Metastable Helium Research Articles

Energy transfer in interaction of a cold atmospheric pressure plasma jet with substrates

The energy flux of a nanosecond pulsed cold atmospheric pressure (CAP) plasma jet in contact with a substrate surface was measured to improve the understanding of the correlation between energy flux, flow dynamics and applied electrical power. The flow pattern properties of the CAP jet were imaged using Rayleigh scattering showing a transition from laminar to turbulent flow at Reynolds number of 700, significantly smaller than the conventional critical Reynolds number of 2040. The energy flux to the surface was determined using a passive thermal probe as a substrate dummy. As expected, the energy flux decreases with increasing distance to the nozzle. Measurements of the floating potential of the probe revealed a strong positive charging (up to 165 V) attributed to ion flux originating mainly from Penning ionization by helium metastables. Negative biasing of the probe doubled the energy flux and showed a significantly increased ion contribution up to a nozzle distance of 6 mm to the surface. For positive biasing an increased contribution of electrons and negative ions was only found at 3 mm distance. The relevance of particle transport to the surface is shown by switching from laminar to turbulent flow resulting in a decreased energy flux. Furthermore, a linear correlation of energy flux and input power was found.

Generation of squeezed light vacuum enabled by coherent population trapping.

We demonstrate the possibility to generate squeezed vacuum states of light by four wave mixing (FWM) enabled coherent population trapping in a metastable helium cell at room temperature. Contrary to usual FWM far detuned schemes, we work at resonance with an atomic transition. We investigate the properties of such states and show that the noise variances of the squeezed and anti-squeezed quadratures cannot be explained by the simple presence of losses. A specific model allows us to demonstrate the role played by spontaneous emitted photons, which experience squeezing while propagation inside of the cell. This theoretical model, which takes into account both residual absorption and spontaneous emission, leads to an excellent agreement with the experimental data without any adjusted parameter.

Metastable Helium Reveals an Extended Atmosphere for the Gas Giant HAT-P-18b

Abstract The metastable helium line at 1083 nm can be used to probe the extended upper atmospheres of close-in exoplanets and thus provide insight into their atmospheric mass loss, which is likely to be significant in sculpting their population. We used an ultra-narrow band filter centered on this line to observe two transits of the low-density gas giant HAT-P-18b, using the 200″ Hale Telescope at Palomar Observatory, and report the detection of its extended upper atmosphere. We constrain the excess absorption to be 0.46% ± 0.12% in our 0.635 nm bandpass, exceeding the transit depth from the Transiting Exoplanet Survey Satellite (TESS) by 3.9σ. If we fit this signal with a 1D Parker wind model, we find that it corresponds to an atmospheric mass loss rate between 8.3 − 1.9 + 2.8 × 10 − 5 M J Gyr−1 and 2.63 − 0.64 + 0.46 × 10 − 3 M J Gyr−1 for thermosphere temperatures ranging from 4000 K to 13,000 K, respectively. With a J magnitude of 10.8, this is the faintest system for which such a measurement has been made to date, demonstrating the effectiveness of this approach for surveying mass loss on a diverse sample of close-in gas giant planets.

Metastable helium Faraday filter for helium lidar to measure the density of the thermosphere.

We demonstrate a metastable helium Faraday optical filter operating on the 23S1 - 23P1 and 23S1 - 23P2 transition at 1083 nm by using a 3 cm long helium cell. The influence of the magnetic field and gas pressure of the helium cell on the filter characteristics is experimental studied. When the magnetic field is 230 Gs and the gas pressure of helium cell is about 110 Pa, the peak transmission corresponding to the two energy level transitions is about 32% and 57%, respectively. The equivalent noise bandwidth (ENBW) under this working condition is about 1.9 GHz. The metastable helium Faraday filter can be used to improve the optical inefficiency of a helium resonance fluorescence lidar to achieve the metastable helium density detection at 200-1000 km thermosphere.

