Accurate calculation of the scattering length for the cooling of hydrogen atoms by lithium atoms

An improved ab initio calculation has been performed for the potential for the LiH a ³Σ⁺ state, using two very large basis sets. The Basis Set Superposition Error (BSSE) correction has been determined for both basis sets and the non-Born-Oppenheimer correction estimated to be negligible. The best potential is approximately 10% deeper than the previous estimate. Vibrational energies and scattering lengths have been calculated for ^6,7LiH(D) with both potentials, with and without the BSSE correction, and also with an estimated potential expected to bracket the true potential. The ⁷LiH scattering length is estimated to be (45 ± 4)a₀ and hence the low-energy cross-section in the best a ³Σ⁺ potential is about half that calculated previously. Enhanced cooling by ⁷Li of trapped H atoms remains feasible. Received 30 April 2001     

Calculation of scattering parameters for ultracold ~6 Li-~7 Li elastic collisions

This paper theoretically studies the elastic scattering properties in a mixture of 6 Li and 7 Li atoms at cold and ultracold temperatures.Based on the constructed accurate interatomic potential of the triplet state for 6 Li 7 Li mixture by the mass scaling method,it calculates the interspecies s-wave scattering lengths and the p-wave scattering lengths by the variable phase method and the semiclassical method,respectively.The scattering length is in good agreement with the experiment.The partial-wave and total cross sections are also calculated and a rich resonance structure is found.     

K-Li冷碰撞散射特性的理论研究

本文应用量子方法和半经典方法计算了表征锂(6Li,7Li) 原子和钾(39K,40K,41K) 原子间超冷碰撞特性的散射参数,如s波散射长度,有效力程,p波散射长度等。超冷温度下6Li-39K单重态和三重态原子间的弹性散射截面主要为s波贡献,随着碰撞能量的增加散射截面有丰富的形状共振出现,计算发现单重态和三重态散射截面均存在显著的i波形状共振. 此外,本文应用简并内态近似获得了超精细态相互作用时的s波散射长度.     

Accurate calculation of the elastic scattering properties of ^7Li atoms at ultralow temperatures

The elastic scattering properties for collisions between two ^7Li atoms are investigated in the cold and ultracold regimes separately. Based on recent theoretical and experimental results, we present the improved hybrid potentials for the singlet X^1 ∑g^＋ and triplet a^3 ∑u^＋ ground states of the Li2, Our calculated values for the scattering lengths α and the effective ranges re are compared with previous ones, and found them to be in good agreement. The scattering lengths are 34.6α0 for the singlet state and -27.6α0 for the triplet state. Shape resonances occur in the collisions at low energies. We also calculate the total cross sections and the energy positions of shape resonances for both X^1 ∑g^＋ and a^3 ∑u^＋ states.     

Ab initio electronic, dynamic, and thermodynamic properties of isotopic lithium hydrides (LiH, LiD, LiT, LiH, LiD, and LiT)

The structural, dielectric, lattice-dynamical, and thermodynamical properties of isotopic lithium hydrides (⁶LiH, ⁶LiD, ⁶LiT, ⁷LiH, ⁷LiD, and ⁷LiT) were investigated within density-functional theory. The atomic structure was fully relaxed and the structural parameters were found to differ by less than 2% from the experimental data. The associated electronic band structure and density of states were also presented. A linear-response approach to the density-functional perturbation theory was employed to work out the Born effective charges, dielectric tensors and phonon frequencies, and thermodynamic properties. The compounds with the heavier Li isotope or H isotope have the lower phonon frequencies; ⁶LiT is more stable than ⁷LiT, ⁶LiD, ⁷LiD, ⁶LiH, and ⁷LiH in the temperature range 0-2700 K. These properties of LiT were predicted for the first time. The results were discussed in terms of the isotope effects on phonon dispersion curves and thermodynamic properties.     

Elastic scattering of sodium and cesium atoms at ultracold temperatures

The elastic scattering properties in a mixture of sodium and cesium atoms are investigated at cold and ultracold temperatures. Based on the accurate interatomic potential for the NaCs mixture, the interspecies s-wave scattering lengths, the effective ranges and the p-wave scattering lengths are calculated by the quantal method and the semiclassical method, respectively. The s-wave scattering lengths are 512.7a₀ for the singlet state and 33.4a₀ for the triplet state. In addition, the spin-change and elastic cross sections are also calculated, and the g-wave shape resonance is found in the total elastic cross sections.     

