Lithium Dimer Research Articles

Di-μ-acetone-κ2 O:O-bis[(acetone-κO)aqualithium(I)] di-μ-acetone-κ2 O:O-bis[diaqualithium(I)] tetrakis{[phthalocyaninato(2−)-κ4 N,N′,N′′,N′′′]lithiate(I)}

The crystals of the title compound, [Li2(C3H6O)4(H2O)2][Li2(C3H6O)2(H2O)4] [Li(C32H16N8)]4, were obtained by a recrystallization of dilithium phthalocyanine from an acetone/water/toluene solution. The title structure consists of two independent Li–phthalocyanine complexes along with acetone-bridged lithium dimers. The lithium dimers are located on inversion centers. The crystal structure is stabilized by a network of hydrogen bonds.

Atom interferometry measurement of the electric polarizability of lithium

Using an atom interferometer, we have measured the static electric polarizability of 7Li α=(24.33 ±0.16)×10-30 m3 = 164.2±1.1 atomic units with a 0.66% uncertainty. Our experiment, which is similar to an experiment done on sodium in 1995 by Pritchard and co-workers, consists in applying an electric field on one of the two interfering beams and measuring the resulting phase-shift. With respect to Pritchard's experiment, we have made several improvements which are described in detail in this paper: the capacitor design is such that the electric field can be calculated analytically; the phase sensitivity of our interferometer is substantially better, near 16 mrad/ $\sqrt{\mbox{Hz}}$ ; finally our interferometer is species selective so that impurities present in our atomic beam (other alkali atoms or lithium dimers) do not perturb our measurement. The extreme sensitivity of atom interferometry is well illustrated by our experiment: our measurement amounts to measuring a slight increase Δv of the atom velocity v when it enters the electric field region and our present sensitivity is sufficient to detect a variation Δv/v ≈6 ×10-13.

Calculation of adiabatic potentials of Li 2

We report adiabatic potential energy curves of the lithium dimer. Our curves are tabulated according to internuclear distance from 3.2 a 0 to 88 a 0. We compare our theoretical results with the ones calculated by other authors and potential energy curves derived from experiments. In our approach we use the configuration interaction method where only the valence electrons of Li atom are treated explicitly. The core electrons are represented by pseudopotential. All calculations are performed by means of MOLPRO program package.

Nature of Bridging Bonds in Lithium and Potassium Acetate Dimers

The structures of lithium and potassium acetates were studied by the RHF/6-31G** quantum-chemical method. The metal-oxygen bonds in the dimers (CH3COOLi)2 and (CH3COOK)2 are electrostatic in nature. The bridging lithium bond is intermediate between hydrogen and ionic, and its peculiarity is determined by the deformation of molecular orbitals in the field of the lithium cation. The resulting data allow the variety of glassy smectic mesophases of lithium acetate.

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.

Isotope effects and Born-Oppenheimer breakdown in excited singlet states of the lithium dimer.

Observation of infrared electronic transitions involving the 1 (1)Deltag state of 7Li2 has instigated an investigation of Born-Oppenheimer breakdown in four singlet electronic states correlating with (2s+2s), (2s+2p), and (2p+2p) lithium atoms. The 1 (1)Deltag state, which correlates at long range with (2p+2p) atoms, has been observed in emission from the (5p) (1)Piu Rydberg state and in 1 (1)Deltag-B (1)Piu bands, in both instances following optical-optical double-resonance excitation. The latter transition was observed previously for the lighter isotopomer, 6Li2 [C. Linton, F. Martin, P. Crozet, A. J. Ross, and R. Bacis, J. Mol. Spectrosc. 158, 445 (1993)]. By analyzing multiple-isotopomer data for several electronic systems simultaneously, we have determined the electronic isotope shifts and the leading vibrational and/or rotational Born-Oppenheimer breakdown terms for the X (1)Sigmag+, A (1)Sigmau+, B (1)Piu, and 1 (1)Deltag states of the lithium dimer. This paper also reports Fourier transform measurements of the B-X absorption spectra of 6Li2 and 7Li2, which were required to better define the bottom portion of the B (1)Piu state potential.

