Metastable Helium Atoms Research Articles

Giant helium dimers produced by photoassociation of ultracold metastable atoms.

We produce giant, purely long-range helium dimers by photoassociation of metastable helium atoms in a magnetically trapped, ultracold cloud. The photoassociation laser is detuned close to the atomic 2(3)S1-2(3)P0 line and produces strong heating of the sample when resonant with molecular bound states. The temperature of the cloud serves as an indicator of the molecular spectrum. We report good agreement between our spectroscopic measurements and our calculations of the five bound states belonging to a 0(+)(u) purely long-range potential well. These previously unobserved states have classical inner turning points of about 150a(0) and outer turning points as large as 1150a(0).

Effects of impurity molecules on the lifetime of antiprotonic helium atoms

Quenching of metastable antiprotonic helium atoms in collisions with hydrogen and deuterium molecules has been studied using laser spectroscopy at CERN’s antiproton decelerator. The temperature dependence of the quenching cross sections of the antiprotonic states ( n, l)=(37,34), (38,35) and (38,37) has been investigated and a deviation from the Arrhenius law was found at low temperatures. In case of the state (38,37) with deuterium, detailed measurements revealed that the quenching cross section levels off at low temperatures indicating a strong quantum tunneling effect.

Measurement of the fluorine atom concentration in a carbon tetrafluoride and helium atmospheric-pressure plasma

The fluorine atom concentration has been measured in the downstream region of a low-temperature, atmospheric-pressure plasma fed with 739.0 Torr helium and 12.6 Torr carbon tetrafluoride (3.1×1017 cm−3). The fluorine atoms were titrated with H2 molecules, and the HF reaction product was detected by infrared spectroscopy. The radio-frequency gas discharge produced 1.2×1015 cm−3 of F atoms, which was about two orders of magnitude higher than that found in low-pressure plasmas. The average electron density and temperature in the plasma was estimated to be 6.1×1011 cm−3 and 2.5 eV, respectively. A numerical model of the plasma indicated that most of the fluorine atoms were generated by the reaction of CF4 with metastable helium atoms.

Theory of metastable deexcitation spectroscopy on simple metals

In this work we investigate the Auger deexcitation process of a slow metastable helium atom $({\mathrm{He}}^{*})$ in its triplet state on a Na surface. The electronic properties and the Auger rates of the interacting atom-metal system are calculated self-consistently at any distance of ${\mathrm{He}}^{*}$ from the surface in the framework of the local spin-density approximation using the embedding Green's function method. The energy distribution of the emitted electrons is obtained by integrating the Auger rates along the classical trajectory in the adiabatic approximation. The analysis of the spin-resolved local density of states shows that the excited atomic states broaden into resonances already at large atom-surface distances. The calculated atom deexcitation rates are in good agreement with experimental results.

Microlithography Using Metastable Helium Atoms: Patterning of Gold Film Coated with Dodecanethiol Self-Assembled Monolayers on Mica

In this paper, we report the establishment of an experimental setup and the procedure used for metastable atom lithography. Using a helium metastable atom beam and dodecanethiol (DDT) self-assembled monolayer (SAM), we carried out the patterning of the gold film on muscovite mica having a flat surface. Large-area intact and flat patterns with a nanoscale width of ∼80–110 nm, of the etched step were obtained. From the analysis of the roughness, it was determined that no remaining gold islands were present in the area exposed to the He* atom beam under the optimized experimental parameters: 40–60 min. of exposure time and 8–10 min. of etching time. In addition, the presence of some flaws in the patterns was considered to be due to the scratches on mica substrate.

Characterization of nanoscale patterns prepared by metastable helium atom beam and butanethiol self-assembled monolayers

By using a helium metastable atom beam and butanethiol (BT) self-assembled monolayers (SAM), the metastable atom lithography to the gold film on muscovite mica having a flat surface was carried out. Large-area intact and clean patterns with a nanoscale width of the edge, approximately 50–70 nm, were obtained. Meanwhile, we found that the shorter chain molecules like the BT can be used as resist and may result in sharper edge of patterns, but the roughness of gold patterns with the BT SAM is larger than that with the Octanethiol (OT) SAM and similar to that with the Dodecanethiol (DDT) SAM. This observation can be interpreted by the presence of the liquid phase in the short-chain homologues. All information indicated that the BT SAMs on atomically flat surfaces could be used as a resist for exposure to metastable atom beams.

