Atom Loss Rate Research Articles

Atom Loss Resonances in a Bose-Einstein Condensate

Atom loss resonances in ultracold trapped atoms have been observed at scattering lengths near atom-dimer resonances, at which Efimov trimers cross the atom-dimer threshold, and near two-dimer resonances, at which universal tetramers cross the dimer-dimer threshold. We propose a new mechanism for these loss resonances in a Bose-Einstein condensate of atoms. As the scattering length is ramped to the large final value at which the atom loss rate is measured, the time-dependent scattering length generates a small condensate of shallow dimers coherently from the atom condensate. The coexisting atom and dimer condensates can be described by a low-energy effective field theory with universal coefficients that are determined by matching exact results from few-body physics. The classical field equations for the atom and dimer condensates predict narrow enhancements in the atom loss rate near atom-dimer resonances and near two-dimer resonances due to inelastic dimer collisions.

Scattering and absorption of ultracold atoms by nanotubes

We investigate theoretically how cold atoms, including Bose–Einstein condensates, are scattered from, or absorbed by, nanotubes with a view to analysing recent experiments. In particular, we consider the role of potential strength, quantum reflection, atomic interactions and tube vibrations on atom loss rates. Lifshitz theory calculations deliver a significantly stronger scattering potential than that found in experiment and we discuss possible reasons for this. We found that the scattering potential for dielectric tubes can be calculated to a good approximation using a modified pairwise summation approach, which is efficient and easily extendable to arbitrary geometries. Quantum reflection of atoms from a nanotube may become a significant factor at low temperatures, especially for non-metallic tubes. Interatomic interactions are shown to increase the rate at which atoms are lost to the nanotube and lead to non-trivial dynamics. Thermal nanotube vibrations do not significantly increase loss rates or reduce condensate fractions, but lower frequency oscillations can dramatically heat the cloud.

Observation of ap-wave optical Feshbach resonance

We demonstrate a p$-wave optical Feshbach resonance (OFR) using purely long-range molecular states of a fermionic isotope of ytterbium ^{171}Yb, following the proposition made by K. Goyal et al. [Phys. Rev. A 82, 062704 (2010)]. The p-wave OFR is clearly observed as a modification of a photoassociation rate for atomic ensembles at about 5 micro-Kelvins. A scattering phase shift variation of \delta \eta=0.022 rad is observed with an atom loss rate coefficient K=28.0*10^{-12} cm^3/s.

Geometric phases generated by the non-trivial spatial topology of static vector fields linearly coupled to a neutral spin-endowed particle: application to 171Yb atoms trapped in a 2D optical lattice

We have constructed the geometric phases emerging from the non-trivial topology of a space-dependent magnetic field B(r), interacting with the spin magnetic moment of a neutral particle. Our basic tool, adapted from a previous work on Berry’s phases, is the space-dependent unitary transformation , which leads to the identity, , at each point r. In the ‘rotated’ Hamiltonian , is replaced by the non-Abelian covariant derivative where can be written as A1(r)Sx + A2(r)Sy + A3(r)Sz. The Abelian differentials Ak(r)·dr are given in terms of the Euler angles defining the orientation of B(r). The non-Abelian field transforms as a Yang–Mills field; however, its vanishing ‘curvature’ reveals its purely geometric character. We have defined a perturbation scheme based upon the assumption that in the longitudinal field A3(r) dominates the transverse field A1, 2(r) contributions, evaluated to second order. The geometry embedded in both the vector field A3(r) and the geometric magnetic field is described by their associated Aharonov–Bohm phase. As an illustration we study the physics of cold 171Yb atoms dressed by overlaying two circularly polarized stationary waves with orthogonal directions, which form a 2D square optical lattice. The frequency is tuned midway between the two hyperfine levels of the (6s6p)3P1 states to protect the optical B(r) field generated by the lattice from the dressed atom instability. The geometric field B3(r) is computed analytically in terms of the Euler angles. The magnitude of the second-order corrections due to the transverse fields can be reduced to the per cent level by a choice of light intensity that keeps the dressed atom loss rate ⩽5 s−1. A second optical lattice can be designed to confine the atoms inside 2D domains where . We extend our analysis to the case of a triangular lattice.

