Metastable Helium Research Articles

Observation of the isotope effect in sub-kelvin reactions

Quantum phenomena in the translational motion of reactants, which are usually negligible at room temperature, can dominate reaction dynamics at low temperatures. In such cold conditions, even the weak centrifugal force is enough to create a potential barrier that keeps reactants separated. However, reactions may still proceed through tunnelling because, at low temperatures, wave-like properties become important. At certain de Broglie wavelengths, the colliding particles can become trapped in long-lived metastable scattering states, leading to sharp increases in the total reaction rate. Here, we show that these metastable states are responsible for a dramatic, order-of-magnitude-strong, quantum kinetic isotope effect by measuring the absolute Penning ionization reaction rates between hydrogen isotopologues and metastable helium down to 0.01 K. We demonstrate that measurements of a single isotope are insufficient to constrain ab initio calculations, making the kinetic isotope effect in the cold regime necessary to remove ambiguity among possible potential energy surfaces.

Observation and measurement of an extra phase shift created by optically detuned light storage in metastable helium

Electromagnetically induced transparency (EIT) in metastable helium at room temperature is experimentally shown to exhibit light storage capabilities for intermediate values of the detuning between the coupling and probe beams and the center of the atomic Doppler profiles. An additional phase shift is shown to be imposed to the retrieved pulse of light when the EIT protocol is performed at non-zero optical detunings. The value of this phase shift is measured for different optical detunings between 0 and 2 GHz, and its origin is discussed.

Observation of the spin polarization of Cs atoms during optical orientation of metastable helium atoms in alkali-helium plasma

We are the first to experimentally observe a magnetic-resonance signal of 62S1/2 Cs atoms by absorption of light from a helium lamp that was used to optically orient metastable 23S1 He atoms. The amplitude of the cesium signal proved to be almost three orders of magnitude lower than the amplitude of the magnetic resonance of 23S1 He atoms. Particular features of the creation and observation of the collisional polarization of cesium under conditions of alkali-helium plasma have been discussed.

Diagnostics of an O2–He RF Atmospheric Plasma Discharge by Spectral Emission

In this paper optical emission spectroscopy (OES) is used as a Diagnostic technique for the measurement of atomic and molecular spectral emissions generated using a helium rf industrial atmospheric plasma jet system. The OES of neutral atomic spectral lines and molecular bands are investigated over a range of plasma process parameters. Wavelength resolve optical emission profiles suggest that the emission of helium’s spectral lines shows that the high energy electrons have a larger influence than helium metastables on the overall spectral emission. Furthermore, the experimental data indicates that the use of high helium flow rates, in any confined open air plasma discharge, limits the significance of air impurities, e.g., nitrogen, for the creation and sustainability of plasma discharges in helium–oxygen gas chemistry.

Mass Spectrometry of Spacecraft Contamination Using Direct Analysis in Real-Time Ion Source

Spacecraft contamination was analyzed using mass spectrometry with the direct analysis in real-time ionization source. This source uses metastable helium for soft ionization and to mediate atmospheric desorption of samples into a mass spectrometer. The sampling methodology allows polymers to be assessed for the presence of vacuum labile components. Vacuum labile residues are significant sources of contamination on spacecraft optics, science instruments, and thermal control surfaces. The methodology also provides sensitive analysis of molecular contamination on spacecraft surfaces using existing spacecraft sampling procedures. This provides identification information for a wide range of molecular components including biomarker compounds.