Kinetic Isotope Effect in Low-Energy Collisions between Hydrogen Isotopologues and Metastable Helium Atoms: Theoretical Calculations Including the Vibrational Excitation of the Molecule.

We present very accurate theoretical results of Penning ionization rate coefficients of the excited metastable helium atoms (4He(23S) and 3He(23S)) colliding with the hydrogen isotopologues (H2, HD, D2) in the ground and first excited rotational and vibrational states at subkelvin regime. The calculations are performed using the current best ab initio interaction energy surface, which takes into account the nonrigidity effects of the molecule. The results confirm a recently observed substantial quantum kinetic isotope effect (Nat. Chem. 2014, 6, 332–335) and reveal that the change of the rotational or vibrational state of the molecule can strongly enhance or suppress the reaction. Moreover, we demonstrate the mechanism of the appearance and disappearance of resonances in Penning ionization. The additional model computations, with the morphed interaction energy surface and mass, give better insight into the behavior of the resonances and thereby the reaction dynamics under study. Our theoretical findings are compared with all available measurements, and comprehensive data for prospective experiments are provided.

Monitoring damage of self-assembled monolayers using metastable excited helium atoms.

The breaking of molecular bonds during exposure to ionizing radiation and electron beams creates irreversible damage in the molecular structure. In some cases, such as lithography, controlled damage of a molecular resist is a desirable process and is the basis for the entire semiconductor industry. In other cases, such as environmental exposure or probing of the molecular structure, the induced damage is a major problem that has limited advances in science and technology. We report here the use of an in situ probe that is minimally invasive to detect real-time damage induced in organic materials. Specifically, we use metastable excited helium atoms in the 3S1 state to characterize the damage caused by a low-energy electron beam ∼30 eV on an organic self-assembled monolayer of 11-bromo-1-undecanethiol on a gold substrate. We were able to monitor the damage caused by the electron beam without introducing any additional observed damage by the probing metastable atoms.

A bright and stable beam of slow metastable helium atoms

A stable high-intensity atomic beam source plays a key role in many precision measurements. The precision spectroscopy of slow metastable (<inline-formula><tex-math id="M6">\begin{document}$2^3{\rm S}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201833_M6.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201833_M6.png"/></alternatives></inline-formula>) helium atoms is of great interest in testing quantum electrodynamics and determining the fine structure constant. By improving the source cavity structure and using laser cooling method, the beam flux is greatly enhanced. The added Zeeman slower reduces the longitudinal velocity of atoms, and at the same time increases the beam brightness of atoms at one single speed. Near the back end of Zeeman slower, a two-dimensional magneto-optical trap is added to collimate and focus the atomic beam. In addition, A beam stabilizing system is developed by using feedback control method. By changing the frequency of transverse cooling laser to change the cooling efficiency, the fluctuation of atomic beam intensity can be compensated in real time, and then the beam intensity can be stabilized at the target number. Experiments show that the continuous beam of metastable helium atoms at a velocity of <inline-formula><tex-math id="M7">\begin{document}$(100\pm 3.6)$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201833_M7.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201833_M7.png"/></alternatives></inline-formula> m/s has an intensity of <inline-formula><tex-math id="M8">\begin{document}$5.8\times10^{12}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201833_M8.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201833_M8.png"/></alternatives></inline-formula> atoms/s/sr and a relative stability of 0.021%. In the experiment of precise spectral measurement based on atomic beam, the narrow longitudinal velocity distribution reduces the lateral Doppler broadening effect, and the lower longitudinal velocity also reasonably reduces the systematic error caused by the first-order Doppler effect. The atomic beam with such high intensity and stability in a single momentum and quantum state obviously improves the signal-to-noise ratio of the spectrum, and further reduces the statistical error of the results in the same detection time. Using this atomic beam, we demonstrated spectroscopy of the <inline-formula><tex-math id="M9">\begin{document}$2^3{\rm S}-2^3{\rm P}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201833_M9.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201833_M9.png"/></alternatives></inline-formula> transition of <inline-formula><tex-math id="M10">\begin{document}$^4{\rm{He}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201833_M10.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201833_M10.png"/></alternatives></inline-formula> under the condition of only 0.1% of the saturated intensity. At this time, the full width at half maximum of the spectral peak is almost close to the natural line width, but the spectral signal-to-noise ratio is still better than 400 and the frequency shift caused by the detection laser power can be less than 1 kHz. This kind of spectral detection at low power can effectively reduce the power-dependent frequency shift, thus obtaining more reliable detection results. This metastable helium atom beam experimental system can also be used as a reference for similar precision measurement experiments.