Avalanche mechanism for atom loss near an atom-dimer Efimov resonance

An Efimov trimer near the atom-dimer threshold can increase the atom loss rate in ultracold trapped atoms through the {\it avalanche mechanism} proposed by Zaccanti et al. A 3-body recombination event creates an energetic atom and dimer, whose subsequent elastic collisions produce additional atoms with sufficient energy to escape from the trapping potential. We use Monte Carlo methods to calculate the average number of atoms lost and the average heat generated by recombination events in both a Bose-Einstein condensate and a thermal gas. We take into account the energy-dependence of the cross sections and the spatial structure of the atom cloud. We confirm that the number of atoms lost can be much larger than the naive value 3 if there is an Efimov trimer near the atom-dimer threshold. This does not produce a narrow loss feature, but it can significantly affect the determination of Efimov parameters.

Trap depth determination using ultracold 87Rb– 40Ar atom collisions

We present a new method for determining the trap depth of an atomic or molecular trap of any type and any depth. This method relies on measurements of the trap loss rate induced by elastic collisions of ultracold Rb atoms with room temperature background gases. Because of the long range nature of the Van der Waals force, these loss rates exhibit significant trap depth dependence over an enormous range of trap depths from a few K to below 100 μK. We compare this method with an independent technique that relies on measurements of atom loss rates during optical excitation of colliding pairs to a repulsive molecular state. The main advantage of the method presented here is its simplicity and applicability to traps of any type requiring only knowledge of the background gas pressure and the long range Van der Waals force between the trapped particles and the background gas particles.

Photoionization in alkali lasers

We have calculated photoionization rates in alkali lasers. The photoionization of alkali atoms in the gain medium of alkali lasers can significantly degrade the laser performance by reducing the neutral alkali density and with it the gain. For a ten atmosphere Rb laser and a Cs exciplex laser, the photoionization induced alkali atom loss rates are greater than 10(5) sec(-1). These high loss rates will quickly deplete the neutral alkali density, reducing gain, and may require fast, possibly, supersonic flow rates to sufficiently replenish the neutral medium for CW operation.

Nondestructive Fluorescent State Detection of Single Neutral Atom Qubits

We demonstrate nondestructive (lossless) fluorescent state detection of individual neutral atom qubits trapped in an optical lattice. The hyperfine state of the atom is measured with a 95% accuracy and an atom loss rate of 1%. Individual atoms are initialized and detected over 100 times before being lost from the trap, representing a 100-fold improvement in data collection rates over previous experiments. Microwave Rabi oscillations are observed with repeated measurements of one and the same single atom.

Spin-dependent inelastic collisions in spin-2 Bose-Einstein condensates

We studied spin-dependent two-body inelastic collisions in $F=2$ $^{87}\text{R}\text{b}$ Bose-Einstein condensates both experimentally and theoretically. The $^{87}\text{R}\text{b}$ condensates were confined in an optical trap and selectively prepared in various spin states in the $F=2$ manifold at a magnetic field of 3.0 G. The measured atom loss rates depend on the spin states of colliding atoms. We measured two fundamental loss coefficients for two-body inelastic collisions with total spins of 0 and 2. The loss coefficients determine the loss rates of all the spin pairs. The experimental results for mixtures of all spin combinations are in good agreement with numerical solutions of the Gross-Pitaevskii equations that include the effect of a magnetic field gradient.

Combination of a magnetic Feshbach resonance and an optical bound-to-bound transition

We use laser light near resonant with an optical bound-to-bound transition to shift the magnetic field at which a Feshbach resonance occurs. We operate in a regime of large detuning and large laser intensity. This reduces the light-induced atom-loss rate by one order of magnitude compared to our previous experiments [D.M. Bauer et al. Nature Phys. 5, 339 (2009)]. The experiments are performed in an optical lattice and include high-resolution spectroscopy of excited molecular states, reported here. In addition, we give a detailed account of a theoretical model that describes our experimental data.

Collision dynamics between stretched states of spin-2 87Rb Bose–Einstein condensates

We experimentally observed the time dependence of the spin populations of spin-2 87Rb Bose–Einstein condensates confined in an optical trap. The condensed atoms were initially populated in the stretched states |F=2,m F =+2〉 and |F=2,m F =−2〉 with several varieties of population imbalances. No spin-exchange collisions were observed in a weak magnetic field of 45 mG. The atom loss rate depended on the observed relative population of spin-states. We calculated the loss rate due to two-body inelastic collisions with the population imbalance using an experimentally estimated rate of 17.0(±1.9)×10−14 cm3 s−1 under the population balance. The calculations were in good agreement with the measurements. Our results show that the dependence of inelastic collisions on spin channels plays an important role in the time-evolution of spin populations.