Fe∖MgO∖Cu-Phthalocyanine 복합구조 계면구조와 그 전자기적 특성

MgO 기반 스핀소자에 유기장벽 Cu-Phthalocyanine(CuPc)가 삽입된 무기<TEX>${\backslash}$</TEX>유기 터널 접합 소자 Fe<TEX>${\backslash}$</TEX>MgO(001)<TEX>${\backslash}$</TEX>CuPc<TEX>${\backslash}$</TEX>Co의 자기 저항 현상과 그 계면 특성의 상관관계에 대한 연구가 진행되었다. 특히 1.6 nm MgO(001)<TEX>${\backslash}$</TEX>x nm CuPc(x = 0~5) 계면의 전자기적 특성을 스핀 편극된 준안정상태 He 원자 분광계(Metastable Helium De-excitation Spectroscopy, MDS)를 이용하여 규명하였다. 에피 성장된 MgO(001) 위에 적층된 약 1.6 nm 두께의 CuPc 층상구조의 표면에서, MgO(001) 하지층의 표면과는 달리, up-spin band와 down-spin band의 비대칭성이 현저해지는 것으로 관찰되었다. 이 결과는 실온과 저온(77 K)에서 ~10 %와 30 %로 각각 측정된 자기저항 현상과 복합장벽을 통과하는 스핀거동을 이해하는데 중요한 단초를 제공해 준다. The influence of insertion of an ultra-thin Cu-Phthalocyanine (CuPc) between MgO barrier and ferromagnetic layer in magnetic tunnel juctions (MTJs) was investigated. In order to understand the relation between the electronic and structural properties of Fe<TEX>${\backslash}$</TEX>MgO<TEX>${\backslash}$</TEX>CuPc, the surface (or interface) analysis was carried out systematically by using spin polarized metastable He de-excited spectroscopy for the CuPc films grown on the Si(001)<TEX>${\backslash}$</TEX>5 nm MgO(001)<TEX>${\backslash}$</TEX>7 nm Fe(001)<TEX>${\backslash}$</TEX>1.6 nm MgO(001) multilayer structure as the thickness of CuPc increases from 0 to 5 nm. In particular, for the 1.6 nm CuPc surface, a rather strong spin asymmetry between up- and down-spin band appears while it becomes weaker or disappears for the CuPc films thinner or thicker than ~1.6 nm. Our results emphasize the importance of the interfacial electronic properties of organic layers in the spin transport of the hybrid MTJs.

On-line coupling of macroporous resin column chromatography with direct analysis in real time mass spectrometry utilizing a surface flowing mode sample holder

A surface flowing mode sample holder was designed as an alternative sampling strategy for direct analysis in real time mass spectrometry (DART-MS). With the sample holder, the on-line coupling of macroporous resin column chromatography with DART-MS was explored and the new system was employed to monitor the column chromatography elution process of Panax notoginseng. The effluent from macroporous resin column was first diluted and mixed with a derivatization reagent on-line, and the mixture was then directly transferred into the ionization region of DART-MS by the sample holder. Notoginsenosides were methylated and ionized in a metastable helium gas stream, and was introduced into MS for detection. The on-line system showed reasonable repeatability with a relative standard deviation of 12.3% for the peak area. Three notoginsenosides, i.e. notoginsenoside R1, ginsenoside Rb1 and ginsenoside Rg1, were simultaneously determined during the eluting process. The alteration of the chemical composition in the effluent was accurately identified in 9min, agreeing well with the off-line analysis. The presented technique is more convenient compared to the traditional UPLC method. These results suggest that the surface flowing mode DART-MS has a good potential for the on-line process monitoring in the pharmaceutical industry.

Multiple (eight) plasma bullets in helium atmospheric pressure plasma jet and the role of nitrogen

As many as eight multiple plasma bullets produced at atmospheric pressure were observed in one voltage period in a capillary helium dielectric barrier plasma jet. We found that the number of the bullets strongly depends on the nitrogen fraction added to the helium supply gas. Using optical emission spectroscopy and ionization rate calculation, this study demonstrates that nitrogen gas plays an important role in the generation and dynamics of multiple plasma bullets through Penning ionization of nitrogen by helium metastables.

Tune-out wavelengths for metastable helium

The six longest tune-out wavelengths for the He($1s2s{\phantom{\rule{4pt}{0ex}}}^{3}{S}_{1}^{e}$) metastable state are determined by explicit calculation. The tune-out wavelength at 413.02 nm is expected to be sensitive to finite mass, relativistic, and quantum electrodynamic effects upon the transition matrix elements and its measurement would provide a nonenergy test of fundamental atomic structure theory.

Luminescence of Oxygen Atoms Stimulated by Metastable Helium at Cryogenic Temperatures

We present investigations of the afterglow of oxygen-helium gas mixtures at cryogenic temperatures. The cooling of a helium jet containing trace amounts of oxygen after passing through a radio frequency discharge zone led to the observation of strong emissions from atomic oxygen. The effect results from the increasing efficiency of energy transfer from metastable helium atoms and molecules to oxygen impurities in the cold dense helium vapor. This effect might find an application for the detection of small quantities of the impurities in helium gas.

