Ground State Of Lithium Research Articles

Spin-state-controlled chemi-ionization reactions between metastable helium atoms and ground-state lithium atoms.

We demonstrate the control of 4He(23S1)-7Li(22S1/2) chemi-ionization reactions by all-optical electron-spin-state preparation of both atomic species prior to the collision process. Our results demonstrate that chemi-ionization is strongly suppressed (enhanced) for non-spin-conserving (spin-conserving) collisions at thermal energies. These findings are in good agreement with a model based on spin angular momentum coupling of the prepared atomic states to the quasi-molecular states. Small deviations from the model indicate the contribution of the 4Σ+ channel to the reaction rate, which is in violation of spin conservation.

Dynamics of positron scattering from lithium, sodium and potassium atoms in hot and dense plasmas

The dynamics of positron scattering from the ground state of lithium, sodium and potassium atoms in hot and dense plasmas has been investigated by applying a two-state approximation that includes elastic scattering and rearrangement scattering. The wave functions and energies of the target alkali atoms have been determined quite accurately within the framework of the ’method of model potential’. An inclusive study was made on the effects of plasma screening on the differential and total cross sections for elastic scattering and positronium formation in the ground state for the incident positron energy lying within 10 eV to 500 eV. For the unscreened case, our present results agree reasonably with the results of other calculations. It was found that the cross sections suffer considerable change due to the effects of the background plasma.

Pressure effect on hyperfine CPT resonance of ground-state Li atoms in glass hot-vapor cell

We report the hyperfine splitting frequency of ground-state lithium atoms. The frequency shift is induced by the collisions to helium and xenon buffer gases. Coherent population-trapping resonance between the hyperfine levels enables the frequency measurements in the range of pressure from 2 to $$54\,\hbox {kPa}$$ and temperature from 260 to $$400\,^{\circ }\hbox {C}$$ in borosilicate-glass cells. The buffer-gas density is self-consistently calibrated from the hyperfine splitting in the sealed cells. The resonance frequency and the line width provide an insight into collision-induced spin dynamics, where hyperfine-shift, spin-rotation, and spin-exchange interactions play an important role in atomic lithium vapor and buffer gas.

Laser continuum source atomic absorption spectroscopy: Measuring the ground state with nanosecond resolution in laser-induced plasmas

A new dual-beam atomic absorption technique is applied to laser-induced plasmas. The technique uses an optical parametric oscillator pseudocontinuum, producing emission that is both wider than the absorption line profile, but narrow enough to allow the use of an echelle spectrograph without order sorting. The dual-beam-in space implementation makes the technique immune to nonspecific attenuation of the probe beam and the structure of the pseudocontinuum. The potential for plasma diagnostics is demonstrated with spatially and temporally resolved measurements of magnesium metastable and lithium ground state optical depths in a laser-induced plasma under reduced pressure conditions. The lithium measurements further demonstrate the technique's potential for isotope ratio measurements.

Prediction of Stable Ground-State Lithium Polyhydrides under High Pressures.

Hydrogen-rich compounds are important for understanding the dissociation of dense molecular hydrogen, as well as searching for room temperature Bardeen-Cooper-Schrieffer (BCS) superconductors. A recent high pressure experiment reported the successful synthesis of novel insulating lithium polyhydrides above 130 GPa. However, the results are in sharp contrast to a previous theoretical prediction by the PBE functional that around this pressure range all lithium polyhydrides (LiHn (n = 2-8)) should be metallic. In order to address this discrepancy, we perform an unbiased structure search with first-principles calculation by including the van der Waals interaction that was ignored in the previous prediction to predict the high pressure stable structures of LiHn (n = 2-11, 13) up to 200 GPa. We reproduce the previously predicted structures, and further find novel compositions that adopt more stable structures. The van der Waals functional (vdW-DF) significantly alters the relative stability of lithium polyhydrides, and predicts that the stable stoichiometries for the ground state should be LiH2 and LiH9 at 130-170 GPa, and LiH2, LiH8, and LiH10 at 180-200 GPa. Accurate electronic structure calculation with GW approximation indicates that LiH, LiH2, LiH7, and LiH9 are insulative up to at least 208 GPa, and all other lithium polyhydrides are metallic. The calculated vibron frequencies of these insulating phases are also in accordance with the experimental infrared (IR) data. This reconciliation with the experimental observation suggests that LiH2, LiH7, and LiH9 are the possible candidates for lithium polyhydrides synthesized in that experiment. Our results reinstate the credibility of density functional theory in the description of H-rich compounds, and demonstrate the importance of considering van der Waals interaction in this class of materials.

