Metastable Helium Atoms Research Articles

Laser Absorption Spectroscopy for Diagnostics of a Neutral Helium Beam

The density resolution of a measurement system using laser absorption spectroscopy is evaluated in order to diagnose a fraction of metastable helium atoms (2 3S1) in a neutral helium beam. Experiments performed in a hollow cathode device show that the density resolution of the present system is about 5 × 1013 m-3 for a 1 m absorption length. Optimization of absorption scheme and improvements of signal detection are suggested, which shows that laser absorption spectroscopy is a promising diagnostic method for a neutral helium beam.

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.

Stimulated Raman adiabatic passage via the ionization continuum in helium: Experiment and theory

We experimentally demonstrate coherent population transfer, driven by stimulated Raman adiabatic passage (STIRAP) between two bound quantum states, coupled via a continuum of states. We present extended numerical and experimental investigations on population transfer from the metastable state 2s 1S0 to the excited state 4s 1S0 in metastable helium atoms. While techniques based on incoherent excitation do not permit any population transfer via rapidly decaying continuum states, our data indicate a maximum transfer efficiency of 20% in coherent excitation by STIRAP. We study the transfer efficiency with respect to the relevant experimental parameters.

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.

Bose–Einstein condensation of metastable helium in a bi-planar quadrupole Ioffe configuration trap

Using a novel magnetic trapping geometry we have evaporatively cooled metastable helium atoms to form a Bose–Einstein condensate containing approximately one million atoms. This is only the fourth demonstration of a metastable condensate and the first realisation of a BEC in a bi-planar quadrupole Ioffe configuration magnetic trap.

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.

Analysis of light-induced frequency shifts in the photoassociation of ultracold metastable helium atoms

We present an exhaustive analysis of the light-induced frequency shifts of the photoassociation lines of ultracold metastable 4He* atoms in a magnetic trap. The measurements of the shifts of several vibrational levels bound in the purely long-range J = 1, 0u+ potential linked to the 23S1–23P0 asymptote were reported in a previous paper (Kim et al 2005 Europhys. Lett. 72 548), and are analysed here. The simplicity of this system makes it very appropriate for a detailed study. Indeed, the purely long-range character of the excited potential allows one to calculate exact excited molecular wavefunctions and to use asymptotic expansions at large internuclear distances of the ground-state wavefunctions appearing in Franck–Condon-type integrals. Moreover, the number of collisional channels to be considered is strongly reduced by the absence of hyperfine structure for 4He* and the use of polarized ultracold atoms and polarized light. This allows us to derive semi-analytical expressions for the shifts showing explicitly their linear dependences on the s-wave scattering length a of spin polarized metastable 4He* atoms. This explains how it is possible to derive the measurement of a from these shifts.

The Role of Metastable Atoms in a Homogeneous Atmospheric Pressure Barrier Discharge in Helium

A one-dimensional fluid model for homogeneous atmospheric pressure barrier discharges in helium is presented by considering elementary processes of excitation and ionization including a metastable atom effect. Using this model we investigate the behaviours of the helium metastable atoms in discharges as well as their influence on the discharge characteristics. It is shown that the metastable atoms with a relatively high concentration during the discharge are mainly produced in the active phase of the discharge and dissolved in the off phase. It is also found that the metastable atom collisions can not only provide seed electrons for discharges but also influence the concentration of ions. A reduction of matestable atom density results in a drop in the charged particle densities and causes a qualitative change in the discharge patterns.

Spin-dipole-induced lifetime of the least-boundΣg+5state ofHe(2S13)+He(2S13)

The properties of the least-bound vibrational level (v=14) of the quintet sigma state formed during the ultracold collision of two spin-polarized metastable helium atoms are crucial to studies of photoassociation spectroscopy of metastable helium. We report a calculation of the autoionization lifetime of this state induced by spin-dipole coupling of the quintet sigma state to the singlet sigma state from which Penning and associative ionization processes are highly probable. We find a lifetime of about 150 microseconds, significantly larger than the recent experimental estimates of (4-5) microseconds.

Interplay between Theoretical Quantum Chemistry and Cold Atom Experiments

In this paper, we briefly summarize the new information which have been deduced on the atom–atom interaction from cold atom experiments. These information are quite unusual and very interesting. Their interpretation is considerably simplified if high quality quantum chemistry calculations are available. After an overview of the domain, we discuss in detail the experiments performed with metastable helium atom, thus showing the variety of new data obtained in this case and the importance of quantum chemistry calculations for their interpretation.

Efficient loading of a He* magneto-optic trap using a liquid He cooled source

We report loading large numbers (up to 3×109) of metastable triplet helium atoms into a magneto-optical trap using an atomic beam derived from a liquid He (LHe) cooled dc discharge source. Moreover, we compare the effect of liquid N2 cooling to LHe cooling the source and demonstrate that LHe cooling offers a significant increase in performance.

Homonuclear ionizing collisions of laser-cooled metastable helium atoms

We present a theoretical and experimental investigation of homonuclear ionizing collisions of laser-cooled metastable helium atoms, considering both the fermionic He-3 and bosonic He-4 isotope. The theoretical description combines quantum threshold behavior, Wigner's spin-conservation rule and quantum statistical symmetry requirements in a single-channel model, complementing a more complete close-coupling theory that has been reported for collisions of metastable He-4 atoms. The model is supported with measurements (in the absence of light fields) of ionization rates in magneto-optically trapped samples, that contain about 3*10^8 atoms of a single isotope. The ionization rates are determined from measurements of trap loss due to light-assisted collisions combined with comparative measurements of the ion production rate in the absence and presence of trapping light. Theory and experiment show good agreement.

