Beam Of Metastable Helium Atoms Research Articles

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.

Selective functionalization of silicon surface controlled by metastable helium atom beam for patterning chemisorbed monolayer molecular assemblies

Metastable neutral helium atoms are highly chemical sensitive to surface dangling bonds. As the basis of a new lithographic technique, a method for fabrication of self‐assembled monolayers (SAMs) and mixed SAMs by the irradiation‐promoted exchange reaction has been recently developed. Metastable helium atoms irradiation is shown to modify organosilane SAM films and Si–H and Si–OH covering surfaces, which render the surface amendable to further SAM modification. The exposure regions can undergo a second chemisorption reaction to produce an assembly of octadecyltrichlorosilane SAMs. Metastable helium atom beam patterned films can be used as templates for selective buildup of fluorophores, amino group, and biological cells to obtain further surface functionalization. Copyright © 2014 John Wiley & Sons, Ltd.

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.

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

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

Precision spectroscopy of helium using a laser-cooled atomic beam

The 23P0,1,2 fine structure interval of 4He can be determined to 10-8 accuracy both theoretically and experimentally. It can be used either to determine the fine structure constant or to test the quantum electrodynamics theory. To reach this goal, it is necessary to measure the fine structure splitting to sub kHz accuracy by increasing the signal-to-noise ratio and eliminating the systematic deviations. In the experimental configuration of present study, transverse laser cooling is used to obtain an intense metastable helium atom beam. The triple state metastable atoms are also bent from the original atomic beam to reduce the background noise. The spectral scanning will be accomplished by tuning the sideband of a frequency-locked diode laser to maintain sufficient frequency stability during the scan. The experimental method has been tested on the setup recently built, and the analysis shows that a sub-kHz precision is feasible.

Spin polarization of the Si(111)-7 × 7 surface: A study with a spin-polarized metastable helium atom beam

We have observed the electron spin polarization of a clean Si(111) surface under external magnetic fields of 0–5 T using a spin-polarized metastable helium atom beam. By making comparisons with metastable deexcitation spectroscopy (MDS) measurements, the spin polarization has been assigned to surface dangling bonds. It has been found that the spin polarization is largely quenched by the adsorption of atomic hydrogen, and that its H-exposure dependence correlates well with that of the MDS spectrum and the low energy electron diffraction pattern.

Recovery of the half-metallicity of anFe3O4(100)surface by atomic hydrogen adsorption

We report experimental evidence that hydrogen termination largely enhances the spin polarization of an ${\text{Fe}}_{3}{\text{O}}_{4}(100)$ surface. An in situ prepared ${\text{Fe}}_{3}{\text{O}}_{4}(100)/\text{MgO}(100)$ film surface was exposed to atomic hydrogen and its surface spin polarization $(P)$ was monitored with a spin-polarized metastable helium-atom beam under external magnetic fields of 0--5 T. The spin asymmetry at the high-energy cutoff, which reflects $P$ at the Fermi level of the topmost surface, increased from $<5\mathrm{%}$ to $>50\mathrm{%}$ at 298 K by H adsorption. The enhancement in $P$ was found to be consistent with the H-induced change in the surface electronic states predicted by a density-functional theory calculation.

Magnetic hexapole lens focusing of a metastable helium atomic beam for UV-free lithography

Sources of rare gas atoms in excited metastable states have been used to expose photoresist-coated substrates to demonstrate atom lithography. These thermal atomic beams are usually created by discharge sources that also produce copious amounts of UV radiation. The UV radiation simultaneously illuminates the substrate and may play a complementary role in altering the photoresist together with the metastable atoms. In the experiments reported here, we have isolated the UV component using a magnetic hexapole lens to focus a thermal beam of metastable helium atoms around a fiducial mask that blocks the UV light. This creates an atom lithography exposure that is the result of illumination by the atoms alone. We have also modelled the performance of the magnetic hexapole lens as a potentially useful device for atom lithography.

An intense cold beam of metastable helium

We have produced and characterised a slow, bright and intense atomic beam of metastable helium atoms, suitable for atomic physics experiments. The maximum continuous flux attained was 2×1010 atoms/s, while a typical longitudinal peak velocity of the beam was ∼26 m/s with a divergence in the range of 15 mrad to 30 mrad.

