Metastable Helium Research Articles

A Novel Metastable Atom Source Based on DC Glow Discharge Pinch

− Metastable atoms of noble gases have many applications in scientific research and industrial application. Though previously many such development of metastable atom sources are reported, in our endeavor for development of a source of this kind, we could develop a novel, simple structure, maintenance free glow discharge source that could produce metastable atoms of noble gases. This source is devised with two cathodes and a cylindrical anode paired with one permanent magnet. The electric field in between cathodes and the anode coupled with the magnetic field produce glow discharge in the centre of the source. The device was tested for various combination of anode voltage and filled helium pressure. The anode voltage was varied from 300 to 2000 V where as the filled pressure was varied from 450 mTorr to 1 Torr. The plasma extends to the outside of the source volume through the aperture in one of the cathode in form of a pinched glow. Due to this pinch, energetic electrons are generated and they excite the neutral atoms in their vicinity. The spectral lines of the glow discharge were observed by one optical spectrometer. Metastable helium atoms were observed at their transition wavelengths of 388.8 and 501.6 nm. The detail designing of the source and experimental results are reported in this article.

Molecular-beam resonance method with Zeeman-decelerated samples: Application to metastable helium molecules

We use a multistage Zeeman decelerator to generate slow beams ($v\ensuremath{\simeq}100\phantom{\rule{0.16em}{0ex}}\mathrm{m}/\mathrm{s}$) of translationally cold, spin-polarized metastable $a\phantom{\rule{0.16em}{0ex}}^{3}\mathrm{\ensuremath{\Sigma}}_{u}^{+}\phantom{\rule{4pt}{0ex}}{\mathrm{He}}_{2}$ molecules and perform a precision measurement of their spin-rotation fine structure. The spin polarization results from the elimination of the high-field-seeking $J=N$ spin-rotational component of each rotational level (rotational quantum number $N$) by the Zeeman deceleration process. By repopulating the $J=N$ component from the $J=N\ifmmode\pm\else\textpm\fi{}1$ low-field-seeking components using radio-frequency radiation, we measured the spin-rotation fine structure of 13 rovibrational levels with $v=0$, 1 and $N=1\text{--}21$. The low beam velocity and the resulting long interaction times with the radio-frequency radiation were exploited to determine the transition frequencies with a precision of 300 Hz.

Approaching the adiabatic timescale with machine learning

The control and manipulation of quantum systems without excitation are challenging, due to the complexities in fully modeling such systems accurately and the difficulties in controlling these inherently fragile systems experimentally. For example, while protocols to decompress Bose-Einstein condensates (BECs) faster than the adiabatic timescale (without excitation or loss) have been well developed theoretically, experimental implementations of these protocols have yet to reach speeds faster than the adiabatic timescale. In this work, we experimentally demonstrate an alternative approach based on a machine-learning algorithm which makes progress toward this goal. The algorithm is given control of the coupled decompression and transport of a metastable helium condensate, with its performance determined after each experimental iteration by measuring the excitations of the resultant BEC. After each iteration the algorithm adjusts its internal model of the system to create an improved control output for the next iteration. Given sufficient control over the decompression, the algorithm converges to a solution that sets the current speed record in relation to the adiabatic timescale, beating out other experimental realizations based on theoretical approaches. This method presents a feasible approach for implementing fast-state preparations or transformations in other quantum systems, without requiring a solution to a theoretical model of the system. Implications for fundamental physics and cooling are discussed.

Observation of polarized alkali atoms in experiments on optical orientation in a He–Rb and He-Cs mixture under conditions of a pulsed gas discharge

It has been demonstrated that optical pumping of alkali atoms (rubidium and cesium) results to a highly efficient indirect optical orientation of metastable He (23S1) atoms. However, in an attempt to perform the inverse experiment (i.e. to receive spin-polarization of alkali atoms upon optical pumping of metastable helium atoms) the result was quite unexpected: the magnetic resonance signal of Cs or Rb atoms was very weak (several hundred times weaker compared to conventional optical pumping of helium in the presence of cesium (rubidium)). We propose an explanation for the unusually small ratio of the magnetic resonance signals from Cs (Rb) and metastable He atoms under condition of optical orientation of He atoms. Except of Cs atoms we are the first to have observed magnetic resonance signals from atoms of 85Rb and 87Rb isotopes when using the indirect optical orientation in conditions of helium-rubidium gas discharge plasma. It is shown that the anomaly in the amplitude ratios of the observed MR signals can be explained by the unidirectional action of three different factors (the presence of a large nuclear spin of alkali atoms, depolarization of alkali atoms between collisions, and spin exchange between atoms of rubidium isotopes (for the He-Rb system).

