Stability of small mixed clusters of 4He and 3He atoms

The existence of small helium clusters containing a variable number of 4He and 3He atoms is studied within a variational Monte Carlo calculation employing the Aziz HFD-B(HE) pair interaction. The clusters 4He(2) support one and two 3He atoms; however, the system with three 3He atoms is metastable, and the next bound system requires at least 18 fermions. All clusters obtained by adding 3He atoms to the trimer 4He(3) and the tetramer 4He(4) are bound, but the clusters 4He(3)-3He(3,4,5) and 4He(3,4)-3He(9) are metastable. All remaining clusters with three or more bosons and any number of fermions are stable.     

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.     

Magneto-optical trapping of Stark-slowed metastable He atoms

We report on the first successful loading of a magneto-optical trap (MOT) with metastable He atoms from a Stark-slower. Thereby, deceleration of the atoms relies on laser-atom interaction in an inhomogeneous electric field. We show that the results obtained are comparable with early results from other groups achieved with a Zeeman slower. The Stark slower, which is able to fully control the final velocity of the atomic He beam, is the first step in achieving complete spin independent kinematic control based solely on electric fields. Received 2 October 2002 / Received in final form 20 January 2003 Published online 29 April 2003 RID="a" ID="a"e-mail: eichmann@mbi-berlin.de     

Production of a bose einstein condensate of metastable helium atoms

We recently observed a Bose-Einstein condensate in a dilute gas of ⁴He in the 2³S₁ metastable state. In this article, we describe the successive experimental steps which led to the Bose-Einstein transition at 4.7 μK: loading of a large number of atoms in a MOT, efficient transfer into a magnetic Ioffé-Pritchard trap, and optimization of the evaporative cooling ramp. Quantitative measurements are also given for the rates of elastic and inelastic collisions, both above and below the transition. Received 15 October 2001     

Accumulation of chromium metastable atoms into an optical trap

We report the fast accumulation of a large number of metastable ⁵²Cr atoms in a mixed trap, formed by the superposition of a strongly confining optical trap and a quadrupolar magnetic trap. The steady state is reached after about 400 ms, providing a cloud of more than one million metastable atoms at a temperature of about 100 μK, with a peak density of 10¹⁸ atoms m^-3. We have optimized the loading procedure, and measured the light shift of the ⁵D₄ state by analyzing how the trapped atoms respond to a parametric excitation. We compare this result to a theoretical evaluation based on the available spectroscopic data for chromium atoms.     

Two body loss rate in a magneto-optical trap of metastable He

We have measured the two body loss rate in a magneto-optical trap containing triplet metastable He atoms. We find a rate constant cm³/s at a -8 MHz detuning, with an uncertainty of a factor 2. This measurement is in disagreement with a recent experiment which measures the absolute, ion-producing collision rate, but agrees with several other published measurements. Received 20 April 1999 and Received in final form 12 July 1999     

Primary populations of metastable antiprotonic (4)He and (3)He atoms

Initial distributions of metastable antiprotonic (4)He and (3)He atoms over principal (n) and angular momentum (l) quantum numbers have been deduced using laser spectroscopy experiments. The regions n = 37-40 and n = 35-38 in the two atoms account for almost all of the observed fractions [(3.0 +/- 0.1)% and (2.4 +/- 0.1)%] of antiprotons captured into metastable states.     

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.     

Effect of optical pumping of metastable helium atoms on the operation of a helium-neon laser

Nonradiative destruction of the upper levels of helium has made possible optical pumping of metastable 2³S He atoms in a helium-neon laser. The changes in gain under various discharge conditions have been determined by introducing an extra loss, which has produced the same drop in generated power as extra pumping from a helium tube. The lifetime of the metastable 2³S He atoms has been measured as a function of the discharge current under a specific set of conditions.     