Bloch oscillations with a metastable helium Bose-Einstein condensate

We have observed Bloch oscillations of a ${}^{4}{\text{He}}^{*}$ Bose-Einstein condensate in an optical lattice at 1557.3 nm. Due to its low mass, metastable helium was efficiently accelerated orders of magnitude faster than demonstrated with other atoms. In a horizontal lattice, we could transfer a total of $800\ensuremath{\hbar}k$ of momentum by shuttling the atomic cloud back and forth 50 times between the $4\ensuremath{\hbar}k$ and $\ensuremath{-}4\ensuremath{\hbar}k$ momentum states with an efficiency of over 99% per Bloch cycle. In a vertical lattice, gravity-induced Bloch oscillations were demonstrated, from which the local gravitational acceleration was derived with a statistical uncertainty of $4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$. A clear advantage of ${\text{He}}^{*}$ over other atoms is that it can be detected with a microchannel plate detector with near unity efficiency, and this enabled observation of Bloch oscillations up to 12 s even though the number of atoms decreased by three orders of magnitude. These results establish ${\text{He}}^{*}$ as a promising candidate for future precision measurements with atom interferometry.

Determining the nature of quantum resonances by probing elastic and reactive scattering in cold collisions.

Scattering resonances play a central role in collision processes in physics and chemistry. They help build an intuitive understanding of the collision dynamics due to the spatial localization of the scattering wavefunctions. For resonances that are localized in the reaction region, located at short separation behind the centrifugal barrier, sharp peaks in the reaction rates are the characteristic signature, observed recently with state-of-the-art experiments in low-energy collisions. If, however, the localization occurs outside of the reaction region, mostly the elastic scattering is modified. This may occur due to above-barrier resonances, the quantum analogue of classical orbiting. By probing both elastic and inelastic scattering of metastable helium with deuterium molecules in merged-beam experiments, we differentiate between the nature of quantum resonances-tunnelling resonances versus above-barrier resonances-and corroborate our findings by calculating the corresponding scattering wavefunctions.

Hanbury Brown and Twiss Bunching of Phonons and of the Quantum Depletion in an Interacting Bose Gas.

We report the realization of a Hanbury Brown and Twiss (HBT)-like experiment with a gas of interacting bosons at low temperatures. The low-temperature regime is reached in a three-dimensional optical lattice and atom-atom correlations are extracted from the detection of individual metastable helium atoms after a long free fall. We observe, in the noncondensed fraction of the gas, a HBT bunching whose properties strongly deviate from the HBT signals expected for noninteracting bosons. In addition, we show that the measured correlations reflect the peculiar quantum statistics of atoms belonging to the quantum depletion and of the Bogoliubov phonons, i.e., of collective excitations of the many-body quantum state. Our results demonstrate that atom-atom correlations provide information about the quantum state of interacting particles, extending the interest of HBT-like experiments beyond the case of noninteracting particles.

Global 3D hydrodynamic modeling of absorption in Lyα and He 10830 A lines at transits of GJ3470b

Abstract Warm Neptune GJ3470b has been recently observed in 23S-23P transition of metastable helium, yielding absorption of about 1 per cent in Doppler velocity range of [-40; 10] km/s. Along with previous detection of absorption in Lyα with depth of 20–40 per cent in the blue and red wings of the line, it offers a complex target for simulation and testing of the current models. Obtained results suggest that absorption in both these lines comes from interaction of expanding upper planetary atmosphere with stellar plasma wind, allowing to constrain the stellar plasma parameters and the helium abundance in planet atmosphere.