Radiative Charge Transfer between Metastable Helium and Lithium Cations

Abstract Depopulation of long-lived metastable helium He(23S) by spontaneous radiative charge transfer in collisions with lithium cations Li+ is investigated using a fully quantal approach. The corresponding transitions start in continuum states of the initial electronic state b 3Σ+ and end in continuum states of the final electronic state a 3Σ+. The process is characterized by cross sections and rate coefficients, which are calculated as functions of initial collision energy and temperature, respectively. Particular consideration is paid to the proper description of the high-energy cross sections in order to include their contributions to the total rate coefficient at high temperatures, where its asymptotic behavior is analyzed. The calculated total rate coefficients are in the range 1.75 × 10−15 – 3.16 × 10−14 cm3 s−1. The comparison with other relevant depopulation mechanisms shows that the radiative processes prevail for temperatures below 3000 K, while at higher temperatures the nonradiative inelastic processes are dominant.

Formation of positive and negative clusters of gold atoms inside helium nanodroplets close to zero K

We report the mass spectrometric detection of positive and negative cluster ions of gold atoms formed by the pickup of gold by helium nanodroplets (HNDs) near zero K and subsequent exposure to electron impact. Both singly and doubly charged positive cluster ions were observed. Ionization is initiated by the helium ions He+ and He*−, metastable helium atoms He* as well as electron attachment. We focus on the size distributions of the ions, electron energy dependence of ion yield, patterns of stability and the presence of “magic” clusters. As reported previously by others using a variety of other methodologies, a clear oscillation in the intensity of (Au)n+/- is seen, especially for n <21, with odd-numbered clusters being more intense than even-numbered clusters. The distributions peak early around n = 8. There is a suggestion of magic numbers at n = 35 and 58. Efficiencies of formation of the gold cluster anions were monitored as a function of the energy of the electrons; the measured intensity distributions exhibit a shift to higher energies with increasing cluster size that is attributed to the need to evaporate helium from the clusters for their observation in the nascent state.

Non-equilibrium excitation of CO2 in an atmospheric pressure helium plasma jet

The energy efficient excitation of CO2 in atmospheric pressure plasmas may be a method to generate solar fuels from renewable energies. This energy efficiency can be very high, if only specific states of the molecules in the plasma are populated creating a strong non-equilibrium. This requires a specific design of the plasma source, method of plasma excitation and choice of gases and admixtures. In this paper, non-equilibrium excitation and dissociation of CO2 in an atmospheric pressure helium RF plasma jet is analysed for varying absorbed plasma power and admixture levels of CO2. The concentrations of CO2 and of CO, as well as the vibrational and rotational temperatures of the possible degrees of freedom of the molecules are evaluated by Fourier transform infrared spectroscopy (FTIR). The molecular rotational vibrational spectra are modelled based on Maxwell–Boltzmann state populations using individual temperatures for each degree of freedom. A strong non-equilibrium excitation of CO2 and CO has been found. Whereas the rotational temperatures are 400 K or below, the vibrational temperature for CO reaches values up to 1600 K and that of the asymmetric vibration of CO2 of 700 K. The dependence of these excitation temperatures on plasma power and admixture level is rather weak. The mass balance, the energy and conversion efficiency are consistent with a very simple chemistry model that is dominated by CO2 dissociation via Penning collisions with helium metastables. A conversion efficiency up to 30% and an energy efficiency up to 10% is observed in the parameter range of the experiment.

Single-atom-resolved probing of lattice gases in momentum space

Measuring the full distribution of individual particles is of fundamental importance to characterize many-body quantum systems through correlation functions at any order. Here we demonstrate the possibility to reconstruct the momentum-space distribution of three-dimensional interacting lattice gases atom-by-atom. This is achieved by detecting individual metastable Helium atoms in the far-field regime of expansion, when released from an optical lattice. We benchmark our technique with Quantum Monte-Carlo calculations, demonstrating the ability to resolve momentum distributions of superfluids occupying $10^5$ lattice sites. It permits a direct measure of the condensed fraction across phase transitions, as we illustrate on the superfluid-to-normal transition. Our single-atom-resolved approach opens a new route to investigate interacting lattice gases through momentum correlations.

