Alkali-metal Molecules Research Articles

Binding properties of some alkali metal vapors: specifically lithium-7 (7Li), soduim-23 (23Na), and potassium-39 (39K)

In this work, the binding energy of three alkali metal molecules, namely, lithium-7 (7Li2), sodium-23 (23Na2), and potassium-39 (39K2), is calculated in both free space and the vapor phase. In free space, the Lippmann–Schwinger equation is solved for negative-definite eigenenergies using a highly effective matrix-inversion method with a symmetrized kernel. In the vapor, two medium effects are taken into account: the effective atomic mass, which is somewhat larger than the “bare” mass and therefore enhances the binding; and the effective binary interaction, with less overall attraction than the “bare” interaction. In free space, the binding energies of the respective molecules are, in units of (103 K), 5.304, 2.936, and 2.522. The corresponding experimental results are 6.150 × 103, 4.275 ×103, and 4.275 × 103 K. The results in the vapor phase are by and large somewhat lower than those in free space.

Magnetic Feshbach resonances in collisions of nonmagnetic closed-shell1Σmolecules

Magnetic Feshbach resonances play a central role in experimental research of atomic gases at ultracold temperatures, as they allow one to control the microscopic interactions between ultracold atoms by tuning an applied magnetic field. These resonances arise due to strong hyperfine interactions between the unpaired electron and the nuclear magnetic moment of the alkali metal atoms. A major thrust of current research is to create an ultracold gas of diatomic alkali-metal molecules in the ground rovibrational state of the ground electronic $^1\Sigma$ state. Unlike alkali metal atoms, $^1\Sigma$ diatomic molecules have no unpaired electrons. However, the hyperfine interactions of molecules may give rise to magnetic Feshbach resonances. We use quantum scattering calculations to study the possible width of these resonances. Our results show that the widths of magnetic Feshbach resonances in ultracold molecule-molecule collisions for $^1\Sigma$ molecules may exceed 1 milliGauss, rendering such resonances experimentally detectable. We hope that this work will stimulate the experimental search of these resonances.

Van der Waals coefficients for systems with ultracold polar alkali-metal molecules

A systematic study of the leading isotropic van der Waals coefficients for the alkali-metal atom + molecule and molecule + molecule systems is presented. Dipole moments and static and dynamic dipole polarizabilities are calculated employing high-level quantum chemistry calculations. The dispersion, induction, and rotational parts of the isotropic van der Waals coefficient are evaluated. The known van der Waals coefficients are then used to derive characteristics essential for simple models of the collisions involving the corresponding ultracold polar molecules.

Formation of ultracold metastable RbCs molecules by short-range photoassociation

Ultracold metastable RbCs molecules are observed in a double species magneto-optical trap through photoassociation near the Rb(5S(1/2)) + Cs(6P(3/2)) dissociation limit followed by radiative stabilization. The molecules are formed in their lowest triplet electronic state and are detected by resonance enhanced two-photon ionization through the previously unobserved (3)(3)Π ← a (3)Σ(+) band. The large rotational structure of the observed photoassociation lines is assigned to the lowest vibrational levels of the 0(+) or 0(-) excited states correlated to the Rb(5P(1/2)) + Cs(6S(1/2)) dissociation limit. This demonstrates the possibility of inducing direct photoassociation in heteronuclear alkali-metal molecules at a short internuclear distance, as pointed out earlier [J. Deiglmayr et al., Phys. Rev. Lett., 2008, 101, 13304].

Full vibrational energy spectra and dissociation energies for some electronic states of diatomic alkali-metal molecules

It is difficult to obtain the accurate high-lying vibrational energies for most of the diatomic electronic states on modern experiments or theoretical computations based on quantum mechanics. Based on the new analytical formula for dissociation energy and algebraic method (AM) generated by Sun et al., the second order perturbation theory are used to study the full vibritional energies{EAMυ} and dissociation energies of the Li2－33Σ+g,Li2－13Δg,Li2－23Πg,Na2－B1Πu and K2－41Σ+g electronic states. The obtained results not only agree well with the experimental data for the low-lying vibrational energies, but also give all high-lying vibrational energies which are still difficult to obtain by experiment at present. These results supply necessary data for the studies which need high-lying vibrational energies and dissociation energies of diatomic alkali-metal molecule.

Reinvestigation of the Rb2 (2)Π3g−a Σ3u+ band on helium nanodroplets

It is well known that alkali-metal molecules are preferentially observed in the weak van der Waals-bound high spin states by helium droplet isolation spectroscopy. In [F. R. Brühl et al., J. Chem. Phys. 115, 10275 (2001)] the Rb2 (2)Π3g−a Σ3u+ band on He droplets was investigated by laser-induced fluorescence and dispersed emission spectroscopy. At that time no information on the magnitude of spin-orbit coupling was available for the (2)Π3g state which connects to the atomic 5s+4d asymptote and it was neglected. In this work we reinvestigate the observed spectra. The dispersed emission spectra, which resulted from free molecules, are consistent with state-of-the-art nonrelativistic potential energy surfaces and effective spin-orbit coupling matrix elements obtained from resonance-enhanced multiphoton ionization spectroscopy of cold Rb dimers [J. Lozeille et al., Eur. Phys. J. D 39, 261 (2006)]. Having validated the theoretical description of the free molecule state, we use the laser-induced fluorescence spectrum to discuss the influence of the He droplet on the excitation band.