Search for helium in the upper atmosphere of the hot Jupiter WASP-127 b using Gemini/Phoenix

Large-scale exoplanet search surveys have shown evidence that atmospheric escape is a ubiquitous process that shapes the evolution and demographics of planets. However, we lack a detailed understanding of this process because very few exoplanets that have been discovered to date could be probed for signatures of atmospheric escape. Recently, the metastable helium triplet at 1.083 μm has been shown to be a viable window for the presence of He-rich escaping envelopes around short-period exoplanets. Our objective is to use, for the first time, the Phoenix spectrograph to search for helium in the upper atmosphere of the inflated hot Jupiter WASP-127 b. We observed one transit and reduced the data manually since no pipeline is available. We did not find a significant in-transit absorption signal indicative of the presence of helium around WASP-127 b, and we set a 90% confidence upper limit for excess absorption at 0.87% in a 0.75 Å passband covering the He triplet. Given the large scale height of this planet, the lack of a detectable feature is likely due to unfavorable photoionization conditions for populating the metastable He I triplet. This conclusion is supported by the inferred low coronal and chromospheric activity of the host star and the old age of the system, which result in a relatively mild high-energy environment around the planet.

Direct observation of a Feshbach resonance by coincidence detection of ions and electrons in Penning ionization collisions

Observation of molecular dynamics with quantum state resolution is one of the major challenges in chemical physics. Complete characterization of collision dynamics leads to the microscopic understanding and unraveling of different quantum phenomena such as scattering resonances. Here we present an experimental approach for observing molecular dynamics involving neutral particles and ions that is capable of providing state-to-state mapping of the dynamics. We use Penning ionization reaction between argon and metastable helium to generate argon ion and ground state helium atom pairs at separation of several angstroms. The energy of an ejected electron carries the information about the initial electronic state of an ion. The coincidence detection of ionic products provides a state resolved description of the post-ionization ion-neutral dynamics. We demonstrate that correlation between the electron and ion energy spectra enables us to directly observe the spin-orbit excited Feshbach resonance state of HeAr+. We measure the lifetime of the quasi-bound HeAr+A2 state and discuss possible applications of our method.

The GAPS programme at TNG

Context. Exoplanets orbiting very close to their parent star are strongly irradiated. This can lead the upper atmospheric layers to expand and evaporate into space. The metastable helium (He I) triplet at 1083.3 nm has recently been shown to be a powerful diagnostic to probe extended and escaping exoplanetary atmospheres. Aims. We perform high-resolution transmission spectroscopy of the transiting hot Jupiter HD 189733 b with the GIARPS (GIANO-B + HARPS-N) observing mode of the Telescopio Nazionale Galileo, taking advantage of the simultaneous optical+near infrared spectral coverage to detect He I in the planet’s extended atmosphere and to gauge the impact of stellar magnetic activity on the planetary absorption signal. Methods. Observations were performed during five transit events of HD 189733 b. By comparison of the in-transit and out-of-transit GIANO-B observations, we computed high-resolution transmission spectra. We then used them to perform equivalent width measurements and carry out light-curves analyses in order to consistently gauge the excess in-transit absorption in correspondence with the He I triplet. Results. We spectrally resolve the He I triplet and detect an absorption signal during all five transits. The mean in-transit absorption depth amounts to 0.75 ± 0.03% (25σ) in the core of the strongest helium triplet component. We detect night-to-night variations in the He I absorption signal likely due to the transit events occurring in the presence of stellar surface inhomogeneities. We evaluate the impact of stellar-activity pseudo-signals on the true planetary absorption using a comparative analysis of the He I 1083.3 nm (in the near-infrared) and the Hα (in the visible) lines. Using a 3D atmospheric code, we interpret the time series of the He I absorption lines in the three nights not affected by stellar contamination, which exhibit a mean in-transit absorption depth of 0.77 ± 0.04% (19σ) in full agreement with the one derived from the full dataset. In agreement with previous results, our simulations suggest that the helium layers only fill part of the Roche lobe. Observations can be explained with a thermosphere heated to ~12 000 K, expanding up to ~1.2 planetary radii, and losing ~1 g s−1 of metastable helium. Conclusions. Our results reinforce the importance of simultaneous optical plus near infrared monitoring when performing high-resolution transmission spectroscopy of the extended and escaping atmospheres of hot planets in the presence of stellar activity.