Inducing a sign inversion in one state of a two-state superposition using ultrafast pulse shaping

An ultrafast pulse shaping scheme is presented that transiently modifies a two-state superposition on the E {sup 1}{sigma}{sub g}{sup +} curve of the lithium dimer. At short time delays, the wave-function amplitude of one of the states is forced to undergo a sign change, while the sign of the second state is programmed to remain static, analogous to the operation of a quantum-computational Z gate. This is observed as a {pi} phase shift in the time-dependent wave-packet signal for pump-probe delays <1.5 ps, relative to longer time delays, greater than 2.5 ps. This shift in sign is accomplished by taking advantage of the separability of the resonant and nonresonant light field effects in the creation of the excited-state wave function. The results show that, for a single state, the resonant and nonresonant light field effects can either be added or subtracted to create the total time evolution of the excited-state wave-function coefficient. If the nonresonant contributions are subtracted from the resonant ones at a time delay when the nonresonant term dominates, then for a short time the excited-state coefficient will have a sign opposite to that at long time, where the resonant term dominates. A desired phase function is derivedmore » to produce the opposition in sign of resonant versus nonresonant contributions, and the experimental transient Z-gate matrix elements are quantified.« less

Laser vaporizedLi2,Na2,K2,and LiNa molecules observed by cavity ring-down spectroscopy

Laser ablation of solid lithium, sodium, potassium, and lithium-sodium alloy was performed using a 308-nm excimer laser at fluences close to $1\mathrm{J}{\mathrm{cm}}^{\ensuremath{-}2}.$ Frequency and time resolved $A\ensuremath{-}X$ and $B\ensuremath{-}X$ electronic transitions of ${}^{6,7}{\mathrm{Li}}_{2},$ ${\mathrm{Na}}_{2},$ LiNa and ${\mathrm{K}}_{2}$ respectively, were observed. Lithium atom, dimer velocities, and influence of the background gas on dimer content of the plume have been measured. The origin of dimers at high densities, with subthermal internal energy, is discussed.

The potential energy surface for spin-aligned Li3(1 4A′) and the potential energy curve for spin-aligned Li2(a 3Σu+)

A global potential energy surface (PES) for the 1 4A′ spin-aligned state of Li3 is presented. The surface is constructed as a many body expansion of the potential which is the sum of pairwise additive two-body potentials plus a three-body term. The two-body potential is that for the a 3Σu+ state of the lithium dimer. It combines the most recent Rydberg–Klein–Rees potential available [A. Ross (private communication)] with well-known short and long range expansions and accurately reproduces all known experimental data. To obtain the three-body contributions, an ab initio PES was computed at 1122 points using full configuration interaction for the three valence electrons with an augmented Gaussian basis and the effective core potentials of Stevens, Basch, and Krauss [W. J. Stevens et al., J. Chem. Phys. 81, 6026 (1984)] for the other electrons. The two-body interactions are also calculated using the same basis and then subtracted from the full interaction to give the three-body term. To construct the three-body potential at arbitrary configurations we use interpolation for small perimeters of the triangle formed by the triatomic system and an analytic fitting function for large perimeters. A switching function guarantees the smoothness of the potential function everywhere. The equilibrium position occurs at D3h symmetry with a bond distance of 5.861a0, nearly 2a0 smaller than the equilibrium value of 7.886a0 of the lithium dimer. The well depth at the equilibrium is 4112.64 cm−1. This is considerably deeper than the well depth of 1001.22 cm−1 for the pairwise additive potential at its equilibrium. Three-body effects are even more important for Li3 than in the recently reported Na3 case [J. Higgins et al., J. Chem. Phys. 112, 5751 (2000)], and the nonadditive three-body term cannot be neglected in any calculation on this system.

Born–Oppenheimer breakdown in a combined-isotopomer analysis of the A 1Σu+–X 1Σg+ system of Li2

New high resolution polarization data have been obtained for the A–X band system of Li2 6,7, and new Fourier transform data for the homonuclear lithium dimers. They are combined with earlier data for Li26,6 and Li2 7,7 in the first systematic combined-isotopomer analysis of data for Li2 . This analysis of 8445 rovibrational transitions yields an improved and internally consistent set of molecular constant for the three Li2 isotopomers, and determines the electronic isotope shift and leading vibrational and rotational Born–Oppenheimer breakdown correction terms for both electronic states.

