Alkali Dimers Research Articles

Full Dynamical and Ab Initio Investigation of the Electron Transfer-Mediated Decay Mechanism of He+ in the Presence of Heavier Alkali Dimers.

We have studied the electron transfer-mediated decay (ETMD) process for the 1s ionized state of the He atom in the presence of a heavier alkali homonuclear dimer (Na2, K2, and Rb2) as well as heteronuclear dimer (LiNa, NaK, and KRb). In our computation, we have considered all the alkali dimers being in the singlet electronic ground state. The electron transfer from the alkali dimer to He (1s-1) leads to the emission of another electron from the alkali dimer into the continuum. We have investigated the impact of the distance of the He atom from the center of mass of the alkali dimer on the ETMD decay width. We also performed the Born-Oppenheimer molecular dynamics simulation to understand the impact of nuclear dynamics on the ETMD process.

Efficient lignin biodegradation triggered by alkali-tolerant ligninolytic bacteria through improving lignin solubility in alkaline solution

Low lignin solubility in aqueous solution is one of the major bottlenecks for lignin biodegradation and bioconversion. Alkaline solution contributes to improving lignin solubility, whereas most microbes can not survive in alkaline conditions. Herein, lignin dissolution behaviors in different pH solutions were systematically investigated, which indicated that solution pH above 10.5 contributed to high solubility of alkali lignin. To match with alkaline lignin aqueous system, several alkali-tolerant ligninolytic bacteria were isolated, most of which are distinct to previously reported ones. Then, the ligninolytic capabilities of these isolates were assessed in different pH conditions by determining their assimilation on alkali lignin, lignin-derived monomers and dimers, their decolorization capabilities, and their lignin peroxidase activities. Thereafter, the underlying ligninolytic and alkali-tolerant mechanisms of Sutcliffiella sp. NC1, an alkalophilic bacterium, was analyzed on the basis of its genome information. The results not only provide valuable information for lignin biodegradation and lignin valorization, but also expand knowledge on alkali-tolerant bacteria.

Computational molecular dynamics simulations of cationic alkali dimers solvated in He clusters: the Li$$_2^+$$ case

Computational molecular dynamics simulations of cationic alkali dimers solvated in He clusters: the Li$$_2^+$$ case

Signatures of Non-universal Quantum Dynamics of Ultracold Chemical Reactions of Polar Alkali Dimer Molecules with Alkali Metal Atoms: Li(2S) + NaLi(a3Σ+) → Na(2S) + Li2(a3Σu+).

Ultracold chemical reactions of weakly bound triplet-state alkali metal dimer molecules have recently attracted much experimental interest. We perform rigorous quantum scattering calculations with a new ab initio potential energy surface to explore the chemical reaction of spin-polarized NaLi(a3Σ+) and Li(2S) to form Li2(a3Σu+) and Na(2S). The reaction is exothermic and proceeds readily at ultralow temperatures. Significantly, we observe strong sensitivity of the total reaction rate to small variations of the three-body part of the Li2Na interaction at short range, which we attribute to a relatively small number of open Li2(a3Σu+) product channels populated in the reaction. This provides the first signature of highly non-universal dynamics seen in rigorous quantum reactive scattering calculations of an ultracold exothermic insertion reaction involving a polar alkali dimer molecule, opening up the possibility of probing microscopic interactions in atom+molecule collision complexes via ultracold reactive scattering experiments.