Laser photoexcitation of Rydberg states in helium with n > 400

Helium atoms travelling in pulsed supersonic beams have been photoexcited from the metastable 1s2s 3S1 level to Rydberg states with principal quantum numbers exceeding 400 by resonance-enhanced two-colour two-photon excitation using narrow-bandwidth CW laser radiation. To achieve this, the photoexcitation region was shielded and stray electric fields cancelled using a cubic arrangement of 80 mm × 80 mm copper plates. The excited Rydberg atoms were detected by pulsed electric field ionisation downstream from this photoexcitation region. Comparison of the experimental spectra with the results of calculations indicate that the stray electric fields at the position of Rydberg state photoexcitation were reduced to as low as 160 μV cm–1. Using helium in these experiments minimises temporal changes in the stray fields resulting from surface adsorption. Measurements performed 70 h apart indicate that the rate of change of these fields was on the order of 2 μV cm–1 per h, making beams of metastable helium atoms ideally suited as probes for high precision electrometry in spectroscopy experiments involving less readily available species such as positronium or antihydrogen.

Measuring the Density of States of the Inner and Outer Wall of Double-Walled Carbon Nanotubes.

The combination of ultraviolet photoelectron spectroscopy and metastable helium induced electron spectroscopy is used to determine the density of states of the inner and outer coaxial carbon nanotubes. Ultraviolet photoelectron spectroscopy typically measures the density of states across the entire carbon nanotube, while metastable helium induced electron spectroscopy measures the density of states of the outermost layer alone. The use of double-walled carbon nanotubes in electronic devices allows for the outer wall to be functionalised whilst the inner wall remains defect free and the density of states is kept intact for electron transport. Separating the information of the inner and outer walls enables development of double-walled carbon nanotubes to be independent, such that the charge transport of the inner wall is maintained and confirmed whilst the outer wall is modified for functional purposes.

Ultra-dense Broad-line Region Scale Outflow in Highly Reddened Quasar SDSS J145057.28+530007.6

Abstract We report the discovery of highly reddening and hydrogen Balmer and metastable helium broad absorption lines in the quasar SDSS J145057.28+530007.6, based on the optical and near-infrared spectra taken from the SDSS-III/BOSS and the TripleSpec observations. The nuclear continuum, Balmer decrement, and absorption-line depth analyses suggest that (1) the accretion disk is completely obscured and the covering factor of the broad-line region (BLR) is only 0.39 ± 0.03, (2) the power-law continuum is reddened by the SMC extinction law of E(B – V) =0.72 ± 0.01 mag and the dusty materials are mainly associated with Ca ii H and K rather than the Balmer and He i* absorption-line system, (3) the unsaturated Balmer (Hβ, Hγ, and Hδ) and He i* λ3889 absorption lines have the same two-Gaussian profiles as the shifts of −931 ± 33 and −499 ± 39 km s−1 and the widths of 121 ± 28 and 196 ± 37 km s−1, respectively. Constrained mutually by the Balmer, He i* absorption lines and undetected Fe ii* λ5169 in the photoionization simulations, the physical properties of the outflow gases are derived as follows: ionization parameter 10−1.4 ≲ U ≲ 10−0.8, density 108.2±0.4 ≲ n H ≲ 109.0±0.4 cm−3, and column density 1022.0±0.2 ≲ N H ≲ 1022.2–22.3 cm−2. We propose that the ultra-dense outflow gases appear in the vicinity of the surface of the BLR or are located at most 3.12 pc away from the engine. That probably implies that the outflow originates from the BLR, and this kind of ultra-dense BLR scale outflow gases can effectively test the physical properties of the outer gases of the BLR.

Two discharge modes of a repetitive nanosecond pulsed helium glow discharge under sub-atmospheric pressure in the repetition frequency range of 20 to 600 kHz

Two discharge modes, α and γ, of a repetitive nanosecond pulsed helium glow discharge at a gas pressure of 10 kPa in the repetition frequency range from 20 to 600 kHz are reported for the first time. The pulsed glow discharge is produced in a pair of parallel plate metal electrodes without insertion of dielectrics. The α mode discharge is volumetrically produced in the electrode gap at a low-repetition frequency, whereas the γ mode discharge is localized at the cathode surface at a high-repetition frequency. At high-repetition frequency, the time interval between voltage pulses is shorter than the lifetime of the afterglow produced by the preceding discharge. Then, the γ mode discharge is maintained by a large number of secondary electrons emitted from the cathode exposed to high-density ions and metastable helium atoms in the afterglow. In the α mode discharge with a low-repetition frequency operation, primary electrons due to gas ionization dominate the ionization process. Thus, a large discharge voltage is needed for the excitation of the α mode discharge. It is established that the bifurcation of α-γ discharge mode, accompanied by a decrease in the discharge voltage, occurs at the high-repetition frequency of ∼120 kHz.

Helium in the eroding atmosphere of an exoplanet.