Lithium Hydrides and Lithium Dimers: Sub‐Doppler Spectroscopy and Dynamics

AbstractDeep insights into molecular structures and dynamics were obtained for lithium dimers and lithium hydrides by sub‐Doppler spectroscopy combining molecular beams and cw tunable laser beams. For 7Li7Li species, accurate spectroscopic studies of the B 1Πu‐X 1Σg+ system were carried out, with the measurement of hundreds of wave numbers and with the determination of dissociation rates due to tunneling through the potential barrier of the B 1πu state. They led to the full description of the B state potential, including its barrier. For 6Li7Li species, weak but non‐negligible dissociation rates in the B 1Πu state were observed, measured and interpreted assuming that an electronic coupling between the B 1Πu and 1 1Πg states takes place through an ungerade‐gerade symmetry breaking. For LiH species, the A 1Σ+‐X 1 Σ+ system was revisited for both isotopomers. For this molecule, dynamical effects are predicted in highly‐excited Rydberg states, that could be studied by use of similar sub‐Doppler techniques.

Measurement of transition dipole moments in lithium dimers using electromagnetically induced transparency.

We have observed electromagnetically induced transparency in a Doppler broadened molecular cascade system using fluorescence detection. We demonstrate that the power-dependent splitting of lines in the upper-level fluorescence excitation spectrum can be used as a new spectroscopic tool for the measurement of molecular transition dipole moment functions.

Observation of lithium dimers in laser produced plume by cavity ring-down spectroscopy

Laser ablation of solid lithium with 308 nm excimer laser is performed. Cavity ring-down absorption spectroscopy is used to detect atoms and molecules in the plume. They are observed at given distances from the target depending on different time delay from vaporization and observation window width. Large ratio of dimer to atom densities is observed compared to the thermalized vapor. Fast dimers appear with rovibrational temperatures about 175 K. Slow dimers exhibit non-Boltzmann rovibrational distribution in which large rotational numbers are preferred.

The E1Σg+ state of lithium dimer revised

This article describes the use of the inverted perturbation approach method for construction of an accurate potential energy curve of the E1Σg+ “shelf” state in Li2. Contrary to previous reports the applied method provides a potential curve free of unphysical oscillations and reproduces energies of all the levels known from previous experiments. Our results confirm the possibility of describing the E state within the Born–Oppenheimer approximation.

Ultrafast spectroscopy of wavelength-dependent coherent photoionization cross sections of Li2 wave packets in the E1Σg+ state: The role of Rydberg states

The significance of Rydberg states in the probing (via ionization) of Li2 wave packets has been studied through quantitative measurements of the relative coherent ionization cross sections in a two-color pump–probe femtosecond experiment. Following the preparation of a single intermediate rovibronic state with a cw laser, a femtosecond pump pulse (around 800 nm) creates a single two-state rotational wave packet by coherent excitation of the E1Σg+ (ν=9; J=27 and 29) states. The wave packet is then probed through ionization using time-delayed, wavelength tunable pulses (in the region 508–690 nm) while the total energy of the system is kept below the dissociation limit of Li2+. The background-free coherent ionization yield (for each probe wavelength) is measured as the relative oscillation amplitude of the single quantum beat time-dependent signal. The experimental results closely follow a relatively simple theoretical model, which is based on the assumption that the coherent ionization predominantly takes place via the excitation of high-n bound singly excited Rydberg states in the ionization continuum converging to the X 2Σg+ ground electronic state of Li2+. The best interpretation is that the high-n Rydberg states (above n∼25) undergo collisional ionization or autoionization and contribute to the measured coherent ionization signal, while the low-n Rydberg states undergo predissociation and do not contribute to the measured signal. An implication of the results is that the final states of the Li2 system, accessed by the above probe pulses, can be better approximated by a corresponding set of isolated discrete levels rather than by a continuum. This conclusion is important to experimental, as well as theoretical, coherent control and wave packet dynamics studies, in particular, when phase- and amplitude-shaped pump and probe pulses are employed. This study is also the first to investigate ionization of lithium dimer slightly below the E 1Σg+ shelf region with visible light.

Energy gap dependence of vibrational dephasing rates in a bath: a semigroup description

The quantum dynamical semigroup formalism provides an appealing general framework for discussing factors that affect pure vibrational dephasing rates in chemical systems. Within this framework, we formulate a Poisson model of pure vibrational dephasing which is more generally applicable than the commonly employed stochastic Gaussian dephasing model. In the limit of small and frequent phase changes, the Poisson model reduces to the stochastic Gaussian form. We find that for certain vibrational states of the lithium dimer in argon, the stochastic Gaussian model is valid, while for other states large and abrupt phase changes clearly require application of the Poisson model. In the former case, dephasing rates increase with the difference in quantum number between constituent vibrational states, while in the latter case, the dependence on quantum number difference or energy gap can become negligible. Recent experimental advances described by Amitay and Leone are expected to permit experimental tests of our theoretical predictions.