Laser-induced Coulomb explosion imaging of alkali-metal dimers on helium nanodroplets

Alkali dimers, $\mathrm{Ak}_2$, residing on the surface of He nanodroplets are doubly ionized due to multiphoton absorption from an intense, 50-fs laser pulse leading to fragmentation into a pair of alkali cations. Based on the measured kinetic energy distributions, $P(E_{\text{kin}})$, of the $\mathrm{Ak}^+$ fragment ions, we retrieve the distribution of internuclear distances, $P(R)$, via the $\mathrm{Ak}_2^{2+}$ potential curve. Results are obtained for $\mathrm{{Na}_2}$, $\mathrm{K}_2$, $\mathrm{Rb}_2$, and $\mathrm{Cs}_2$ in both the 1 $^1\Sigma_{g}^+$ ground state and in the lowest-lying triplet state 1 $^3\Sigma_{u}^+$, and for $\mathrm{Li}_2$ in the 1 $^3\Sigma_{u}^+$ state. For $\mathrm{Li}_2$, $\mathrm{K}_2$, and $\mathrm{Rb}_2$, the center of the measured $P(R)$'s is close to the center of the wave function, $\Psi(R)$, of the vibrational ground state in the 1 $^1\Sigma_{g}^+$ and 1 $^3\Sigma_{u}^+$ states, whereas for $\mathrm{{Na}_2}$ and $\mathrm{{Cs}_2}$ small shifts are observed. For all the $\mathrm{{Ak}_2}$, the width of the measured $P(R)$ is broader than $|\Psi(R)|^2$ by a factor of 2-4. We discuss that resonance effects in the multiphoton ionization and interaction of the $\mathrm{Ak}^+$ ion with the He droplet give rise to the observed deviations of $P(R)$ from $|\Psi(R)|^2$. Despite these deviations, we deem that timed Coulomb explosion will allow imaging of vibrational wave packets in alkali dimers on He droplets surfaces.

Author Correction: Highly efficient double ionization of mixed alkali dimers by intermolecular Coulombic decay

Author Correction: Highly efficient double ionization of mixed alkali dimers by intermolecular Coulombic decay

Non-Relativistic Electronic-Structure Computation of Neutral and Cationic Systems [Fr2, Fr-AEM+ (AEM= Ca, Sr, Ba)

The experimental field of ultracold ion-atom mixtures including an alkali-metal atom and an alkaline-earth-metal ion as well as of homonuclear alkali dimers has paved the way for creating and manipulating the ultracold molecules. The present paper is focused on a study of molecules such us francium dimer and a comparative spectroscopic investigation of the cationic systems Fr-(Ca+, Sr+, Ba+). We adopt a computational scheme without spin-orbit coupling reposed on the full configuration interaction and semi-empirical pseudo-potential theory of the atomic cores Fr+, Ca2+, Sr2+, and Ba2+ with extended and optimized basis sets. We have determined the adiabatic potentials with their relative spectroscopic constants, the electric dipole moments and the vibrational levels spacings for the 1,3Σu,g+ and 1,3Σ+ electronic states for Fr2 and Fr-AEM+, respectively, correlated toward {Fr(7s) + Fr(7s, 7p, 6d, 8s, 8p)}, {Ca(4s2, 4s4p, 4s3d), Sr(5s2, 5s5p, 5s4d), Ba(6s2, 6s6p, 6s5d) + Fr+}, and {Ca+(4s, 3d), Sr+(5s, 4d), Ba+(6s, 5d) + Fr(7s, 7p)}. The accuracy and reliability of the current results are discussed by comparing with theoretical data available in the literature. The occurrence of some avoided crossings between the neighboring electronic states is leading to a charge or excitation transfer for atom-ion collisions in the diverse charge or excited states. The Σ+-Σ+ transitions are determined in order to evaluate the future radiative lifetimes of vibrational states serving the direct laser cooling.

Resonant Ionic, Covalent Bond, and Steric Characteristics Present in 1Σu+ States of Li2

The molecular bonding in the excited states of the alkali dimers involves the resonant ionic, covalent bond and steric interactions. We show here the case of the 1Σu+ states of Li2 by ab initio calculation. These interactions as functions of the internuclear distance lead to complex potential energy curves, providing an important application for high resolution laser spectroscopy. The spectroscopic constants for the 4 and 5 1Σu+ states are obtained for the first time.