Ab-initio reconstruction of metastable-induced electron-emission spectra (MIES) for molecules

We developed a quantum-chemical approach for ab initio calculations of metastable-induced electron-emission spectra (MIES). In contrast to earlier work, our calculation method accounts for the anisotropy of the MIES spectra with respect to not only the direction of incidence of metastable helium but also to that of electron detection. Since we can freely choose both, incidence and detection direction in our simulation, our method is able to reconstruct MIES spectra which are dominated by Auger deexcitation for any experimental setups and orientation of the studied molecules. Here, we study the dependence of the MIES spectrum on the incidence and detection direction and other experimental parameters and compare the results for the case of adsorbed benzene with experimental data.

Constraints on Metastable Helium in the Atmospheres of WASP-69b and WASP-52b with Ultranarrowband Photometry

Abstract Infrared observations of metastable 23S helium absorption with ground- and space-based spectroscopy are rapidly maturing, as this species is a unique probe of exoplanet atmospheres. Specifically, the transit depth in the triplet feature (with vacuum wavelengths near 1083.3 nm) can be used to constrain the temperature and mass-loss rate of an exoplanet’s upper atmosphere. Here, we present a new photometric technique to measure metastable 23S helium absorption using an ultranarrowband filter (FWHM 0.635 nm) coupled to a beam-shaping diffuser installed in the Wide-field Infrared Camera on the 200 inch Hale Telescope at Palomar Observatory. We use telluric OH lines and a helium arc lamp to characterize refractive effects through the filter and to confirm our understanding of the filter transmission profile. We benchmark our new technique by observing a transit of WASP-69b and detect an excess absorption of 0.498% ± 0.045% (11.1σ), consistent with previous measurements after considering our bandpass. We then use this method to study the inflated gas giant WASP-52b and place a 95th percentile upper limit on excess absorption in our helium bandpass of 0.47%. Using an atmospheric escape model, we constrain the mass-loss rate for WASP-69b to be ( ) at 7000 K (12,000 K). Additionally, we set an upper limit on the mass-loss rate of WASP-52b at these temperatures of ( ). These results show that ultranarrowband photometry can reliably quantify absorption in the metastable helium feature.

Speed-adjustable atomic beam of metastable helium for precision measurement

A bright beam of metastable helium atoms is useful in various studies. Laser spectroscopy of helium in an atomic beam can provide a determination of the fine-structure constant and the nuclear charge radius, towards a test of quantum electrodynamics (QED). In these measurements, it is necessary to control not only the internal quantum state but also the translation motion of the helium atom. Here we present a setup producing an intense and speed-adjustable continuous beam of helium atoms. By using a combination of laser cooling, focusing, and deflection, helium atoms are prepared at the single quantum state of ${2}^{\phantom{\rule{0.16em}{0ex}}3}{S}_{1}$ ($m=0$), with a very narrow distribution of the longitudinal speed ($\ensuremath{\delta}{v}_{z}l\ifmmode\pm\else\textpm\fi{}4.5$ m/s). At the same time, we obtain a flux of $1.8\ifmmode\times\else\texttimes\fi{}{10}^{13}$ (atoms/s)/sr with a fractional fluctuation below 0.02%. The atomic-beam quality was investigated by laser spectroscopy, indicating a considerable improvement over those used in previous helium precision-spectroscopy measurements.

Laser spectroscopy of pionic helium atoms.