Loss of hydrogen and oxygen from the upper atmosphere of Venus

Atmospheric escape from the upper atmosphere of Venus is mainly influenced by the loss of hydrogen and oxygen caused by the interaction of solar radiation and particle flux with the unprotected planetary environment. Because one main aim of the ASPERA-4 particle/plasma and VEX-MAG magnetic field experiments on board of ESA's forthcoming Venus Express mission is the investigation of atmospheric erosion processes from the planet's ionosphere–exosphere environment, we study the total loss of hydrogen and oxygen and identified the efficiency of several escape mechanisms involved. For the estimation of pick up loss rates we use a gas dynamic test particle model and obtained average loss rates for H + , and O + pick up ions of about 1 × 10 25 s - 1 and about 1.6 × 10 25 s - 1 , respectively. Further, we estimate ion loss rates due to detached plasma clouds, which were observed by the pioneer Venus orbiter and may be triggered by the Kelvin–Helmholtz instability of about 0.5 – 1 × 10 25 s - 1 . Thermal atmospheric escape processes and atmospheric loss by photo-chemically produced oxygen atoms yield negligible loss rates. Sputtering by incident pick up O + ions give O atom loss rates in the order of about 6 × 10 24 s - 1 . On the other hand, photo-chemically produced hot hydrogen atoms are a very efficient loss mechanism for hydrogen on Venus with a global average total loss rate of about 3.8 × 10 25 s - 1 , which is in agreement with Donahue and Hartle [1992. Solar cycle variations in H + and D + densities in the Venus ionosphere: implications for escape. Geophys. Res. Lett. 12, 2449–2452] and of the same order but less than the estimated H + ion outflow on the Venus nightside of about 7.0 × 10 25 s - 1 due to acceleration by an outward electric polarization force related to ionospheric holes by Hartle and Grebowsky [1993. Light ion flow in the nightside ionosphere of Venus. J. Geophys. Res. 98, 7437–7445]. Our study indicates that on Venus, due to its larger mass and size compared to Mars, the most relevant atmospheric escape processes of oxygen involve ions and are caused by the interaction with the solar wind. The obtained results indicate that the ratio between H/O escape to space from the Venusian upper atmosphere is about 4, and is in a much better agreement with the stoichiometrically H/O escape ratio of 2:1, which is not the case on Mars. However, a detailed analysis of the outflow of ions from the Venus upper atmosphere by the ASPERA-4 and VEX-MAG instruments aboard Venus Express will lead to more accurate atmospheric loss estimations and a better understanding of the planet's water inventory.

Three-body recombination in two-dimensional atomic hydrogen gas

We study three-body recombination in the gas of spin-polarized atomic hydrogen adsorbed on the surface of superfluid helium at temperatures from $45\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}130\phantom{\rule{0.3em}{0ex}}\mathrm{mK}$. The two-dimensional gas is thermally compressed to densities up to $\ensuremath{\approx}4\ifmmode\times\else\texttimes\fi{}{10}^{12}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$ using the ``cold spot'' method which makes the three-body process the dominant decay channel in the system. We measure the loss rate and surface density of atoms directly and independently and observe the former to be proportional to the third power of the latter. The result for the surface three-body recombination rate constant at $4.6\phantom{\rule{0.3em}{0ex}}\mathrm{T}$, ${L}_{s}=2.0(7)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}25}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{4}∕\mathrm{s}$, significantly reduces the discrepancy between the theory and earlier measurements where the surface density was inferred from the adsorption isotherm. We also measured the three-body rate constant on a 0.1% $^{3}\mathrm{He}\text{\ensuremath{-}}^{4}\mathrm{He}$ film and found ${L}_{s}=1.3(4)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}24}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{4}∕\mathrm{s}$. This larger value is attributed to an increased delocalization of adsorbed hydrogen atoms in the direction normal to the surface.

Photoassociation dynamics in a Bose-Einstein condensate

A dynamical many-body theory of single color photoassociation in a Bose-Einstein condensate is presented. The theory describes the time evolution of a condensed atomic ensemble under the influence of an arbitrarily varying near-resonant laser pulse, which strongly modifies the binary scattering properties. In particular, when considering situations with rapid variations and high light intensities the approach described in this article leads, in a consistent way, beyond standard mean-field techniques. This allows one to address the question of limits to the photoassociation rate due to many-body effects which has caused extensive discussions in the recent past. Both the possible loss rate of condensate atoms and the amount of stable ground-state molecules achievable within a certain time are found to be stronger limited than according to mean-field theory. By systematically treating the dynamics of the connected Green's function for pair correlations the resonantly driven population of the excited molecular state as well as scattering into the continuum of noncondensed atomic states are taken into account. A detailed analysis of the low-energy stationary scattering properties of two atoms modified by the near resonant photoassociation laser, in particular of the dressed state spectrum of the relative motion prepares for the analysis of the many-body dynamics. The consequences of the finite lifetime of the resonantly coupled bound state are discussed in the two-body as well as in the many-body context. Extending the two body description to scattering in a tight trap reveals the modifications to the near resonant adiabatic dressed levels caused by the decay of the excited molecular state.