Optimized loading of an optical dipole trap for the production of chromium BECs

We report on a strategy to maximize the number of chromium atoms transferred from a magneto-optical trap into an optical trap through accumulation in metastable states via strong optical pumping. We analyse how the number of atoms in a chromium Bose–Einstein condensate can be raised by a proper handling of the metastable state populations. Four laser diodes have been implemented to address the four levels that are populated during the MOT phase. The individual importance of each state is specified. To stabilize two of our laser diode, we have developed a simple ultrastable passive reference cavity whose long term stability is better than 1 MHz.     

Laser cooling without repumping: a magneto-optical trap for erbium atoms

We report on a novel mechanism that allows for strong laser cooling of atoms that do not have a closed cycling transition. This mechanism is observed in a magneto-optical trap (MOT) for erbium, an atom with a very complex energy level structure with multiple pathways for optical-pumping losses. We observe surprisingly high trap populations of over 10(6) atoms and densities of over 10(11) atoms cm(-3), despite the many potential loss channels. A model based on recycling of metastable and ground state atoms held in the quadrupole magnetic field of the trap explains the high trap population, and agrees well with time-dependent measurements of MOT fluorescence. The demonstration of trapping of a rare-earth atom such as erbium opens a wide range of new possibilities for practical applications and fundamental studies with cold atoms.     

Metastable (2S) helium atom scattering from NiO(100) and Cu(100) surfaces

The scattering of metastable 2³S He atoms (He*) from cleaved NiO(100) as well as from clean and CO-covered Cu(100) surfaces has been studied. For these varied surfaces, which were characterized in situ by ground state He atom scattering, only broad He* angular distributions without any diffraction peaks were observed. For metastable He atoms scattered from the clean Cu(100) surface a total survival probability of 1×10⁻⁶ was determined. For NiO(100) and the CO-covered Cu(100) surface values of about 1×10⁻⁵ were obtained. Time-of-flight spectra of the surviving He* atoms revealed a substantial energetic broadening which increases with the substrate temperature. This behaviour indicates a large well depth for the He*–surface interaction potential and is discussed in terms of an enhanced multiphonon excitation and/or trapping probability upon the scattering.     

从大气压到超低气压范围的氦等离子体发射光谱的基本特性

采用三种实验装置(介质阻挡放电装置、空心阴极放电装置和彭宁放电装置)分别测量了不同压强范围内氦等离子体的发射光谱。通过对氦等离子体发射光谱的分析,已观察到一个共同的特点,就是在三种放电条件下产生的氦等离子体中31P1→21S0的谱线强度总是最强,可以推测亚稳态氦原子的含量相当显著,但不同的装置也有不同的特点,介质阻挡放电装置能够产生准辉光放电,谱线中氦原子的谱线强度很低,而空心阴极放电与彭宁放电装置能够产生稳定均匀的等离子体,且发射足够强的光辐射。我们已对所拍摄的光谱的谱线都进行了辨认,所有结果表明原子发射光谱分析法是研究不同条件下氦等离子体状态的一种十分有效的手段。     

Trapping of ultracold atoms in a 3He/4He dilution refrigerator

We describe the preparation of ultracold atomic clouds in a dilution refrigerator. The closed-cycle ³He/⁴He cryostat was custom made to provide optical access for laser cooling, optical manipulation and detection of atoms. We show that the cryostat meets the requirements for cold atom experiments, specifically in terms of operating a magneto-optical trap, magnetic traps and magnetic transport under ultrahigh vacuum conditions. The presented system is a step toward the creation of a quantum hybrid system combining ultracold atoms and solid-state quantum devices.     

Optical measurement of <Superscript>3</Superscript>He nuclear polarization for metastable exchange optical pumping studies at high magnetic field

An accurate optical method to measure the nuclear polarization of ³He atoms in the 1¹S ground state is described. The absorption of a weak, probe laser beam is used to measure the relative populations of two hyperfine sublevels of the 2³S metastable state that are not addressed by the pumping laser beam. Since a common spin temperature between the ground and metastable states is established by metastable exchange collisions, the nuclear polarization can be derived from these absorption measurements. The method is highly sensitive, robust, and can be used to monitor the dynamics of optical pumping and relaxation processes without interfering with them. It was successfully implemented and tested in the 0.45–2.0 T magnetic field range at the ³He gas pressure up to 67 mbar.     