Electron Paramagnetic Resonance of Alkali Metal Atoms and Dimers on Ultrathin MgO.

Electron paramagnetic resonance (EPR) can provide unique insight into the chemical structure and magnetic properties of dopants in oxide and semiconducting materials that are of interest for applications in electronics, catalysis, and quantum sensing. Here, we demonstrate that EPR in combination with scanning tunneling microscopy (STM) allows for probing the bonding and charge state of alkali metal atoms on an ultrathin magnesium oxide layer on a Ag substrate. We observe a magnetic moment of 1 μB for Li2, LiNa, and Na2 dimers corresponding to spin radicals with a charge state of +1e. Single alkali atoms have the same charge state and no magnetic moment. The ionization of the adsorbates is attributed to charge transfer through the oxide to the metal substrate. Our work highlights the potential of EPR-STM to provide insight into dopant atoms that are relevant for the control of the electrical properties of surfaces and nanodevices.

Quantum-State-Sensitive Detection of Alkali Dimers on Helium Nanodroplets by Laser-Induced Coulomb Explosion.

Rubidium dimers residing on the surface of He nanodroplets are doubly ionized by an intense femtosecond laser pulse leading to fragmentation into a pair of Rb^{+} ions. We show that the kinetic energy of the Rb^{+} fragment ions can be used to identify dimers formed in either the X ^{1}Σ_{g}^{+} ground state or in the lowest-lying triplet state, a ^{3}Σ_{u}^{+}. From the experiment, we estimate the abundance ratio of dimers in the a and X states as a function of the mean droplet size and find values between 4∶1 and 5∶1. Our technique applies generally to dimers and trimers of alkali atoms, here also demonstrated for Li_{2}, Na_{2}, and K_{2}, and will enable femtosecond time-resolved measurements of their rotational and vibrational dynamics, possibly with atomic structural resolution.

Reaction microscope for investigating ionization dynamics of weakly bound alkali dimers.

We report on the implementation of a far-off-resonant, optical dipole force trap in a reaction microscope combined with a magneto-optical trap. Kinematically complete multi-photon ionization experiments were performed on optically trapped 6Li atoms and photo-associated 6Li2 molecules in their highest vibrational state. The apparatus allows us to distinguish different ionization mechanisms related to the presence of the IR field of the optical dipole trap that can occur during ionization of 6Li and 6Li2 in strong fields. In a series of proof-of-principle experiments, we detect weakly bound dimers via three-photon ionization with femtosecond pulses (τ = 30 fs) at a central wavelength of 780 nm and measure directly the momenta of the photoelectrons in coincidence with recoil ions.

Theory of double ionization of a neighboring molecule by interatomic Coulombic decay

Several known processes, such as single-photon double ionization and double Auger decay, result in correlated emission of two electrons from an atom or molecule. The ratio of double to single ionization in these processes usually amounts only to several percent. Recently, an experiment was reported in helium droplets doped with alkali dimers, where double ionization of the dimer after excitation of the helium proceeds via interatomic Coulombic decay and occurs with an efficiency comparable to that of single ionization via the usual interatomic Coulombic decay. Motivated by these experimental results, we investigate here the theory of this double interatomic Coulombic decay (dICD) process. First, we develop an explicit asymptotic formula for the decay width of dICD based on the assumption that the electronically excited system providing the necessary excess energy and its neighbor are spatially well separated. This formula contains only quantities accessible experimentally for the separated entities -- the system and its neighbor. Second, we derive a general analytical expression for the decay width of dICD by using many-body perturbation theory. Finally, we investigated the efficiency of dICD for experimentally realizable small atomic and molecular clusters employing the asymptotic formula.