Microfabrication of Silicon Using Self-Assembled Monolayer Resist and Metastable Helium Beam

We herein report on the microfabrication of a Si(111) surface with a negative/positive contrast by atom lithography using a neutral metastable helium atom beam (He-MAB) and a self-assembled monolayer (SAM) of octadecyltrichlorosilane (OTS). The OTS SAM bonded directly to the silicon surface as a resist and was exposed to He-MAB through a stencil mask to yield a latent image in it. Using chemical etching to develop and transfer the latent image directly onto the underlying silicon substrate, a square silicon micromesa and a microwell matrix with a nanoscale edge resolutions of approximately 100 nm on the Si(111) surface were fabricated. The negative/positive patterning mechanism was discussed in terms of the damage of the SAM resist under the irradiation of He-MAB and the possible effects of contamination.

Extremely surface-sensitive hysteresis loop measurement with a spin-polarized metastable helium atom beam

The sample current induced by the deexcitation of He* atoms on a ferromagnetic surface was found to depend on the He* spin direction. This spin dependence was used for measuring the hysteresis loop of an Fe film on Cu(100). The hysteresis loop is extremely surface specific because the spin dependence is due to the electron emission via the He* deexcitation that occurs on the vacuum side of the topmost surface layer. The hysteresis loop for a 2.2ML Fe film on Cu(100) has been found to agree well with that measured with the surface magneto-optic Kerr effect.

Nanolithography with metastable helium atoms in a high-power standing-wave light field

We have created periodic nanoscale structures in a gold substrate with a lithography process using metastable triplet helium atoms that damage a hydrophobic resist layer on top of the substrate. A beam of metastable helium atoms is transversely collimated and guided through an intense standing-wave light field. Compared to commonly used low-power optical masks, a high-power light field (saturation parameter of 107) increases the confinement of the atoms in the standing wave considerably, and makes the alignment of the experimental setup less critical. Due to the high internal energy of the metastable helium atoms (20 eV), a dose of only one atom per resist molecule is required. With an exposure time of only eight minutes, parallel lines with a separation of 542 nm and a width of 100 nm (one-eleventh of the wavelength used for the optical mask) are created.

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.

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.

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.

Precision microwave measurement of the 2(3)P(1)-2(3)P(0) interval in atomic helium: a determination of the fine-structure constant.

The 2(3)P(1)-to- 2(3)P(0) interval in atomic helium is measured using a thermal beam of metastable helium atoms excited to the 2(3)P state using a 1.08-microm diode laser. The 2(3)P(1)-to- 2(3)P(0) transition is driven by 29.6-GHz microwaves in a rectangular waveguide cavity. Our result of 29,616,950.9+/-0.9 kHz is the most precise measurement of helium 2(3)P fine structure. When compared to precise theory for this interval, this measurement leads to a determination of the fine-structure constant of 1/137.0359864(31).

A source for a high-intensity pulsed beam of metastable helium atoms

The operational characteristics and technical details of ahigh-intensity discharge source for a pulsed beam with a high flux of heliumatoms in their metastable states (exceeding 4×1015atoms sr-1 s-1) are discussed. Owing to the high ionizationpotential of helium, stable operation of a pulsed discharge in an atomic beamis difficult to achieve. In this work we describe a simple, robust andeasy-to-operate design of a reliable high-intensity pulsed source formetastable helium atoms. The principle of operation relies on injectionseeding the discharge with electrons emitted from a hot wire, permitting astrong enhancement both in efficiency and instability of theproduction of metastable atoms.

Precision microwave measurement of the 2 3P1-2 3P2 interval in atomic helium

The 2 3P1-to- 2 3P2 interval in atomic helium is measured using a thermal beam of metastable helium atoms excited to the 2 3P state using a 1.08-&mgr;m diode laser. The 2 3P1-to- 2 3P2 transition is driven by 2.29-GHz microwaves in a coaxial transmission line. Our result of 2 291 174.0+/-1.4 kHz is the most precise measurement of helium 2 3P fine structure. This measurement plays a key role in obtaining a new value for the fine-structure constant.

Sharp edged silicon structures generated using atom lithography with metastable helium atoms

By combining atom lithography and plasma etching technology in a two-step process, we demonstrate the transfer of sharp edged structures into silicon with a depth of 580 nm and an inclination of better than 86°. A self-assembled monolayer resist deposited on a Au-coated Si surface is damaged by a beam of metastable helium atoms through a physical mask. A wet etching process removes Au in the damaged regions, resulting in an intermediate mask of patterned Au on Si. Low-pressure plasma etching is then used to transfer the pattern of the Au mask into the Si. This plasma etching process shows a selectivity greater than 19 with respect to the Au mask.