Magneto-optical trap for metastable helium at 389 nm

We have constructed a magneto-optical trap (MOT) for metastable triplet helium atoms utilizing the $2{}^{3}{S}_{1}\ensuremath{\rightarrow}3{}^{3}{P}_{2}$ line at 389 nm as the trapping and cooling transition. The far-red-detuned MOT (detuning $\ensuremath{\Delta}=\ensuremath{-}41\mathrm{MHz})$ typically contains few times ${10}^{7}$ atoms at a relatively high $(\ensuremath{\sim}{10}^{9}{\mathrm{cm}}^{\ensuremath{-}3})$ density, which is a consequence of the large momentum transfer per photon at 389 nm and a small two-body loss rate coefficient $(2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}{\mathrm{cm}}^{3}/\mathrm{s}<\ensuremath{\beta}<1.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}{\mathrm{cm}}^{3}/\mathrm{s}).$ The two-body loss rate is more than five times smaller than in a MOT on the commonly used $2{}^{3}{S}_{1}\ensuremath{\rightarrow}2{}^{3}{P}_{2}$ line at 1083 nm. Furthermore, laser cooling at 389 nm results in temperatures somewhat lower than those achieved using 1083 nm. The 389-nm MOT exhibits small losses due to two-photon ionization, which have been investigated as well.

Effects of discharge current and voltage on the high density of metastable helium atoms

Both hollow-cathode and Penning-type discharges were adopted to excite helium atoms to a metastable state. Experimental data indicate that Penning discharge is more suitable for generating high fractions of metastables in a low-density helium beam for laser-induced fluorescence technique in measuring electric fields at the edge of a plasma. The metastable density increases with increasing helium gas pressure in the range of 1.33×10-2-66.7Pa. The highest metastable density of 3.8×1016m-3 is observed at a static gas pressure of 66.7Pa. An approximately linear relationship between the density of metastable helium atoms and the plasma discharge current is observed. Magnetic field plays a very important role in producing a high density of metastable atoms in Penning discharge.

Metastable-Atom-Induced Dissociation of Dodecanethiolate Self-Assembled Monolayers on Gold Substrates

Emission of H + and CH X + is observed for dodecanethiolate SAMs on Au( 11) surfaces under irradiation of the slow helium metastable atom beam at a thermal energy. Metastable atom deexcitation spectroscopy measurements show that impinging metastable helium atoms predominantly interact with ap orbitals constituted mainly of carbon 2p and hydrogen 1s atomic states of the methyl group at the top end of the alkane chain of the dodecanethiolate molecule. These results indicate that the hole created in the ap orbitals leads not only to C-H bond scission followed by H + desorption but also to C-C bond scission between the methyl group and the neighboring carbon atom followed by CH x + desorption.

Insulated surface discharge for metastables driven processing at atmospheric pressure

The insulated surface discharge (ISD) in helium under atmospheric pressure operated with voltage of 1–3 kV in frequency range from 1 to 10 kHz is investigated. The influence of the insulating film on the operation of ISD is discussed. The presence of poly-vinyl-chloride (PVC) foil or silicon polymer film on the surface of the discharge electrode prolongs the lifetime of metastable excited helium atoms up to 17 or 50 μs, respectively.

Using ion production to monitor the birth and death of a metastable helium Bose Einstein condensate

We discuss observations of the ion flux from a cloud of trapped 23S1 metastable helium atoms. Both Bose–Einstein condensates (BEC) and thermal clouds were investigated. The ion flux is compared with time-of-flight observations of the expanded cloud. We show data concerning BEC formation and decay, as well as measurements of two-and three-body ionization rate constants. We also discuss possible improvements and extensions of our results.

