Stern-Gerlach magnet can selectively remove $2{\phantom{\rule{0.16em}{0ex}}}^{3}\phantom{\rule{-0.16em}{0ex}}S$ atoms. As a result, we are able to efficiently detect either $2{\phantom{\rule{0.16em}{0ex}}}^{1}\phantom{\rule{-0.16em}{0ex}}S$ or $2{\phantom{\rule{0.16em}{0ex}}}^{3}\phantom{\rule{-0.16em}{0ex}}S$ atoms (or both) without a UV background.

The Improved Self-assembled Monolayer of Octadecyltrichlorosilane as Positive Resist for Patterning Silicon Surface by Metastable Helium Atom Beam Lithography

We utilized a beam of helium atoms in a metastable excited state to expose an improved ultrathin self-assembled monolayer (SAM) of octadecyltrichlorosilane (OTS) acted as positive resist grown directly on silicon wafer substrate. The metastable helium beam incident on the samples was patterned with a mask, which was placed in gentle contact with the substrate surface. A three-step wet-chemical etching process was successfully developed to transfer mask patterns into the underlying silicon substrate. Only positive patterns were fabricated on silicon surface by using the improved positive resist and controlling of the exposure dosage and the etching time. The sizes of positive patterns on silicon were successfully decreased from micrometer scale to 100 nm.

Lyman-α radiation of a metastable hydrogen beam to measure electric fields

The interaction between a metastable H(2s) atomic hydrogen beam and an external electric field leads to the emission of the Lyman-α line. It originates in the Stark mixing of the near-degenerate 2s1/2 and 2p1/2 levels separated by the Lamb shift. The quenched radiation proportional to the square of the electric field amplitude is recovered in vacuum by using such an atomic probe beam. We observe the strong enhancement of the signal when the field is oscillating at the Lamb shift frequency. This technique is applied in a plasma, offering an alternative way to measure weak electric fields by direct and non-intrusive means.

Parity nonconservation in hydrogen

We discuss the prospects for parity violation experiments in atomic hydrogen and deuterium to contribute to testing the Standard Model (SM). We find that, if parity experiments in hydrogen can be done, they remain highly desirable because there is negligible atomic-physics uncertainty and low energy tests of weak neutral current interactions are needed to probe for new physics beyond the SM. Analysis of a generic APV experiment in deuterium indicates that a 0.3% measurement of C1D requires development of a slow (77K) metastable beam of ≈ 5×1014D(2S)s − 1 per hyperfine component. The advent of UV radiation from free electron laser (FEL) technology could allow production of such a beam.

Spin-polarized metastable He study of surface magnetic order in Ho thin films

A spin-polarized metastable helium beam operating in magnetic fields of up to 5 T has been used to investigate the spin polarization at the surface of thin films of Ho(0001) grown on W(110) substrates. The extreme surface sensitivity of this technique allows a direct probe of the various magnetic structures that form as a function of film thickness, temperature, and applied magnetic field. Evidence for a transition from a conical ferromagnetic structure to a helical antiferromagnetic (AFM) phase is observed at low temperatures with a further transition to paramagnetism at close to the AFM ordering temperature, TN. In addition, the spin polarization of the 6s CE that mediate the exchange interaction between localized Ho 4f moments (the RKKY interaction) is found to be positive, supporting earlier observations.

Thermal desorption metastable atom bombardment ionization aerosol mass spectrometer

A metastable atom bombardment (MAB) ionization source has been coupled to an existing thermal desorption aerosol mass spectrometer. The design allows real-time alternation between MAB and electron ionization (EI). A jet of metastable species produced in a DC discharge is directed at the ionization volume of the mass spectrometer, where Penning ionization is thought to be the dominant mechanism. Performance is characterized in experiments with oleic acid particles. By changing discharge gases between N 2, Kr, and Ar, the excited state energy of the metastable species can be adjusted in the range 8.5–11.7 eV. For vaporization at 180 °C, all gases yield significantly less fragmentation than EI, which could improve results of factor analysis. Fragmentation increases with vaporization temperature, but generated fragments have higher average mass than those produced by EI. Analyte signal levels are 0.1% and 0.006% of equivalent analysis with EI when using Ar * and Kr *, respectively. These sensitivities are not practical for ambient studies, but are sufficient for source measurements, as demonstrated with direct measurements of biomass burning emissions. The measured Ar * flux of 3.6 × 10 13 sr −1 s −1 is ∼30 times lower than the best literature values for similar metastable beam sources, suggesting that sensitivity can be increased by source design improvements.