Deceleration and trapping of a fast supersonic beam of metastable helium atoms with a 44-electrode chip decelerator

Deceleration and trapping of a fast supersonic beam of metastable helium atoms with a 44-electrode chip decelerator

Deceleration and trapping of a fast supersonic beam of metastable helium atoms with a 44-electrode chip decelerator

A surface-electrode decelerator consisting of 44 electrodes has been used to decelerate supersonic beams of helium Rydberg atoms moving with an initial velocity of 1200 m/s. Prior to the deceleration, the helium atoms were excited from the $1s2s{\phantom{\rule{0.16em}{0ex}}}^{1}{S}_{0}$ metastable state to selected Rydberg-Stark states with a narrow-band tunable UV laser. Complete deceleration could be achieved over a distance of 36 mm and in 60 $\ensuremath{\mu}$s, corresponding to an acceleration of $\ensuremath{-}2.0\ifmmode\times\else\texttimes\fi{}{10}^{7}$ m/s${}^{2}$. After deceleration, the atoms were held in stationary electric traps above the chip surface, reaccelerated off the chip, and detected by pulsed electric-field ionization. The decelerator was also used to generate helium Rydberg-atom beams with a final velocity tunable between 0 and 1200 m/s. Transitions between low- and high-field-seeking Rydberg-Stark states were observed during trap loading and are attributed to adiabatic Landau-Zener dynamics at electric fields exceeding the Inglis-Teller field. By comparing the experimental results with the results of particle-trajectory simulations, the velocity distribution of the decelerated atoms was found to be characterized by temperatures ranging between 50 and 200 mK, depending on the magnitude of the electric dipole moment of the Rydberg-Stark states selected prior to the deceleration.

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.

Polarization-dependent manipulation of optical properties in a tripod system

We analyze the dependence of the transmission profiles of an atom in a tripod configuration on the polarizations of the coupling and the probe beams and use room-temperature metastable helium (${}^{4}$He*) as a model system. We show that, by rotating the orthogonally polarized coupling-probe beams with respect to an applied small magnetic field, one can manipulate the detuned peaks due to electromagnetically induced transparency [Kumar, Laupr\^etre, Ghosh, Bretenaker, and Goldfarb, Phys. Rev. A 84, 023811 (2011)] and the central peak arising because of ground-state coherent population oscillations [Laupr\^etre, Kumar, Berger, Faoro, Ghosh, Bretenaker, and Goldfarb, Phys. Rev. A 85, 051805(R) (2012)] observed earlier separately. Our experimental results match well with our numerical simulation using the Floquet method.

Scalar Magnetometers for Space Applications

A survey of existing instrumentation and developments is presented emphasizing instrumentation for in-flight calibration of vector magnetometers on magnetic mapping missions. Proton free or forced precession magnetometers are at the focus as calibration references, because the proton gyromagnetic ratio is a basic atomic constant for the SI units of magnetic field and electric current. The classical proton free precession, the Overhauser forced oscillation and a new field cycling Overhauser magnetometer are presented. Alkali metal vapor magnetometers, although not absolute in the same sense as the classical proton magnetometer, offer stability and resolution well suited for the calibration purposes. Recent developments are discussed. The metastable Helium magnetometer also offers quasi-absolute scalar measurements, and the use of semiconductor tuned lasers replacing an RF-excited Helium lamp holds great promise for improved accuracy and reduced power consumption.

Photodissociation and Radiative Association of HeH+ in the Metastable Triplet State

We investigate the photodissociation of HeH(+) in the metastable triplet state as well as its formation through the inverse process, radiative association. In models of astrophysical plasmas, HeH(+) is assumed to be present only in the ground state, and the influence of the triplet state has not been explored. It may be formed by radiative association during collisions between a proton and metastable helium, which are present in significant concentrations in nebulae. The triplet state can also be formed by association of He(+) and H, although this process is less likely to occur. We compute the cross sections and rate coefficients corresponding to the photodissociation of the triplet state by UV photons from a central star using a wave packet method. We show that the photodissociation cross sections depend strongly on the initial vibrational state and that the effects of excited electronic states and nonadiabatic couplings cannot be neglected. We then calculate the cross section and rate coefficient for the radiative association of HeH(+) in the metastable triplet state.