High resolution laser spectroscopy of spatially restricted hot alkali atomic and dimer vapor

In this paper we discuss a novel use of a Cs-filled optical cell with micrometric thickness. The cell intrinsically contains an extremely small number of Rb atoms. When heated up to temperatures around 350 $$^\circ $$C, the vapor in the cell consists of: (i) Cs atoms, (ii) an order of magnitude lower concentration $$\hbox {Cs}_2$$ dimers and (iii) a small number of Rb atoms. This mixture is a high density but optically thin alkali vapor layer, and the produced complex ensemble of $$\hbox {Cs}_2$$ diatomic molecules, together with Rubidium and Cesium atoms can be investigated by means of linear/nonlinear spectroscopy. In this work we study the fluorescence and selective reflection spectral profiles of Rb atoms in the presence of high-density Cs atoms considered as a buffer gas for the Rb atoms and Cs dimers. In addition, the Rb hyperfine transition spectrum is used as a precise frequency reference for the measurement of the spectral profile widths and the frequency positions of the experimentally registered spectral lines of $$\hbox {Cs}_2$$ molecules in the spectral region around the Rb $$\hbox {D}_2$$ resonance line.

Comparative analysis of nonrelativistic and relativistic calculations of electric dipole moments and polarizabilities of heteronuclear alkali-metal dimers

We analyze the molecular electric dipole moments (PDMs) and static electric dipole polarizabilities of heteronuclear alkali dimers in their ground states by employing coupled-cluster theory, both in the non-relativistic and four-component relativistic frameworks. The roles of electron correlations as well as relativistic effects are demonstrated by studying them at different levels of theory, followed by a comprehensive treatment of error estimates. We compare our obtained values with the previous non-relativistic calculations, some of which include lower-order relativistic corrections, as well as with the experimental values, wherever available. We find that the PDMs are very sensitive to relativistic effects, as compared to polarizabilities; this aspect can explain the long-standing question on the difference between experimental values and theoretical results for LiNa. We show that consideration of relativistic values of PDMs improves significantly the isotropic Van der Waals $C_6$ coefficients of the investigated alkali dimers over the previously reported non-relativistic calculations. The dependence of dipole polarizabilities on molecular volume is also illustrated.

Electron transfer mediated decay of alkali dimers attached to He nanodroplets.

Alkali metal dimers attached to the surface of helium nanodroplets are found to be efficiently doubly ionized by electron transfer mediated decay (ETMD) when photoionizing the helium droplets. This process is evidenced by detecting in coincidence two energetic ions created by Coulomb explosion and one low-kinetic energy electron. The kinetic energy spectra of ions and electrons are reproduced by simple model calculations based on diatomic potential energy curves, and are in agreement with ab initio calculations for the He-Na2 and He-KRb systems. This work demonstrates that ETMD is an important decay channel in heterogeneous nanosystems exposed to ionizing radiation.

Double-minimum 3 Σu+1 state in Rb2 : Spectroscopic study and possible applications for cold-physics experiments

The double-minimum 3 $^{1}\mathrm{\ensuremath{\Sigma}}_{\mathrm{u}}^{\phantom{\rule{0.28em}{0ex}}\phantom{\rule{0.16em}{0ex}}+}$ state in $^{85}\mathrm{Rb}_{2}$ and $^{85}\mathrm{Rb}\phantom{\rule{0.16em}{0ex}}^{87}\mathrm{Rb}$ was investigated by analyzing rotationally resolved spectra of the 3 $^{1}\mathrm{\ensuremath{\Sigma}}_{\mathrm{u}}^{\phantom{\rule{0.28em}{0ex}}\phantom{\rule{0.16em}{0ex}}+}\ensuremath{\longleftarrow}\mathrm{X}^{1}\mathrm{\ensuremath{\Sigma}}_{\mathrm{g}}^{+}$ band system simplified by polarization labeling. A total of 3665 transitions to rovibrational levels located in the inner well and above the internal barrier were identified and an accurate potential energy curve of the state reaching up to $R\ensuremath{\approx}21$ \AA{} was constructed using the inverted perturbation approach, making this state the best characterized double-minimum state in alkali dimers. The possibility of formation of ultracold rubidium dimers in the absolute ground state by photoassociation into levels of the 3 $^{1}\mathrm{\ensuremath{\Sigma}}_{\mathrm{u}}^{\phantom{\rule{0.28em}{0ex}}\phantom{\rule{0.16em}{0ex}}+}$ state above the internal barrier followed by spontaneous or stimulated emission is discussed.