A metastable-induced electron spectroscopy study on the process of oxygen adsorption at a Ni(1 1 0) surface

The electronic structure of oxygen-adsorbed Ni(110) surfaces has been studied by metastable-induced electron spectroscopy (MIES). Helium metastable atoms are de-excited both at clean and at oxygen-adsorbed Ni(110) surfaces through the process of resonance ionization followed by Auger neutralization (AN). The intensity of electron emission due to AN was increased by lower exposures of oxygen (∼0.5l), which is attributed to an enhanced contribution of sp-state electrons at the Ni surface to the AN process. At moderate exposures of oxygen (1–3l) where the low-energy electron diffraction (LEED) pattern showed (2×1 ) reconstruction, the MIES spectra exhibited an O 2p-induced peak, caused by inter-atomic Auger transition between O 2p and Ni 3d states. With increasing exposure to 4l, the LEED pattern changed to (3×1 ), while the O 2p-induced peak position moved to lower energies. Variations in the local electronic structure of oxidized top-layers of the Ni(110) surface and the process of metastable-induced electron emission are discussed in connection with the oxygen-induced structural change.

High-density metastable helium atoms produced by Penning-type discharges

A high density of metastable helium atoms has been created using a Penning-type discharge. Experimental data indicate that the metastable density increases with the increase in the helium gas pressures in the range from 0.1 mTorr to 0.5 Torr. The highest metastable density of 3.5×1016 m−3 is observed at a steady-state gas pressure of 0.5 Torr. The relationships between the densities of the metastable helium atoms and the plasma discharge current are also obtained.Based on the results obtained using a spectral absorption technique and a theoretical collisional–radiative model, the data of the helium plasma spectroscopic emissions have been used in diagnosing plasma electron density and electron temperature. The results suggest the possibility of generating high-density metastable atoms with a high discharge current.

Patterning of gold film on muscovite mica by using a helium-metastable atom beam and an octanethiol self-assembled monolayer

Using a helium-metastable atom beam and an octanethiol (OT) self-assembled monolayer (SAM), we carried out atom lithography to the gold film on muscovite mica having a flat surface. A large intact area and a clear pattern with a nanoscale width ∼80–100 nm of the etched step was obtained. The gold surface, as well as the surface of the mica substrate, was flat. From the analysis of the roughness, appreciable gold islands were not detected in the area exposed to the He* atom beam. This finding demonstrates that the OT SAMs on atomically flat surfaces can be used as a resist for exposure to metastable-atom beams.

Metastable-atom-stimulated desorption from hydrogen-passivated silicon surfaces

Ion desorption stimulated by a slow metastable helium atom beam is investigated for hydrogen-passivated Si(1 1 1) surfaces using a pulsed discharge metastable-atom source, which also generates photons (He I) and fast neutrals (He 0). The time-of-flight spectrum of desorbed positive ions shows peaks corresponding to H + stimulated by metastable-helium atoms and to ions sputtered by fast neutrals but no peak relevant to the photons. The lack of photon-stimulated desorption excludes the possible contribution of secondary electrons to the metastable-atom-stimulated desorption (MSD). The narrow energy distribution of MSD ions and the metastable atom deexcitation spectroscopy measurement prove that the MSD is induced by electronic transitions initiated by the abstraction of electrons from the valence band of the silicon hydrides during the Auger-type decay of metastable helium atoms.

Analog Cherenkov detectors used in laser spectroscopy experiments on antiprotonic helium