Accurate Determination of the Scattering Length of Metastable Helium Atoms Using Dark Resonances between Atoms and Exotic Molecules

We present a new measurement of the s-wave scattering length a of spin-polarized helium atoms in the 2(3)S1 metastable state. Using two-photon photoassociation spectroscopy and dark resonances, we measure the energy E(nu)=14= -91.35+/- 0.06 MHz of the least-bound state nu = 14 in the interaction potential of the two atoms. We deduce a value of a=7.512+/-0.005 nm, which is at least 100 times more precise than the best previous determinations and is in disagreement with some of them. This experiment also demonstrates the possibility to create exotic molecules binding two metastable atoms with a lifetime of the order of 1 micros.

Experimental demonstration of population inversion driven by retroreflection-induced bichromatic adiabatic passage

We present experimental data to demonstrate coherently driven population inversion by retroreflection-induced bichromatic adiabatic passage in metastable helium atoms. Complete and robust population transfer from an initial to a target state is induced by coherent interaction of the atoms in a supersonic beam with two counterpropagating and temporally delayed laser pulses of different intensities. The radiation fields intersect the atomic beam slightly tilted away from normal incidence, thereby inducing Doppler shifts of the atomic resonance between the initial and the target state. Thus the laser pulses produce a bichromatic field in the rest frame of each atom, which induces complete coherent population transfer by an adiabatic passage process.

Effects of adsorbate-induced states on the metastable He atom scattering from water- and benzene-adsorbed Cu(1 0 0)

The intensity of metastable helium (He*) atoms which survive during the scattering from water- and benzene-adsorbed Cu(1 0 0) surfaces was measured. The survival probability (SP) of He* was found to be sensitive to the electronic states at around the Fermi level, which is derived from the adsorbate/metal hybridization and extend toward the vacuum. The SP is likely to depend largely on the kinetic energy of the He* atoms. The kinetic energy dependence can be understood on the basis of the He* decay mechanism. Metastable-atom deexcitation spectroscopy (MDS) and surface electronic structure calculation have been used for discussing the results for the He* SP.

Metastable helium spectroscopy on simple metals: Comparison between low and high work function substrates

In this work we investigate the deexcitation process of a slow metastable helium atom (He*) in its triplet state on simple metal surfaces. We compute the electronic properties of the interacting He*–metal (jellium-like) system, using the embedding Green’s function method, within the framework of density functional theory. The Auger deexcitation rates are calculated at different distances from the surface using the Fermi golden rule, and the energy distribution of the emitted electrons is obtained by integrating the Auger rates along the classical trajectory followed by the atom in the scattering process. We present the results for three metal surfaces (Al, Na, and Cs) which are sufficiently different to span the distinct deexcitation mechanisms taking place when the He* interacts with the surface: resonant ionization + Auger neutralization for high work function substrates, and Auger deexcitation for low work function ones. Our results for the deexcitation spectral profiles are in good agreement with the experimental ones.

Electron Collisions with Laser Cooled and Trapped Metastable Helium Atoms: Total Scattering Cross Sections

Absolute measurements of total scattering cross sections for low energy (5-70 eV) electrons by metastable helium (2(3)S) atoms are presented. The measurements are performed using a magneto-optical trap which is loaded from a laser-cooled, bright beam of slow He(2(3)S) atoms. The data are compared with predictions from convergent close coupling and R matrix with pseudostate calculations, and we find good agreement between experiment and theory.

Improved metastable de-excitation spectrometer using laser-cooling techniques

Details of a new approach for performing metastable de-excitation spectroscopy are given. A beam of metastable (2S3) helium atoms, produced in a hollow cathode dc discharge, is collimated and subsequently focused using Doppler cooling of the 2S13–2P23 transition at 1083nm, forming an intense probe of up to 1×1012atomss−1cm−2. The large distance (2.5m) between source and sample means that the beam is relatively free of UV photons and 2S1 metastable atoms, removing the need for quench lamps and chopper wheels. As well as providing a clean high intensity source, the well defined nature of the beam is a necessary step towards using more sophisticated laser-cooling techniques with the ultimate aim of producing a metastable helium microscope. MDS and UPS spectra from Si(111) are shown.

Metastable helium: Atom optics with nano-grenades

In this, the hundredth year since Einstein first postulated the existence of photons, the successful application of wave - particle duality to matter has seen an explosion of activity in the field of atom optics and Bose - Einstein condensation (BEC). This article provides a brief introduction to atom optics, illustrated with applications taken from experiments using helium atoms in long-lived (metastable) excited states. Metastable helium atoms store the greatest amount of energy (∼20 electron volts) in any atomic or molecular system. They behave like nano-hand grenades, making it easy to detect single atoms, opening up promising applications as well as fundamental studies of the quantum statistical properties of atomic systems.

High-power frequency-stabilized laser for laser cooling of metastable helium at 389 nm

A high-power, frequency-stabilized laser for cooling of metastable helium atoms using the 2S13→3P23 transition at 389 nm has been developed. The 389 nm light is generated by frequency doubling of a titanium:sapphire laser in an external enhancement cavity containing a lithium–triborate nonlinear crystal. With a maximum conversion efficiency of 75%, 1 W of useful 389 nm power is produced out of 2 W at 778 nm. While being stabilized to the 2S13→3P23 transition, the 389 nm frequency is tunable over ±150MHz with respect to the field-free atomic resonance frequency. This is accomplished by Zeeman tuning of the absorption lines used in the frequency-stabilization scheme. The setup for saturated absorption spectroscopy in an rf discharge cell, used to stabilize the 389 nm laser to the atomic transition, is described in detail.