Bonding of alkali-alkaline earth molecules in the lowest Σ+ states of doublet and quartet multiplicity

\begin{wrapfigure}{r}{0pt} \includegraphics[scale=0.6]{ak_ake.eps} \end{wrapfigure} In the present study the ground state as well as the lowest $^4\Sigma^+$ state were determined for 16 AK-AKE molecules.\footnote{ J. V. Pototschnig, A. W. Hauser and W. E. Ernst, Phys. Chem. Chem. Phys., 2016,18, 5964-5973} Multireference configuration interaction calculations were carried out in order to understand the bonding of diatomic alkali-alkaline earth (AK-AKE) molecules. The correlations between molecular properties (disociation energy, bond distances, electric dipole moment) and atomic properties (electronegativity, polarizability) will be discussed. A correlation between the dissociation energy and the dipole moment of the lowest $^4\Sigma^+$ state was observed, while the dipole moment of the lowest $^2\Sigma^+$ state does not show such a simple dependency. In this case an empirical relation could be established. The class of AK-AKE molecules was selected for this investigation due to their possible applications in ultracold molecular physics

RYDBERG STATES OF ALKALI METAL ATOMS ON SUPERFLUID HELIUM DROPLETS - THEORETICAL CONSIDERATIONS

The bound states of electrons on the surface of superfluid helium have been a research topic for several decades. One of the first systems treated was an electron bound to an ionized helium cluster.footnote{A. Golov and S. Sekatskii, Physica B, 1994, 194, 555-556} Here, a similar system is considered, which consists of a helium droplet with an ionized dopant inside and an orbiting electron on the outside. In our theoretical investigation we select alkali metal atoms (AK) as central ions, stimulated by recent experimental studies of Rydberg states for Na,footnote{E. Loginov, C. Callegari, F. Ancilotto, and M. Drabbels, J. Phys. Chem. A, 2011, 115, 6779-6788} Rb,footnote{F. Lackner, G. Krois, M. Koch, and W. E. Ernst, J. Phys. Chem. Lett., 2012, 3, 1404-1408} and Csfootnote{F. Lackner, G. Krois, M. Theisen, M. Koch, and W. E. Ernst, Phys. Chem. Chem. Phys., 2011, 13, 18781-18788} attached to superfluid helium nanodroplets. Experimental spectra , obtained by electronic excitation and subsequent ionization, showed blueshifts for low lying electronic states and redshifts for Rydberg states._x000d_ _x000d_ In our theoretical treatment the diatomic AK$^+$-He potential energy curves are first computed with textit{ab initio} methods. These potentials are then used to calculate the solvation energy of the ion in a helium droplet as a function of the number of atoms. Additional potential terms, derived from the obtained helium density distribution, are added to the undisturbed atomic pseudopotential in order to simulate a 'modified' potential felt by the outermost electron. This allows us to compute a new set of eigenstates and eigenenergies, which we compare to the experimentally observed energy shifts for highly excited alkali metal atoms on helium nanodroplets._x000d

SIGN CHANGES IN THE ELECTRIC DIPOLE MOMENT OF EXCITED STATES IN RUBIDIUM-ALKALINE EARTH DIATOMIC MOLECULES

In a recent series of combined experimental and theoretical studies we investigated the ground state and several excited states of the Rb-alkaline earth molecules RbSrfootnote{F. Lackner, G. Krois, T. Buchsteiner, J. V. Pototschnig, and W. E. Ernst, Phys. Rev. Lett., 2014, 113, 153001; G. Krois, F. Lackner, J. V. Pototschnig, T. Buchsteiner, and W. E. Ernst, Phys. Chem. Chem. Phys., 2014, 16, 22373; J. V. Pototschnig, G. Krois, F. Lackner, and W. E. Ernst, J. Chem. Phys., 2014, 141, 234309} and RbCa.footnote{J. V. Pototschnig, G. Krois, F. Lackner, and W. E. Ernst, J. Mol. Spectrosc., in Press (2015), doi:10.1016/j.jms.2015.01.006} The group of alkali-alkaline earth (AK-AKE) molecules has drawn attention for applications in ultracold molecular physics and the measurement of fundamental constantsfootnote{M. Kajita, G. Gopakumar, M. Abe, and M. Hada, J. Mol. Spectrosc., 2014, 300, 99-107} due to their large permanent electric and magnetic dipole moments in the ground state. These properties should allow for an easy manipulation of the molecules and simulations of spin models in optical lattices.footnote{A. Micheli, G. K. Brennen, and P. Zoller, Nature Physics, 2006, 2, 341-347} In our studies we found that the permanent electric dipole moment points in different directions for certain electronically excited states, and changes the sign in some cases as a function of bond length. We summarize our results, give possible causes for the measured trends in terms of molecular orbital theory and extrapolate the tendencies to other combinations of AK and AKE - elements