Collisional stability of localized Yb(3P2) atoms immersed in a Fermi sea of Li

We establish an experimental method for a detailed investigation of inelastic collisional properties between ytterbium (Yb) in the metastable state and ground state lithium (Li). By combining an optical lattice and a direct excitation to the state we achieve high selectivity on the collisional partners. Using this method we determine inelastic loss coefficients in collisions between 174Yb() with magnetic sublevels of mJ = 0 and −2 and ground state 6Li to be and , respectively. Absence of spin changing processes in Yb()–Li inelastic collisions at low magnetic fields is confirmed by inelastic loss measurements on the mJ = 0 state. We also demonstrate that our method allows us to look into loss processes in few-body systems separately.

Comparison of experimental and theoretical fully differential cross sections for single ionization of the2sand2pstates of Li byO8+ions

This paper presents a full three-dimensional (3D) comparison between experiment and theory for 24 MeV ${\mathrm{O}}^{8+}$ single ionization of the $2s$ ground state of lithium and the $2p$ excited state. Two theoretical approximations are examined: the three-body continuum distorted-wave (3DW) and three-body continuum distorted-wave--eikonal initial state (3DW-EIS). Normally, there is a significant difference between these two approaches and the 3DW-EIS is in much better agreement with experiment. In this case, there is very little difference between the two approaches and both are in very good agreement with experiment. For the excited $2p$ state, the 3D cross sections would exhibit a mirror symmetry about the scattering plane if all three magnetic sublevels were excited in equal proportions. For the present experiment, the $2{p}_{+1}\phantom{\rule{4pt}{0ex}}(m=+1)$ sublevel is dominantly excited (quantization axis is the incident beam direction) and for this case there is a magnetic dichroism which is observed both experimentally and theoretically.

Ground-State Structures of Atomic Metallic Hydrogen

Ab initio random structure searching using density functional theory is used to determine the ground-state structures of atomic metallic hydrogen from 500 GPa to 5 TPa. Including proton zero-point motion within the harmonic approximation, we estimate that molecular hydrogen dissociates into a monatomic body-centered tetragonal structure near 500 GPa (r(s)=1.23) that remains stable to 1 TPa (r(s)=1.11). At higher pressures, hydrogen stabilizes in an …ABCABC… planar structure that is similar to the ground state of lithium, but with a different stacking sequence. With increasing pressure, this structure compresses to the face-centered cubic lattice near 3.5 TPa (r(s)=0.92).

Cold collisions of alkali-metal atoms and chromium atoms

We present ab initio potentials for ground state lithium, sodium, potassium and rubidium atoms interacting with ground state chromium atoms via the 6Σ+ and 8Σ+ states of the corresponding dimers. Each potential is matched to the leading van der Waals dispersion energy −C6/R6 − C8/R8 and an exchange energy; we list the values of C6, C8 and the exchange fitting parameters. We present calculated values from quantal and semi-classical approximations for the s-wave scattering length and effective range and the p-wave scattering volume for collisions of each of the alkali-metal atoms lithium, sodium, potassium and rubidium with 52chromium atoms and comment on s-wave scattering by 53chromium atoms.

Two-electron photoionization of ground-state lithium

We apply the convergent close-coupling (CCC) formalism to single-photon two-electron ionization of the lithium atom in its ground state. We treat this reaction as single-electron photon absorption followed by inelastic scattering of the photoelectron on a heliumlike ${\text{Li}}^{+}$ ion. The latter scattering process can be described accurately within the CCC formalism. We obtain integrated cross sections of single photoionization leading to the ground and various excited states of the ${\text{Li}}^{+}$ ion as well as double photoionization extending continuously from the threshold to the asymptotic limit of infinite photon energy. Comparison with available experimental and theoretical data validates the CCC model.