Charged pions1 are the lightest and longest-lived mesons. Mesonic atoms are formed when an orbital electron in an atom is replaced by a negatively charged meson. Laser spectroscopy of these atoms should permit the mass and other properties of the meson to be determined with high precision and could place upper limits on exotic forces involving mesons (as has been done in other experiments on antiprotons2-9). Determining the mass of the π- meson in particular could help to place direct experimental constraints on the mass of the muon antineutrino10-13. However, laser excitations of mesonic atoms have not been previously achieved because of the small number of atoms that can be synthesized and their typically short (less than one picosecond) lifetimes against absorption of the mesons into the nuclei1. Metastable pionic helium (π4He+) is a hypothetical14-16 three-body atom composed of a helium-4 nucleus, an electron and a π- occupying a Rydberg state of large principal (n≈16) and orbital angular momentum (l≈n-1) quantum numbers. The π4He+ atom is predicted to have an anomalously long nanosecond-scale lifetime, which could allow laser spectroscopy to be carried out17. Its atomic structure is unique owing to the absence of hyperfine interactions18,19 between the spin-0 π- and the 4He nucleus. Here we synthesize π4He+ in a superfluid-helium target and excite the transition (n,l)=(17,16)→(17,15) of the π--occupied π4He+ orbital at anear-infrared resonance frequency of 183,760 gigahertz. The laser initiates electromagnetic cascade processes that end with the nucleus absorbing the π- and undergoing fission20,21. The detection of emerging neutron, proton and deuteron fragments signals the laser-induced resonance in the atom, thereby confirming the presence of π4He+. This work enables the use of theexperimental techniques of quantum opticsto study ameson.

Evidence for He i 10830 Å Absorption during the Transit of a Warm Neptune around the M-dwarf GJ 3470 with the Habitable-zone Planet Finder

Abstract Understanding the dynamics and kinematics of outflowing atmospheres of hot and warm exoplanets is crucial to understanding the origins and evolutionary history of the exoplanets near the evaporation desert. Recently, ground-based measurements of the meta-stable helium atom’s resonant absorption at 10830 Å has become a powerful probe of the base environment which is driving the outflow of exoplanet atmospheres. We report evidence for the He i 10830 Å in absorption (equivalent width ∼0.012 ± 0.002 Å) in the exosphere of a warm Neptune orbiting the M-dwarf GJ 3470, during three transits using the Habitable Zone Planet Finder near-infrared spectrograph. This marks the first reported evidence for He i 10830 Å atmospheric absorption for a planet orbiting an M-dwarf. Our detected absorption is broad and its blueshifted wing extends to −36 km s−1, the largest reported in the literature to date. We modeled the state of helium atoms in the exosphere of GJ3470b based on assumptions on the UV and X-ray flux of GJ 3470, and found our measurement of flux-weighted column density of meta-stable state helium , derived from our transit observations, to be consistent with the model, within its uncertainties. The methodology developed here will be useful to study and constrain the atmospheric outflow models of other exoplanets like GJ 3470b, which are near the edge of the evaporation desert.

Transitions between States of the Hyperfine Structure of Antiprotonic $${}^{\mathbf{4}}$$He in Collisions with Atoms of the Medium: Interaction Ab Initio

Collisions of the metastable antiprotonic helium atoms with the atoms of the medium cause, among other processes, transitions between hyperfine structure (HFS) states, as well as shifts and broadening of microwave M1 spectral lines. To obtain the interaction potential matrix ( $$\bar{p}$$ $$\bar{He}^{+}){-}\textrm{He}$$ , the potential energy surface (PES) is calculated by the unrestricted Hartree–Fock method with consideration of the electron correlations within the second-order of perturbation theory (MP2). With this potential, the system of equations of close coupling of the HFS channels is numerically solved; the cross sections and transition rates, shifts, and broadenings of M1 spectral lines are calculated; they are then used to solve the kinetic equation that determines the time evolution of the HFS state density matrix. The results of these calculations are compared with the experimental data and with the results of model calculations.