Implementation of relativistic coupled cluster theory for massively parallel GPU-accelerated computing architectures

In this paper, we report a reimplementation of the core algorithms of relativistic coupled cluster theory aimed at modern heterogeneous high-performance computational infrastructures. The code is designed for efficient parallel execution on many compute nodes with optional GPU coprocessing, accomplished via the new ExaTENSOR back end. The resulting ExaCorr module is primarily intended for calculations of molecules with one or more heavy elements, as relativistic effects on electronic structure are included from the outset. In the current work, we thereby focus on exact 2-component methods and demonstrate the accuracy and performance of the software. The module can be used as a stand-alone program requiring a set of molecular orbital coefficients as starting point, but is also interfaced to the DIRAC program that can be used to generate these. We therefore also briefly discuss an improvement of the parallel computing aspects of the relativistic self-consistent field algorithm of the DIRAC program

Electronic Excitations Of Alkali-alkaline Earth Diatomic Molecules - Results From Ab Initio Calculations

Recently interest in polar diatomic molecules with a magnetic dipole moment has been growing. An example for such molecules is the combination of an alkali metal atom and an alkaline earth metal atom. These systems are quite small, containing only three valence electrons. Nevertheless calculations of excited states are challenging. Ab initio calculations for two sample systems, LiCa\footnote{G. Krois, J.V. Pototschnig, F. Lackner and W.E. Ernst, J. Phys. Chem. A, 117, 13719-13731 (2013)} and RbSr, will be presented. The potential energy curves and transition dipole moments for the ground state and several excited states were determined, up to 25000 \wn for LiCa and up to 22000 \wn for RbSr. Multireference configuration interaction calculations (MRCI) based on complete active space self-consistent field wave functions (CASSCF) were used to determine the properties of the system as implemented in the MOLPRO software package.\footnote{H.-J. Werner and P. J. Knowles and G. Knizia and F. R. Manby and M. {Sch\"{u}tz} et al., MOLPRO, version 2010.1, see http://www.molpro.net/} Effective core potentials (ECPs) and core polarization potentials (CCPs) were applied to reduce the computational effort, while retaining accuracy. The similarities and differences of the two systems will be discussed. In both systems the accurate description of the asymptotic values of the PECs corresponding to atomic D-states proved to be difficult. The results will be compared to recent experiments, showing that a combination of theory and experiment gives a reliable description of the systems

EXCITED STATES OF THE DIATOMIC MOLECULE CrHe

Author Institution: Institute of Experimental Physics, TU Graz, Petersgasse 16, 8010 Graz, AustriaChromium (Cr) atoms embedded in superfluid helium nanodroplets (He$_N$) have been investigated by laser induced fluorescence, beam depletion and resonant two-photon ionization spectroscopy in current experiments at our institute. Cr is found to reside inside the He$_N$ in the a$^7$S ground state. Two electronically excited states, z$^7$P and y$^7$P, are involved in a photoinduced ejection process which allowed us to study Fano resonances in the photoionisation spectra. The need for a better understanding of the experimental observations triggered a theoretical approach towards the computation of electronically excited states via high-level methods of computational chemistry. Two well-established, wave function-based methods, CASSCF and MRCI, are combined to calculate the potential energy curves for the three states involved. The character of the two excited states z$^7$P and y$^7$P turns out to be significantly different. Theory predicts the ejection of the Cr atom in the case of an y$^7$P excitation as was observed experimentally. The quasi-inert helium environment is expected to weaken spin selection rules, allowing a coupling between different spin states especially during the ejection process. We therefore extend our theoretical analysis to the lowest state in the triplet- and quintet- manifold. Most of these alternative states show very weak bonding of only a few \wn

Electronic spectra of ytterbium fluoride from relativistic electronic structure calculations