High-light-intensity photoassociation in a Bose-Einstein condensate

We investigate theoretically the molecular yield in photoassociation of Bose-Einstein condensed sodium atoms for light intensities of the order of and above those applied in a recent experiment. Our results show that the rate at which ground-state molecules may be formed saturates at high light intensities whereas the loss rate of condensate atoms does not. This is caused by the opposing roles of the short- and long-range pair correlations present near resonance under the influence of the laser and is crucial for the development of efficient photoassociation procedures in a condensate.

Magnetic field dependence of the dynamics ofRb87spin-2 Bose-Einstein condensates

We experimentally studied the spin-dependent collision dynamics of $^{87}\mathrm{Rb}$ spin-2 Bose-Einstein condensates confined in an optical trap. The condensed atoms were initially populated in the $\ensuremath{\mid}F=2,{m}_{F}=0⟩$ state, and their time evolutions in the trap were measured in the presence of external magnetic field strengths ranging from 0.1 to $3.0\phantom{\rule{0.3em}{0ex}}\mathrm{G}$. The atom loss rate due to inelastic two-body collisions was found to be $1.4(2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}\phantom{\rule{0.3em}{0ex}}{\text{cm}}^{3}\phantom{\rule{0.3em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$. Spin mixing in the $F=2$ manifold developed dramatically for the first few tens of milliseconds, and the oscillations in the population distribution between different magnetic components were observed over a limited range of magnetic field strengths. The antiferromagnetic property of this system was deduced from the magnetic field dependence on the evolution of relative populations for each ${m}_{F}$ component.

Diffusion model of the transport of sputtered metal atoms in a cylindrical vessel

Measurement of the diffusion loss rate of metal atoms sputtered by a pulsed gas discharge in a vessel represents a convenient method for determination of the diffusion coefficient of metal atoms in gases (J. Phys. D 30 (1997) 1878). This measurement should be combined with an appropriate diffusion model, which takes into account the experimental geometry and the interactions of the diffusing particles with the walls of the container. The effect of the latter is usually accounted for through the boundary condition used. In this work we consider a boundary condition based on a thermodynamic approach. The obtained diffusion model for cylindrical geometry is compared with experimental results for the diffusion of ground-state Mg atoms in Kr, as well as with some other model solutions.

Hydrogen Atom Reactions with Molecular Halogens: The Rate Constants for H + F2 and H + Cl2 at 298 K

The rate constants for H atom reactions with Cl2 and F2 have been measured by monitoring the loss rate of hydrogen atoms in the presence of excess [Cl2] and [F2] via time-resolved, laser-induced Lyman-α fluorescence. The rate constants for H + F2 and H + Cl2 were found to be 2.4 ± 0.4 (2σ) × 10-12 and 2.52 ± 0.18 (2σ) × 10-11 cm3 s-1, respectively. The result for H + F2 is consistent with the recommendation of Baulch et al. [J. Phys. Chem. Ref. Data 1981, 10 (Suppl. 1)], and our k(H + Cl2) value is consistent with the majority of previous measurements.

Interstitial oxygen loss and the formation of thermal double donors in Si

The combination of first-principles total energy calculations and a general kinetic model, which takes into account all processes of association, dissociation, and restructuring, is used to study the kinetics of thermal double donors (TDDs) in silicon over the temperature range of 300–650 °C. A strong correlation is found between the formation rate of TDDs and the loss rate of interstitial oxygen atoms. Also, a close agreement with experiments is obtained. It is found that TDDs grow via consecutive reactions where fast diffusing oxygen dimers and all TDDs capture interstitial oxygen atoms.

Excitation-assisted inelastic processes in trapped Bose-Einstein condensates

We find that inelastic collisional processes in Bose-Einstein condensates induce local variations of the mean-field interparticle interaction, and are accompanied by the creation and annihilation of elementary excitations. The physical picture is demonstrated for the case of three-body recombination in a trapped condensate. For a high trap barrier the production of high-energy trapped single-particle excitations results in a strong increase of the loss rate of atoms from the condensate.