Few‐parameter exponentially correlated wavefunctions for the ground state of lithium

AbstractCompact, but relatively accurate wavefunctions for the ground state of the Li atom were obtained through the use of a limited basis of exponentially correlated functions with optimized nonlinear parameters. In contrast to our earlier work, the basis contains pre‐exponential factors that improve the rate of convergence of the basis‐set expansion. The matrix elements needed in the present work were evaluated analytically using recursive methods reported recently by one of us; a check on the programming was provided by comparison with numerical evaluations carried out by Turbiner and Guevara. The rate of convergence of the expansion is compared with those of Hylleraas‐basis computations, and a comparison is also made with exponentially correlated studies of He‐like systems. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009

Sum-over-states calculations of the polarizabilities of the ground and first excited states of the lithium atom

Approximate moments S(k) of the oscillator strength distribution are calculated by a finite sum over states. Small configuration interaction (CI) wave functions arc constructed in such a manner that the length and velocity formulas for S(−2), the static polarizability, give essentially identical results. The oscillator strengths are used to obtain the frequency dependent polarizability. Both the static and dynamic results for the lithium ground state compare favorably with values obtained with extensive CI wave functions. The method is also applied to the first excited (2P) state of lithium.

The Coulomb four-body problem in a classical framework: triple photoionization of lithium

Formulating a quasiclassical approach we determine the cross section for the complete four-body break-up of the lithium ground state following single photon absorption from threshold up to 220 eV excess energy. In addition, we develop a new classification scheme for three-electron ionizing trajectories in terms of electron–electron collisions, thereby identifying two main ionization paths which the three electrons in the ground state of lithium follow to escape to the continuum. The dominant escape paths manifest themselves in a characteristic ‘T-shape’ break-up pattern of the three electrons which implies observable structures in the electronic angular correlation probability. This break-up pattern prevails for excess energies so low that the Wannier threshold law σ ∝ Eα describes already the triple ionization cross section, whose predicted value α = 2.16 we can confirm quantitatively.

Ab initio study of low-lying triplet states of the lithium dimer

Observation of Bose–Einstein condensation in Li 2 7 initiated the interest in the scattering length of two ground state lithium atoms when they approach each other as a radical pair triplet a Σ u + 3 state. But some properties of this state are still unknown. In present work, a number of low-lying triplet states of lithium molecule are calculated by multi-configuration self-consistent field (MCSCF) and response techniques with account of spin–orbit coupling, spin–spin coupling and some other magnetic perturbations. The singlet–triplet transition probabilities to the ground state are also presented. Most results are connected with the weakly bound lowest triplet a Σ u + 3 state, whose radiative lifetime and spin-splitting are unknown so far in spite of its great importance in Bose–Einstein condensation. Calculations indicate that this state has a very small spin-splitting, λ ss = − 0.01 cm −1, which is negligible in comparison with the line-width in experimental Fourier transform spectra published so far. Similar splitting is obtained for the upper state of the 1 Σ g + 3 − − a Σ u + 3 transition. This is in agreement with experimental rovibronic analysis of the 1 Σ g + 3 − − a Σ u + 3 band system in which the triplet structure was not resolved. The radiative lifetime of the a Σ u + 3 state is predicted to exceed 10 h.

Unified treatment of chemical and van der Waals forces via symmetry-adapted perturbation expansion.

We propose a symmetry-adapted perturbation theory (SAPT) expansion of the intermolecular interaction energy which in a finite order provides the correct values of the constants determining the asymptotics of the interaction energy (the van der Waals constants) and is convergent when the energy of the interacting system is submerged in the continuum of Pauli-forbidden states-the situation common when at least one of the monomers has more than two electrons. These desirable features are achieved by splitting the intermolecular electron-nucleus attraction terms of the Hamiltonian into regular (long-range) and singular (short-range) parts. In the perturbation theory development, the regular part is treated as in the conventional polarization theory, which guarantees the correct asymptotics of the interaction energy, while the singular part is weakened sufficiently by an application of permutational symmetry projectors so that a convergent perturbation series is obtained. The convergence is demonstrated numerically, for both the chemical and van der Waals minima, by performing high-order calculations of the interaction energy of the ground-state lithium and hydrogen atoms-the simplest system for which the physical ground state is submerged in the Pauli-forbidden continuum. The obtained expansion enables a systematic extension of SAPT calculations beyond second order with respect to the intermolecular interaction operator.