We report an investigation of the low-lying excited states of the YbF molecule-a candidate molecule for experimental measurements of the electron electric dipole moment-with 2-component based multi-reference configuration interaction (MRCI), equation of motion coupled cluster (EOM-CCSD) and the extrapolated intermediate Hamiltonian Fock-space coupled cluster (XIHFS-CCSD). Specifically, we address the question of the nature of these low-lying states in terms of configurations containing filled or partially-filled Yb 4f shells. We show that while it does not appear possible to carry out calculations with both kinds of configurations contained in the same active space, reliable information can be extracted from different sectors of Fock space-that is, by performing electron attachment and detachment IHFS-CCSD and EOM-CCSD calculation on the closed-shell YbF+ and YbF- species, respectively. From these calculations we predict Ω = 1/2, 3/2 states, arising from the 4f13σ26s, 4f145d1/6p1, and 4f135d1σ16s configurations to be able to interact as they appear in the same energy range around the ground-state equilibrium geometry. As these states are generated from different sectors of Fock space, they are almost orthogonal and provide complementary descriptions of parts of the excited state manifold. To obtain a comprehensive picture, we introduce a simple adiabatization model to extract energies of interacting Ω = 1/2, 3/2 states that can be compared to experimental observations

Electronic spectra of ytterbium fluoride from relativistic electronic structure calculations

We report an investigation of the low-lying excited states of the YbF molecule--a candidate molecule for experimental measurements of the electron electric dipole moment--with 2-component based multi-reference configuration interaction (MRCI), equation of motion coupled cluster (EOM-CCSD) and the extrapolated intermediate Hamiltonian Fock-space coupled cluster (XIHFS-CCSD). Specifically, we address the question of the nature of these low-lying states in terms of configurations containing filled or partially-filled Yb $4f$ shells. We show that while it does not appear possible to carry out calculations with both kinds of configurations contained in the same active space, reliable information can be extracted from different sectors of Fock space--that is, by performing electron attachment and detachment IHFS-CCSD and EOM-CCSD calculation on the closed-shell YbF$^+$ and YbF$^-$ species, respectively. From these we observe $\Omega = 1/2, 3/2$ states that arise from the $4f^{13}\sigma_{6s}^2$, $4f^{14}5d$/$6p$, and $4f^{13}5d\sigma_{6s}$ configurations appear in the same energy range around the ground-state equilibrium geometry and they are therefore able to interact. As these states are generated from different sectors of Fock space, they are almost orthogonal and provide complementary descriptions of parts of the excited state manifold. To obtain a comprehensive picture, we introduce a simple adiabatization model to extract energies of interacting $\Omega = 1/2, 3/2$ states that can be compared to experimental observations.Comment: 28+57 pages, 8+18 tables, 5+34 figure

Implementation of relativistic coupled cluster theory for massively parallel GPU-accelerated computing architectures

International audienceIn this paper, we report a reimplementation of the core algorithms of relativistic coupled cluster theory aimed at modern heterogeneous high-performance computational infrastructures. The code is designed for efficient parallel execution on many compute nodes with optional GPU coprocessing, accomplished via the new ExaTENSOR back end. The resulting ExaCorr module is primarily intended for calculations of molecules with one or more heavy elements, as relativistic effects on electronic structure are included from the outset. In the current work, we thereby focus on exact 2-component methods and demonstrate the accuracy and performance of the software. The module can be used as a stand-alone program requiring a set of molecular orbital coefficients as starting point, but is also interfaced to the DIRAC program that can be used to generate these. We therefore also briefly discuss an improvement of the parallel computing aspects of the relativistic self-consistent field algorithm of the DIRAC program

Implementation of relativistic coupled cluster theory for massively parallel GPU-accelerated computing architectures

International audienceIn this paper, we report a reimplementation of the core algorithms of relativistic coupled cluster theory aimed at modern heterogeneous high-performance computational infrastructures. The code is designed for efficient parallel execution on many compute nodes with optional GPU coprocessing, accomplished via the new ExaTENSOR back end. The resulting ExaCorr module is primarily intended for calculations of molecules with one or more heavy elements, as relativistic effects on electronic structure are included from the outset. In the current work, we thereby focus on exact 2-component methods and demonstrate the accuracy and performance of the software. The module can be used as a stand-alone program requiring a set of molecular orbital coefficients as starting point, but is also interfaced to the DIRAC program that can be used to generate these. We therefore also briefly discuss an improvement of the parallel computing aspects of the relativistic self-consistent field algorithm of the DIRAC program