Structure and bonding of methyl alkali metal molecules

We have carried out a theoretical investigation of the methyl alkali metals CH3 M with M=Li, Na, K and Rb using density functional theory (DFT) at the BP86/TZ2P level. Our purpose is to determine how the structure and thermochemistry (e.g., C-M bond lengths and strengths) of these organoalkali metal compounds depend on the metal atom, and to understand the emerging trends in terms of quantitative Kohn-Sham molecular orbital (KS-MO) theory. The C-M bond becomes longer and weaker if one goes from Li to the more electropositive Rb. Also, the polarity of the C-M bond increases along this series but it preserves a strong intrinsic preference to homolytic over ionic dissociation in the gas phase. We show that a description of the bonding mechanism in terms of a polar C-M electron-pair bond between the methyl radical and alkali metal atom is just as natural as an ionic description (i.e., in terms of CH3-+M+) and that it provides a straightforward way of understanding all observed trends.

Energy Levels and Line Intensities of Diatomic Molecules. Application to Alkali Metal Molecules

Abstract Calculation of rotational level energies and line intensities of diatomic molecules and its application to alkali metal diatomic molecules are reported. The effects of spin–orbit interaction, rotational interaction, spin–rotation interaction, spin–spin interaction, hyperfine interaction, and Zeeman interaction are considered.

Pseudopotential investigation of some alkali metal molecules

AbstractThe effect of electron correlation on the results of pseudopotential calculations was examined using a simple analytical semiempirical pseudopotential and a correlated floating‐type one‐center wave function. Investigations were performed for the XH alkali metal hydride molecules (X  Na, K, Rb, Cs). The inclusion of the electron correlation in the ground state proved important for the calculation of the dissociation and ionization energies, but it is less significant for the determination of the equilibrium nuclear distances. The ground state potential energy curves are also determined.

A mass spectrometric study of heteronuclear diatomic alkali metal molecules. Dissociation energies and ionization potentials of NaLi, KLi, and NaK.

The gaseous molecules NaLi, KLi, and NaK were studied by equilibrium and electron impact techniques. The dissociation energies of these molecules were determined from the second and the third law enthalpies of various equilibria, and the following values were obtained: D00(NaLi) =20.7±1.5 kcal/mole, D00 KLi) =18.7±1.0 kcal/mole, and D00(NaK) =14.6±1.0 kcal/mole. Electron impact measurements yield values of 4.94±0.10, 4.69±0.10, and 4.57±0.20 eV for the ionization potentials of NaLi, KLi, and NaK molecules, respectively. The results are compared with spectroscopic data and recent theoretical calculations. The relation of these results to the bonding in heteronuclear diatomic alkali metal molecules is discussed.

Simplified SCF calculations on the diatomic alkali metal molecules

Simplified SCF calculations on the diatomic alkali metal molecules

Further consideration of the estimation of spectroscopic trends by a perturbation method using Hellmann pseudopotentials

The use of new values of Hellmann pseudopotential parameters and wave functions is shown to yield much better predictions by the method previously proposed. Corrected results are presented for predictions of ionization potentials and the lowest 1Σ- 1Σ transitions of alkali metal molecules. New predictions are made for the lowest 1Π- 1Σ transitions of alkali metal molecules.

Emission Spectra of Alkali-Metal Molecules Observed with a Heat-Pipe Discharge Tube

Intense molecular emission spectra from vapors of each of the alkali metals with the exception of lithium have been observed using a new device, the heat-pipe discharge tube. For Na2, K2, and Br2 the total emission appears to be almost entirely comprised of transitions Σu+1→1Σg+ from the first excited singlet state. For Cs2, emission from a low-lying Πu3 state appears to contribute as well.

Relations between electronegativity and potential energy in the alkali metal molecules

Potential energy functions can be constructed by the aid of parameters other than spectroscopic constants to render the possibility of linking different areas in chemical physics. By using a formula connecting force constants of diatomic molecules and bonds in polyatomics, a potential energy function can be obtained by replacing some of the spectroscopic parameters by expressions based on the use of electronegativities.

Excited Electronic States of Lithium and Sodium Molecules

THE electronic spectra of the alkali metal molecules are of interest for several reasons: for example, (1) Li2 is sufficiently simple to be the subject of quantitative wave-mechanical calculations1,2, and (2) these spectra may be expected to be somewhat similar to that of H2, for which reliable analyses of many systems are now available3,4. Furthermore, the excited singlet ungerade states of the alkali metals are readily accessible, since they may be observed in combination with the ground states at λ > 2300 A. It is therefore surprising that for Li2 and Na2 only the lowest excited singlet states, A 1Σ+ u and B 1IIu, have been well characterized5: several systems at shorter wave-lengths have been reported5,6, but the vibrational analyses of some of these are uncertain, and only for the system C—X of Na2 has a rotational analysis been attempted7.

Absorption Bands of Li2, Na2, K2 and NaK

Although fairly complete information is available regarding the first two systems of bands of Li2, Na2 and K2, the position with regard to the higher systems cannot yet be considered satisfactory. For this reason the spectra of these alkali-metal molecules, in the regions corresponding to the third, fourth and higher systems of bands, have been investigated in absorption, employing a quartz E1 spectrograph for lithium, and a 21-ft. concave grating (Eagle mounting), having a dispersion of 1.3 A. per mm., for sodium and potassium. A hydrogen discharge tube has been used as source of continuum below λ 3600 and a tungsten lamp above that.