Helium is the second-most abundant element in the Universe after hydrogen and is one of the main constituents of gas-giant planets in our Solar System. Early theoretical models predicted helium to be among the most readily detectable species in the atmospheres of exoplanets, especially in extended and escaping atmospheres 1 . Searches for helium, however, have hitherto been unsuccessful 2 . Here we report observations of helium on an exoplanet, at a confidence level of 4.5 standard deviations. We measured the near-infrared transmission spectrum of the warm gas giant 3 WASP-107b and identified the narrow absorption feature of excited metastable helium at 10,833 angstroms. The amplitude of the feature, in transit depth, is 0.049 ± 0.011 per cent in a bandpass of 98 angstroms, which is more than five times greater than what could be caused by nominal stellar chromospheric activity. This large absorption signal suggests that WASP-107b has an extended atmosphere that is eroding at a total rate of 1010 to 3 × 1011 grams per second (0.1-4 per cent of its total mass per billion years), and may have a comet-like tail of gas shaped by radiation pressure.

Influence of the chemical potential on the Casimir-Polder interaction between an atom and gapped graphene or a graphene-coated substrate

We present a formalism based on first principles of quantum electrodynamics at nonzero temperature which permits to calculate the Casimir-Polder interaction between an atom and a graphene sheet with arbitrary mass gap and chemical potential, including graphene-coated substrates. The free energy and force of the Casimir-Polder interaction are expressed via the polarization tensor of graphene in (2+1)-dimensional space-time in the framework of the Dirac model. The obtained expressions are used to investigate the influence of the chemical potential of graphene on the Casimir-Polder interaction. Computations are performed for an atom of metastable helium interacting with either a free-standing graphene sheet or a graphene-coated substrate made of amorphous silica. It is shown that the impacts of the nonzero chemical potential and the mass gap on the Casimir-Polder interaction are in opposite directions by increasing and decreasing the magnitudes of the free energy and force, respectively. It turns out, however, that the temperature-dependent part of the Casimir-Polder interaction is decreased by a nonzero chemical potential, whereas the mass gap increases it compared to the case of undoped, gapless graphene. The physical explanation for these effects is provided. Numerical computations of the Casimir-Polder interaction are performed at various temperatures and atom-graphene separations.

Ionization Mechanism of Positive-Ion Nitrogen Direct Analysis in Real Time.

Nitrogen can be an inexpensive alternative to helium used by direct analysis in real time (DART), especially in consideration of the looming helium shortage. Therefore, the ionization mechanism of positive-ion N2 DART has been systematically investigated. Our experiments suggest that a range of metastable nitrogen species with a variety of internal energies existed and all of them were less energetic than metastable helium atoms. However, compounds with ionization energies (IE) equal to or lower than 10.2 eV (all organic compounds except the extremely small ones) can be efficiently ionized. Because N2 DART was unable to efficiently ionize ambient moisture and common organic solvents such as methanol and acetonitrile, the most important ionization mechanism was direct Penning ionization followed by self-protonation of polar compounds generating [M+H]+ ions. On the other hand, N2 DART was able to efficiently ionize ammonia, which was beneficial in the ionization of hydrogen-bonding compounds with proton affinities (PA) weaker than ammonia generating [M+NH4]+ ions and large PAHs generating [M+H]+ ions through proton transfer. N2 DART was also able to efficiently ionize NO, which led to the ionization of nonpolar compounds such as alkanes and small aromatics generating [M-(2m+1)H]+ (m=0,1…) ions. Lastly, metastable nitrogen species was also able to produce oxygen atoms, which resulted in increased oxygen adducts as the polarity of organic compounds decreased. In comparison with He DART, N2 DART was approximately one order of magnitude less sensitive in generating [M+H]+ ions, but could be more sensitive in generating [M+NH4]+ ions. Graphical Abstract ᅟ.

Measurement of He neutral temperature in detached plasmas using laser absorption spectroscopy

The reduction of the heat load onto plasma-facing components by plasma detachment is an inevitable scheme in future nuclear fusion reactors. Since the control of the plasma and neutral temperatures is a key issue to the detached plasma generation, we have developed a laser absorption spectroscopy system for the metastable helium temperature measurements and used together with a previously developed laser Thomson scattering system for the electron temperature and density measurements. The thermal relaxation process between the neutral and the electron in the detached plasma generated in the linear plasma device, NAGDIS-II was studied. It is shown that the electron temperature gets close to the neutral temperature by increasing the electron density. On the other hand, the pressure dependence of electron and neutral temperatures shows the cooling effect by the neutrals. The possibility of the plasma fluctuation measurement using the fluctuation in the absorption signal is also shown.