A full description of the potential curve of the B 1Πu state of Li27

An accurate potential curve for the bound and quasibound region of the B 1Πu state of Li27 is produced by analyzing high resolution sub-Doppler B 1Πu←X 1Σg+ excitation spectra in conjunction with lower resolution data of Hessel and Vidal [J. Chem. Phys. 70, 4439 (1979)] and of Russier et al. [J. Mol. Spectrosc. 168, 39 (1994)]. The bound and quasibound part of the curve is generated by the direct fit of molecular energies to a numerical potential; the outermost (repulsive) part of the curve is calculated by an asymptotic method using Coulombic and exchange parameters determined from the 1 1Πg and A 1Σu+ states of the lithium dimer. The full potential energy curve of the B 1Πu state reproduces all measured energies of f parity, and the widths of the predissociated lines, to within the experimental accuracy.

Phase-tailoring molecular wave packets to time shift their dynamics

Time shifting (up to a global arbitrary phase) the dynamics of molecular wave packets, i.e, | Ψ( t)〉→| Ψ( t− t shift)〉, is demonstrated using a high degree of state selective coherent phase control with shaped femtosecond laser pulses. The benchmark system for the present work is the lithium dimer molecule. The phase-tailored Li 2 wave packets are composed of several rovibrational states of the electronic E 1Σ g + shelf state excited from a single rovibrational level (selected using a cw laser) of the A 1Σ u + state. The time-shifting operation has implications for the experimental implementation of coherent control, as well as for the use of the control ability to study coherent configurations and dynamics that otherwise would be difficult (sometimes impossible) to access experimentally. This is due, for example, to dephasing and/or depopulation of the wave packet in combination with long recurrence times. One such inaccessible coherent configuration of the present Li 2 wave packets corresponds to the global maximum of their ionization yield (as probed in the present experiment).

Predissociation induced by ungerade-gerade symmetry breaking in theB1Πustate of the6Li7Limolecule

Unlike the ${}^{6}{\mathrm{Li}}^{6}\mathrm{Li}$ and ${}^{7}{\mathrm{Li}}^{7}\mathrm{Li}$ isotopomers, the ${}^{6}{\mathrm{Li}}^{7}\mathrm{Li}$ molecule exhibits surprisingly high dissociation rates for the levels that lie just above the dissociation limit of the $B{}^{1}{\ensuremath{\Pi}}_{u}$ state. As shown by Cacciani and Kokoouline [Phys. Rev. Lett. 84, 5296 (2000)] they can be attributed to a coupling between the $B{}^{1}{\ensuremath{\Pi}}_{u}$ and $1{}^{1}{\ensuremath{\Pi}}_{g}$ states, through the breaking of the ungerade-gerade symmetry. Systematic measurements of these dissociation rates have been performed by discriminating atomic fluorescence from molecular fluorescence in a sub-Doppler experiment involving a beam of lithium dimers and a cw tunable laser beam. Calculations of dissociation rates have been performed in the framework of a perturbative approach, by solving the coupled Schr\odinger equations through the use of the mapped Fourier grid representation method. Measured and calculated dissociation rates are in excellent agreement for a unique value of the coupling strength $h=0.144$ ${\mathrm{cm}}^{\ensuremath{-}1}.$

Quantum dynamics simulation of the ultrafast photoionization of Li2

The Li2 species offers an ideal system to compare experimental pump/probe ultrafast photoionization with quantum dynamical calculations on well characterized potential energy surfaces. The present work utilizes the best available potential energy surfaces and appropriate quantum dynamical methods to analyze the photoionization and dynamics of a wave packet prepared in the E 1Σg+ shelf state of lithium dimer. A direct comparison between calculated (ab initio) and measured quantum dynamics is made for signals obtained with different laser pulse shapes, intensities, and chirps, and the validity of the theoretical model is considered, as well as the applicability and failure of perturbation theory. The results illustrate the high sensitivity of the time-dependent pump/probe ionization transient signals to the detailed modeling of both the pump and probe stages. They also show some of the inadequacies of the current potential surfaces and dipole moment matrix elements of lithium dimer.