The potential energy curve of the ground state of lithium, [formula omitted] Li 2

Seven reported potential energy curves (RKR, IPA, and rotationless) are compared for the ground states of 7Li 2, 6Li 7Li, and 6Li 2. Discrepancies among them, particularly in the repulsive (inner) limb, are evaluated. Irregularities in the behavior of the “spectroscopic” parameter β of the Morse function are found with some of them. The “universal” potential energy function that produces a priori potentials, without prior knowledge of them [J. Am. Chem. Soc. 113 (1991) 4755], yields a curve that lies generally between the seven spectroscopic potentials over most of their domain. The a priori potential appears as a consensus and is judged to be the best estimate of the potential energy curve of Li 2. Values of distances obtained from it are presented at the reported energy levels of 7Li 2.

Convergence behavior of the symmetry-adapted perturbation theory for states submerged in Pauli forbidden continuum

The polarization expansion and the symmetry-adapted perturbation theory (SAPT) in the symmetrized Rayleigh–Schrödinger (SRS) and the Hirschfelder–Silbey (HS) formulations are applied through high order to the medium- and long-range interaction of the ground-state lithium and hydrogen atoms. The interaction energies obtained by perturbation theory are compared with the counterpoise-corrected full configuration interaction results. It is shown that the SRS and HS expansions diverge as a result of the presence of the Pauli forbidden continuum in which the physical eigenstates of the perturbed Hamiltonian are submerged. Despite this divergence, the SAPT expansions give accurate results in low orders and excellent results when summed up in a standard way of assigning a sum to an asymptotically convergent series. The polarization expansion is found to diverge as well, with its asymptotic limit equal to the arithmetic mean of singlet and triplet energies. Unlike the case of simpler systems, for the interaction of lithium and hydrogen atoms the Hirschfelder–Silbey method does not provide any improvement over the much simpler SRS approach.

Calculations of magnetic moments for three-electron atomic systems.

The first fully correlated calculations of the magnetic moment in lithium are presented. Relative to the free-electron value, the Zeeman gJ factor for the ground state lithium gJ/g(e)-1 is calculated to a computational accuracy of 200 parts in 10(9), including relativistic and radiative corrections of orders alpha2, alpha2m/M, and alpha3. The isotope shifts in gJ are predicted precisely for various isotopes. The extensions to the first excited S state of lithium and the ground state of Be+ are made.

Cross Sections for Photon Emission Following Electron Capture by C2+, N3+, N4+ and O3+ Ions in Collision with Li(2s) Atoms

We report measurements of photon emission cross sections following electron capture of C2+, N3+, N4+ and O3+ ions (obtained from an ECR ion source) in collision with ground state lithium atoms at keV energies. We examine the sources of error and obtain best estimates of the relative values of the cross sections having accuracies in the range from 15% to 50%. The accuracy of the absolute cross sections may be lower for capture by N3+ and N4+ ions because of the uncertainty in the fraction of metastable ions. We discuss this problem.

Lithium vapour excitation at 2S $\mathsf{\rightarrow}$ 3D two-photon resonance

We report study of processes which occur in lithium vapour under two-photon excitation of the Li(3D) state at 639.1 nm. A time-resolved technique has been used to measure the fluorescence from the Li(3D), Li(2P) and Li(3P) states. We have determined radiation rates for lithium atom densities in the range 1013–1014 cm−3 and laser powers (105–106 Wcm−2). The ground-state lithium atom density was determined by knowing temperature and vapour pressure in a modified heat-pipe oven. The contribution to radiation rates from different processes and prospect for cross-section determination of homonuclear reverse energy-pooling are discussed.