超冷温度下钾和铯原子间弹性散射特性的精确计算

本文分别用量子方法和半经典方法计算了超冷钾和铯原子之间弹性碰撞的s波散射长度,有效力程和p波散射长度等散射参数. 超冷温度下³⁹K-¹³³Cs原子间的弹性散射截面主要为s波贡献,随着碰撞能量的增加散射截面有丰富的形状共振出现, 计算发现单重态和三重态截面分别存在显著的g波和d波形状共振.另外,本文应用简并内态近似方法获得了⁴¹K-¹³³Cs 超精细态相互作用时的s波散射长度.     

Precise determination of 6Li cold collision parameters by radio-frequency spectroscopy on weakly bound molecules

We employ radio-frequency spectroscopy on weakly bound (6)Li(2) molecules to precisely determine the molecular binding energies and the energy splittings between molecular states for different magnetic fields. These measurements allow us to extract the interaction parameters of ultracold (6)Li atoms based on a multichannel quantum scattering model. We determine the singlet and triplet scattering lengths to be a(s) = 45.167(8)a(0) and a(t) = -2140(18)a(0) (1a(0) = 0.052 917 7 nm), and the positions of the broad Feshbach resonances in the energetically lowest three s-wave scattering channels to be 83.41(15), 69.04(5), and 81.12(10) mT.     

Calculation of the elastic collision properties of Na and Li atoms at ultracold temperature

This paper firstly reports a theoretical study of elastic scattering properties in a mixture of ²³Na and ⁷Li atoms at cold and ultracold temperatures in detail. Based on the new constructed accurate singlet X¹∑_g⁺ and the triplet a³∑_u⁺ states interatomic potentials for ²³Na⁷Li mixture, it calculates the scattering lengths and the effective ranges by three computational methods, and obtains good agreements. Using the mass scaling method, it also calculates ²³Na⁶Li scattering lengths and s-wave and total elastic cross sections, whose rich resonance structures were found and interpreted in terms of quasibound diatomic levels trapped behind a centrifugal barrier.     

超冷钠原子弹性散射特性的精确计算

基于精确的原子之间相互作用势,系统研究了钠原子在超冷温度下的弹性散射特性,精确计算了钠原子间碰撞时的s波散射长度、有效力程、p波散射长度以及束缚态数目等散射参数.超冷温度下单重态和三重态原子间的弹性散射截面主要为s波贡献,随着碰撞能量的增加散射截面有丰富的形状共振出现,计算发现单重态和三重态散射截面分别存在显著的f波和i波形状共振.应用简并内态近似方法获得了超精细态相互作用时的s波散射长度,所得结果与精确值比较符合.     

Mössbauer spectra obtained using β − γ coincidence method after 57Mn implantation into LiH and LiD

Highly energetic ⁵⁷Mn (T_1/2 = 1.45 m) was generated by nuclear projectile fragmentation in a heavy-ion accelerator, and implanted into lithium hydride (LiH) and lithium deuteride (LiD) at 578 K. Mössbauer spectroscopy with β ? γ coincidence detection was then carried out on the ⁵⁷Fe obtained from β^?decay of the ⁵⁷Mn to study the time dependence of the site distributions and coordination environments of dilute Fe atoms implanted in the LiH and LiD. The results suggest that the Fe atoms can substitute for either the Li and H or D atoms within 100 ns. Additionally, the displacement behavior of the substitutional ⁵⁷Fe atoms on the lattice sites is discussed.     

The scattering lengths for alkali-metal atoms

The collision of alkali-metal atoms at ultralow temperatures have been studied, The Scattering lengths and the effective range are calculated for 7Li, 23Na, 39K, 87Rb, and 133Cs.     

Thermal expansion and mode Grüneisen parameters of LiH and LiD

The third order contribution of the shell-shell interaction between next nearest neighbours has been determined from a calculation of the coefficient of linear thermal expansion of LiH and ⁷LiD. The Coulomb anharmonicity is included. A shell model fitted to phonon energies from inelastic neutron scattering in ⁷LiD was used. From this model then averaged Grüneisen constants and individual mode Grüneisen parameters have been deduced.     

Scattering length for fermionic alkali atoms

We examine the use of the WKB approximation to determine the p-wave scatteringlength. For this we solve the p partial wave Schrödingerequation and analyse the validity of adopting the semiclassical solution toevaluate the constant factors in the solution. We also calculate the p-wavescattering lengths of ⁶Li and ⁴⁰K for the a³Σ⁺ _u andX¹Σ⁺ _g states respectively using the variable phase method. Thep-wave scattering lengths of ¹³²Cs and ¹³⁴Cs are also calculated.Based on our calculations, the value of the p-wave scattering lengths of⁶Li and ⁴⁰K are -36a_o and -95a_o respectively.     