Simple Closed-Form Expression for Penning Reaction Rate Coefficients for Cold Molecular Collisions by Non-Hermitian Time-Independent Adiabatic Scattering Theory.

We present a simple expression and its derivation for reaction rate coefficients for cold anisotropic collision experiments based on adiabatic variational theory and time-independent non-Hermitian scattering theory. We demonstrate that only the eigenenergies of the resulting one-dimensional Schrödinger equation for different complex adiabats are required. The expression is applied to calculate the Penning ionization rate coefficients of an excited metastable helium atom with molecular hydrogen in an energy range spanning from hundreds of kelvins down to the millikelvin regime. Except for trivial quantities like the masses of the nuclei and the bond length of the diatomic molecule participating in the collision, one needs as input data only the complex potential energy surface (CPES). In calculations, we used recently obtained ab initio CPES by D. Bhattacharya et al. ( J. Chem. Theory Comput. 2017 , 13 , 1682 - 1690 ) without fitting parameters. The results show good accord with current measurements ( Nat. Phys. 2017 , 13 , 35 - 38 ).

Electron-ion momentum vector coincidences in electron collisions with atoms and molecules

We review experimental studies of electron impact ionization of atoms and molecules which became feasible by applying the electron-ion momentum vector coincidence technique. Besides briefly discussing excitation of metastable helium we present studies of single ionization of helium and argon and also mention a pilot experiment for positron impact ionization. The main strength of the method is revealed if three or more particles are to be detected and if the cross section of the reaction is small. Exemplarily helium double ionization close to threshold is shown. Studies of molecular ionization reveal a molecular alignment dependent collision dynamics for the molecular hydrogen target. For larger molecules ionized electronic orbital and ionic fragment can be correlated.

Multistage Zeeman decelerator for molecular-scattering studies

We present a new concept for a multi-stage Zeeman decelerator that is optimized particularly for applications in molecular beam scattering experiments. The decelerator consists of a series of alternating hexapoles and solenoids, that effectively decouple the transverse focusing and longitudinal deceleration properties of the decelerator. It can be operated in a deceleration and acceleration mode, as well as in a hybrid mode that makes it possible to guide a particle beam through the decelerator at constant speed. The deceleration features phase stability, with a relatively large six-dimensional phase-space acceptance. The separated focusing and deceleration elements result in an unequal partitioning of this acceptance between the longitudinal and transverse directions. This is ideal in scattering experiments, which typically benefit from a large longitudinal acceptance combined with narrow transverse distributions. We demonstrate the successful experimental implementation of this concept using a Zeeman decelerator consisting of an array of 25 hexapoles and 24 solenoids. The performance of the decelerator in acceleration, deceleration and guiding modes is characterized using beams of metastable Helium ($^3S$) atoms. Up to 60 % of the kinetic energy was removed for He atoms that have an initial velocity of 520 m/s. The hexapoles consist of permanent magnets, whereas the solenoids are produced from a single hollow copper capillary through which cooling liquid is passed. The solenoid design allows for excellent thermal properties, and enables the use of readily available and cheap electronics components to pulse high currents through the solenoids. The Zeeman decelerator demonstrated here is mechanically easy to build, can be operated with cost-effective electronics, and can run at repetition rates up to 10 Hz

Laser diagnostics on atmospheric pressure discharge plasmas, including cryoplasmas, in environments around room and cryogenic temperature

Cryoplasmas, the plasma gas temperature (Tg) of which can be controlled continuously below room temperature, show various unique and advantageous properties depending on Tg. Recently, the Tg dependence of plasma chemistry related to metastable helium (Hem) has been revealed in helium cryoplasmas. However, Tg was only estimated by thermal calculation from the temperature outside the plasmas. In this study, for better evaluation of Tg, near-infrared laser heterodyne interferometry (NIR-LHI) measurements were conducted in atmospheric pressure helium pulsed discharge plasmas at around room and cryogenic ambient temperatures (Ta). The maximum difference between Tg and Ta was evaluated as 47 K at Ta = 300 K with 282 mW power consumption. To further investigate the Tg dependence of plasma chemical reactions related to Hem, laser absorption spectroscopy (LAS) was performed on the same discharge plasmas to measure the Hem density and lifetime. The Hem lifetime was longer at lower Tg, i.e. the lifetime at Tg = 145 K (9.6 µs) was seven times longer than that at Tg = 386 K (1.4 µs). By comparing the results with the numerically simulated rates of Hem quenching reactions taking Tg into account, the mechanism of the Hem quenching reaction was revealed to vary with Tg even though the main quenching reaction was a three-body collision at all Tg. In this manner, the combination of NIR-LHI with LAS led directly to the Tg dependence of Hem quenching reactions.