Real Time in Situ Chemical Characterization of Submicrometer Organic Particles Using Direct Analysis in Real Time-Mass Spectrometry

Direct analysis in real time mass spectrometry (DART-MS) is used to analyze the surface chemical composition of nanometer-sized organic aerosol particles in real time at atmospheric pressure. By introducing a stream of particles in between the DART ionization source and the atmospheric pressure inlet of the mass spectrometer, the aerosol is exposed to a thermal flow of helium or nitrogen gas containing some fraction of metastable helium atoms or nitrogen molecules. In this configuration, the molecular constituents of organic particles are desorbed, ionized, and detected with reduced molecular ion fragmentation, allowing for compositional identification. Aerosol particles detected include alkanes, alkenes, acids, esters, alcohols, aldehydes, and amino acids. The ion signal produced by DART-MS scales with the aerosol surface area rather than volume, suggesting that DART-MS is a viable technique to measure the chemical composition of the particle interface. For oleic acid, particle size measurements of the aerosol stream exiting the ionization region suggest that the probing depth depends upon the desorption temperature, and the probing depth is estimated to be on the order of 5 nm for a 185 nm diameter particle at a DART heater temperature of 500 °C with nitrogen as the DART gas. The reaction of ozone with submicrometer oleic acid particles is measured to demonstrate the ability of this technique to identify products and quantify reaction rates in a heterogeneous reaction.

The Unusual Absorption Line Spectrum of Quasar SDSS J2220+0109

We report the Balmer broad absorption lines (BALs) in the quasar SDSS J2220+0109 discovered from the SDSS data, and present a detailed analysis of the peculiar absorption line spectrum, including the He I* multiplet at λλ3189, 3889 arising from the metastable 23s-state helium and the Balmer Hα and Hβ lines from the excited hydrogen H I of n=2 level, which are rarely seen in quasar spectra, as well as many absorption lines arising from the excited Fe II* of the levels 7 955 cm−1, 13 474 cm−1 and 13 673 cm−1 in the wavelength range 3100∼3300 Å. Ca II H, K absorption line doublets also clearly appear in the SDSS spectrum. All absorption lines show a similar blueshifted velocity structure of Δv ≈−1500 ∼ 0 km·s−1 relative to the quasar's systematic redshift determined from the emission lines. Detailed analysis suggests that the Balmer absorption lines should arise from the partially ionized region with a column density of NHI ≈ 1021 cm−2 for an electron density of ne ∼ 106 cm−3; and that the hydrogen n=2 level may be populated via collisional excitation with Lyα pumping.In addition to its peculiar absorption features, SDSS J2220+0109 also shows unusual Fe II emission lines very likely originated from the low-density gas of ne≈106cm−3.

Third-order spatial correlations for ultracold atoms

We present here the first measurement of the third-order spatial correlation function for atoms, made possible by cooling a metastable helium cloud to create an ultracold thermal ensemble just above the Bose–Einstein condensation point. The resulting large correlation length well exceeds the spatial resolution limit of the single-atom detection system, and enables extension of our earlier temporal measurements to evaluate the third-order correlation function in the spatial plane of the detector. The enhancement of the spatial third-order correlation function above a value of unity demonstrates the presence of spatial three-atom bunching, as expected for an incoherent source.

Production of metastable 23S1 helium in a laser produced plasma at low pressures

We used cavity ringdown spectroscopy to study how the laser ablation of various materials (graphite, aluminum and iron) influences production of 23S1 metastable helium in the regime of low pressure helium gas. Plasma was produced by nanosecond Nd:YAG laser at 1064nm with irradiance from 0.8 to 10.9×109W/cm2. Helium pressure ranged from 0.03 to 0.2mbar. It was found that at higher irradiances, metastable production for all materials reaches saturation due to plasma shielding and is more pronounced for graphite. The highest yield of metastables was found in the case of aluminum target (0.05%). For metals, production rate is discussed in terms of work function and ionization potential.