Laser trapping of 225Ra and 226Ra with repumping by room-temperature blackbody radiation

We have demonstrated Zeeman slowing and capture of neutral 225Ra and 226Ra atoms in a magneto-optical trap. The intercombination transition 1S0-->3P1 is the only quasicycling transition in radium and was used for laser-cooling and trapping. Repumping along the 3D1-->1P1 transition extended the lifetime of the trap from milliseconds to seconds. Room-temperature blackbody radiation was demonstrated to provide repumping from the metastable 3P0 level. We measured the isotope shift and hyperfine splittings on the 3D1-->1P1 transition with the laser-cooled atoms, and set a limit on the lifetime of the 3D1 level based on the measured blackbody repumping rate. Laser-cooled and trapped radium is an attractive system for studying fundamental symmetries.     

Laser spectroscopic determination of the 6He nuclear charge radius

We have performed precision laser spectroscopy on individual 6He (t(1/2)=0.8 s) atoms confined and cooled in a magneto-optical trap, and measured the isotope shift between 6He and 4He to be 43 194.772+/-0.056 MHz for the 2(3)S1-3(3)P2 transition. Based on this measurement and atomic theory, the nuclear charge radius of 6He is determined for the first time in a method independent of nuclear models to be 2.054+/-0.014 fm. The result is compared with the values predicted by a number of nuclear structure calculations and tests their ability to characterize this loosely bound halo nucleus.     

Laser spectroscopy of nuclear reaction products: recent results and future prospects

Laser spectroscopic observations of nuclear reaction products produced with intensities of less than 10⁴ atoms/second are now possible with several different methods. We describe the recoil into gas method which has recently been successful. This method is not Dopplerfree, but can give reasonable spectra if the resolution requirements of the spectra are not too high. It has the great advantage that it very efficiently uses the atoms, and spectra have been observed with primary production rates of less than 10³ atoms/sec. Our recent work has concentrated on developing the recoil into gas method for the refractory element Hf. In order that the atoms could be cycled to produce many fluorescence photons, nitrogen and hydrogen impurity gases were added to the argon buffer gas to quench metastable levels to the ground state. In this way spectra could be obtained with fluxes of 10⁴ atoms/second. Future prospects for trapping radioactive atoms in a magneto-optic trap will be discussed.     

Space resolved density measurements of argon and helium metastable atoms in radio-frequency generated He-Ar micro-plasmas

Space resolved concentrations of helium He* (³S₁) and argon Ar* (³P₂) metastable atoms in an atmospheric pressure radio frequency micro-plasma jet were measured using tunable diode laser absorption spectroscopy. Even small absorptions down to 10^-4 could be measured using lock-in technique. The absolute density of metastable atoms densities at different rf-power, flow rate and gas mixture was deduced from measured absorption rates. Metastable concentrations range from 10⁹ to 10¹¹ cm^-3. Analysis of spectral profiles provided the pressure broadening coefficients of both metastable atoms by helium. The spatial distribution of metastable atoms in the plasma volume was obtained for various discharge conditions. Density profiles between the electrodes reveal the sheath structure and reflect the plasma excitation distributions in the discharge volume. It reveals the dominance of the α-mode discharge.     

Metastable helium atom stimulated desorption of H+ Ion

H+ desorption induced by the impact of metastable helium atoms has been found for H(2)O/Na/Ni(110) coadsorption systems. The measurements were carried out using a time-of-flight technique and a pulsed-discharge type metastable helium atom ( He(*)) source. It is concluded that the H+ desorption by He(*) is induced by a hole created on the valence levels via the Auger deexcitation of He(*). The H+ desorption by He(*) may be understood within the framework of the Menzel-Gomer-Readhead model.