We describe some acrylic Cherenkov detectors read out by gateable fine-mesh photomultipliers, used in laser spectroscopy experiments of metastable antiprotonic helium ( p ̄ He + ) atoms carried out at the LEAR and AD facilities at CERN. The atoms were produced by stopping pulsed antiproton beams in a helium target. Charged particles emerging from the antiproton annihilations produced Cherenkov light in the detector, the time envelope of which consisted of a strong flash from the promptly-annihilating antiprotons, followed by a much longer but less intense tail from the delayed annihilations of the metastable atoms . The photomultiplier was turned off during the initial light flash by reversing the electric potential on its dynodes, thus allowing only the delayed annihilations to be recorded as an analog pulse. The atoms were irradiated with a laser pulse tuned to the characteristic wavelength which stimulated antiproton transitions from a metastable state to a state with a short lifetime against annihilation. The resonance condition between the laser beam and the atom was thus revealed as a sharp spike superimposed on the Cherenkov light pulse. We tested Cherenkov radiators of various sizes and different types of photomultipliers in order to (i) suppress the spurious afterpulsing in the photomultiplier signal caused by the strong flash of Cherenkov light during the prompt annihilation, (ii) evaluate the background caused by π + →μ + →e + decay, and (iii) optimize the linearity, dynamic range, and response time of the detector.

Perturbation calculations of the van der Waals potentials of He(2 3S, 2 1S) metastable atoms with rare gas atoms

Perturbative expressions are presented for calculating the adiabatic potential curves between metastable He(2 3S, 2 1S) helium atoms and the rare gas atoms Ne, Ar, Kr and Xe. The first-order terms are obtained by assuming that the main contribution comes from the 2s-wave electron of the metastable helium atom and the s-wave electrons in the outermost shell of the rare gas atom. With the second-order van der Waals terms modified with Tang–Toennies damping functions, a simple analytical expression is obtained for the entire potential curves. The van der Waals well depths and positions are in good agreement with the experiments.

Energy Transfer Kinetics of N2(X ,v) and SiH4

We have studied the kinetics of N2(X,v) with silane. These experiments were done in a discharge-flow reactor and used N2(X,v) diagnostics employing metastable energy transfer reactions. To investigate vibrational levels greater than five, and up to at least thirteen, we analyze N2(B3Πg,v‘≤12) fluorescence excited in the interaction between metastable N2(A3 ) and N2(X1 ,v). N2+ (B2 ,v‘=0−8) fluorescence, excited when metastable helium atoms collide with N2(X,v), diagnoses vibrational levels between one and six. The rate coefficients for quenching vibrationally excited nitrogen by silane range from 1 × 10-13 to 6 × 10-13 cm3 molecule-1 s-1, being higher for the higher vibrational levels. Observations at 4500 nm show that at least one channel for the N2(X,v) quenching process involves excitation of SiH4(ν3). Comparison of the relative excitation rate of SiH4(ν3) with that for N2O(ν3) excitation when N2O is added to the reactor, however, indicates that at most 15% of the N2(X,v) quenching events result in ν3 excitation. Presumably the rest of the energy is absorbed by other vibrational modes.

He(11S)–He(23S)collision and radiative transition at low temperatures

There has been considerable recent interest in low-temperature interrogation of quantum helium nanodroplets. We consider the interaction of helium ground and metastable states, employing the best known molecular dimer potentials. Radiative transitions between different metastable rovibrational levels are compared with the discharge emission transition lines and good agreement is found. Collisional interactions between helium ground and metastable atoms are studied and elastic scattering, metastability excitation transfer and diffusion cross sections and rate coefficients are calculated at ultralow energies. The integral elastic cross section at ultralow energies is about $5000 {\mathrm{\AA{}}}^{2}.$ Effect of shape resonances that form in the potential barriers, on the cross sections is investigated. We find that some shape resonances live for longer than 100 s. Long-range potential wells could support ``shelf'' levels. We discuss their collisional and spectroscopic properties.

Large-angle adjustable coherent atomic beam splitter by Bragg scattering

Using a ``monochromatic'' (single-axial-velocity) and slow (250 m/s) beam of metastable helium atoms, we realize up to eighth-order Bragg scattering and obtain a splitting angle of 6 mrad at low laser power (3 mW). This corresponds to a truly macroscopic separation of 12 mm on the detector. For fifth-order scattering, we have observed several oscillations of the splitting ratio when varying the laser power (``Pendell\osung oscillations''). The large splitting angle, the adjustable splitting ratio, and the cleanness of the split beams, with $200\ensuremath{-}\ensuremath{\mu}\mathrm{m}$ rms width each, make the beam splitter ideal for a large-enclosed-area atom interferometer.