Quantum degenerate mixtures of alkali and alkaline-earth-like atoms

We realize simultaneous quantum degeneracy in mixtures consisting of the alkali and alkaline-earth-like atoms Li and Yb. This is accomplished within an optical trap by sympathetic cooling of the fermionic isotope ?Li with evaporatively cooled bosonic 1??Yb and, separately, fermionic 1?3Yb. Using cross-thermalization studies, we also measure the elastic s-wave scattering lengths of both Li-Yb combinations, |a(?Li-1??Yb)| = 1.0 ± 0.2 nm and |a(?Li-1?3Yb)| = 0.9 ± 0.2 nm. The equality of these lengths is found to be consistent with mass-scaling analysis. The quantum degenerate mixtures of Li and Yb, as realized here, can be the basis for creation of ultracold molecules with electron spin degrees of freedom, studies of novel Efimov trimers, and impurity probes of superfluid systems.     

Quantum dynamics studies on the non-adiabatic effects of H + LiD reaction

After the Big Bang, chemical reactions of hydrogen with LiH and its isotopic variants played an important role in the late stage of recombination. Moreover, these reactions have attracted the attention of experts in the field of molecular dynamics because of its simple structure. Electronically non-adiabatic effects play a key role in many chemical reactions, while the related studies in LiH₂ reactive system and its isotopic variants are not enough, so the microscopic mechanism of this system has not been fully explored. In this work, the microscopic mechanism of H + LiD reaction are performed by comparing both the adiabatic and non-adiabatic results to study the non-adiabatic effects. The reactivity of R1 (H + LiD → Li + HD) channel is inhibited, while that of R2 (H + LiD → D + LiH) channel is enhanced when the non-adiabatic couplings are considered. For R1 channel, a direct stripping process dominates this channel and the main reaction mechanism is not influenced by the non-adiabatic effects. For R2 channel, at relatively low collision energy, the dominance changes from a rebound process to the complex-forming mechanism when the non-adiabatic effects are considered, whereas the rebound collision approach still dominates the reaction at relatively high collision energy in both calculations. The presented results provide a basis for further detailed study on this importantly astrophysical reaction system.     

Neutron scattering length of lithium and boron and their isotopes

Coherent neutron scattering lengths and free cross sections were measured in order to determine the spin dependent scattering lengths of the isotopes of Li and B. The transmission measurements on Li at neutron energies of 0.51 meV, 1.26 eV and 5.19 eV delivered data for the⁶Li-abundances in the samples and for the free scattering cross section of⁷Li. By means of experiments on 15 different samples of lithium compounds we obtained the complex spin state scattering lengths for the bound atoms to be: (ie81-01) Measurements on 3 boron samples with different enrichments led to (ie81-02) A review on slow neutron scattering and resonance data shows an over all consistency of all values.     

Positron scattering by atoms

We describe the present status of coupled-state calculations for positron scattering by ‘one-electron’ atoms. We show how pseudostates are used to represent the continuum channels. Illustrative results from positron scattering by atomic hydrogen and the alkali metals are presented.     

Quantum-degenerate mixture of fermionic lithium and bosonic rubidium gases

We report on the observation of sympathetic cooling of a cloud of fermionic 6Li atoms which are thermally coupled to evaporatively cooled bosonic 87Rb. Using this technique we obtain a mixture of quantum-degenerate gases, where the Rb cloud is colder than the critical temperature for Bose-Einstein condensation and the Li cloud is colder than the Fermi temperature. From measurements of the thermalization velocity we estimate the interspecies s-wave triplet scattering length |amx|=20(+9)(-6)aB. We found that the presence of residual rubidium atoms in the |2, 1> and the |1, -1> Zeeman substates gives rise to important losses due to inelastic collisions.     

超低温下钠原子散射特性研究

超低温下钠原子的散射特性对原子间的相互作用势非常敏感.本文基于构造精确的原子间相互作用势,详细研究了在超冷温度下处在超精细态|F=2,mF=2〉下23Na原子的弹性散射特性.我们分别用Numerov和半经典方法计算了散射长度和有效程,得到了较满意的结果.低能散射截面有丰富的共振现象产生,我们发现并给出了d-波和g-波形状共振的能级位置和共振宽度;此外,我们还对仅两个分波有贡献情况下的微分散射截面进行了理论计算.