Quasi-one-dimensional ultracold rigid-rotor collisions: Reactive and nonreactive cases

We study polar alkali dimer scattering in a quasi-1D geometry for both reactive and non-reactive species. Elastic and reactive rates are computed as a function of the amplitude of a static electric field within a purely long-range model with suitable boundary conditions at shorter range. We describe the diatomic molecules as rigid rotors and results are compared to the fixed-dipole approximation. We show in particular that for molecules with a sufficiently strong induced dipole moment oriented perpendicular to the trap axis, the long-range repulsive interaction leads to the suppression of short-range processes. Such shielding effect occurs for both reactive and non-reactive molecules, preventing two-body reactions as well as losses due to "sticky processes" [Phys. Rev. A 85, 062712 (2012)] from occurring. The present results demonstrate the possibility to suppress loss rates in current ultracold molecule experiments using 1D confinement.

An accurate semi-empirical potential model for the a 3Σu + state of the alkali dimers Na2, K2, Rb2, and Cs2 which reproduces the scattering length.

A new four parameter Tang-Toennies type potential model is described for the a 3Σu + triplet state of the alkali dimers Na2, K2, Rb2, and Cs2. Compared to an earlier three parameter semi-empirical model based on the experimental well depth, De, well location, Re, and the harmonic vibrational frequency, ωe [Lau et al., J. Chem. Phys. 145, 194308 (2016)], the new model is also adjusted to be consistent with the scattering length. The results are shown to have a similar good agreement with the spectroscopic term values as the earlier model with the advantage that the scattering length is properly described. The deviations from recent potentials for Cs2 are discussed.

The low-lying adiabatic states of the K2+ alkali dimer

Despite the simplicity of the K2+ alkali dimer with a single active electron, comparison of the ab initio results of Berriche et al. (2008) with those of Magnier and Frécon (2003) based on a model potential approach reveals a number of serious disagreement concerning several excited states. In particular, the 52Σu+, 62Σg+, 62Σu+, 72Σg+, 72Σu+, 32Πg, 42Πg and 22Δu states which are found to be repulsive by Magnier and Frécon, but attractive when ab initio techniques are employed. To clarify the origin of this disagreement, the adiabatic energies and spectroscopic constants are re-computed for the low-lying states of the K2+ alkali dimer within a model potential framework. Contrary to the claims of Magnier and Frécon, the new results based on a model potential approach agree well with the ab initio ones.

Highly efficient double ionization of mixed alkali dimers by intermolecular Coulombic decay

As opposed to purely molecular systems where electron dynamics proceed only through intramolecular processes, weakly bound complexes such as He droplets offer an environment where local excitations can interact with neighbouring embedded molecules leading to new intermolecular relaxation mechanisms. Here, we report on a new decay mechanism leading to the double ionization of alkali dimers attached to He droplets by intermolecular energy transfer. From the electron spectra, the process is similar to the well-known shake-off mechanism observed in double Auger decay and single-photon double ionization1,2, however, in this case, the process is dominant, occurring with efficiencies equal to, or greater than, single ionization by energy transfer. Although an alkali dimer attached to a He droplet is a model case, the decay mechanism is relevant for any system where the excitation energy of one constituent exceeds the double ionization potential of another neighbouring molecule. The process is, in particular, relevant for biological systems, where radicals and slow electrons are known to cause radiation damage3. Using alkali metal dimers attached to helium droplets, a new decay mechanism for intermolecular Coulombic decay is demonstrated. The process leads to previously unresolved double ionization for excitation energies exceeding double ionization energies.