Magnetic fields in the spiral arms of galaxies

Ordered magnetic fields observed in the Galaxy are approximately aligned along the axis of the spiral arms. The magnetic field strength is 3 microgauss but the total energy of the galactic magnetic field is as high as 1055ergs. The magnetic field direction is observed to reverse between adjacent spiral arms and this configuration requires an electric current sheet to loop each arm. Mechanisms for creating an electric current are discussed and attention is focused on the possibility of a net proton current. This requirement may be compatible with the indications that partially ionized gas is rolling round the spiral arms as they orbit the Galaxy. There is evidence which suggests that the direction of roll of the gas also reverses between adjacent spiral arms. There is a well known coincidence between the energy density of the galactic magnetic field and the energy density of turbulent gas motions, both are close to 1eVcm–3. Possible origins for these energies and the implications of this coincidence are discussed.     

Electrostatic potentials and fields from density expansions of deformed atoms in molecules

The exact representation of the molecular density by means of atomic expansions, consisting in spherical harmonics times analytical radial factors, is employed for the calculation of electrostatic potentials, fields, and forces. The resulting procedure is equivalent to an atomic multipolar expansion in the long-range regions, but works with similar efficiency and accuracy in the short-range region, where multipolar expansions are not valid. The performances of this procedure are tested on the calculation of the electrostatic potential contour maps and electrostatic field flux lines of water and nitrobenzene, computed from high-quality molecular electron densities obtained with Slater basis sets.     

Tune-out and magic wavelengths for hydrogenlike and screened-hydrogenlike atoms

The tune-out and magic wavelengths (TMWs) for the hydrogenlike and screened-hydrogenlike atoms (Z = 1-8) are calculated using a variational method. The first three tune-out wavelengths for each of the nS (n = 1, 2, 3) states and the first three magic wavelengths for each of the 1S-nS (n = 2, 3) transitions of the hydrogenlike atoms in terms of nuclear charge Z, and the first TMWs for the above mentioned states or transitions of the screened-hydrogenlike atoms in terms of the screening parameter μ and Z are presented. All the results for the TMWs are reported for the first time in the literature except the magic wavelengths for the hydrogen atom. The magic wavelengths for hydrogen atom are in agreement with the available data. The precise estimation of the TMWs would be helpful for the design of the ion-trapping experiment of the hydrogenic systems. Our study is expected to be useful for the measurements of the hydrogenic 1S-2S and 1S-3S transitions spectroscopy for the improved accuracy.     

Valence p functions for alkali and alkaline-earth atoms

Contracted Gaussian-type function (CGTF) basis sets are reported for valence p orbitals of the six alkali and alkaline-earth atoms Li, Be, Na, Mg, K, and Ca for molecular applications. These sets are constructed by Roothaan–Hartree–Fock calculations for the ns → np excited states of atoms, in which both linear and nonlinear parameters of CGTFs are variationally optimized. The present CGTF sets reproduce well the numerical Hartree–Fock ns → np excitation energies: the largest error is 0.0009 hartrees for Li. New CGTFs are tested with diatomic Li₂, Na₂, K₂, and MH molecules, where M = Li, Be, Na, Mg, K, and Ca, by self-consistent-field (SCF) and multiconfiguration SCF calculations. The resultant spectroscopic constants compare well with those of more elaborate calculations and are sufficiently close to experimental values, supporting the efficiency of the present set for the valence p orbitals. Received: 9 July 1998 / Accepted: 17 September 1998 / Published online: 1 February 1999     

Participation of preadsorbed atoms in heterogeneous recombination of hydrogen atoms on semiconductor surfaces

Atoms trapped from the vapor phase in the preadsorbed state were found to provide the catalytic activity of semiconductors in the heterogeneous recombination of hydrogen atoms.     

Sizes of atoms and ions and covalency of bonding in molecules and crystals

A new system of atomic radii for the elements up to barium inclusive is constructed. Values of the radii are chosen so as the dependence between the dissociation energy of diatomic homonuclear molecules and a depth of atom overlapping is monotonous, and the scatter of data is minimal. The depth of overlapping is calculated as a difference between the sum of atomic radii and an experimental interatomic distance. Conclusions are made that: the radii of free atoms and ions are determined by the value of the electron density equal to 0.01 au; they considerably change in molecules and crystals only as a result of the charge transfer from cation to anion; covalent bonding is well described by the overlapping of free atoms (ions), confined by the surface of the given radius, and its energy depends upon the depth of overlapping of valence electron densities of atoms. A method of overlapping atoms is proposed for the approximate estimation of ionic sizes and charges in bound systems.     

Cocondensation reactions of heterocyclic aromatic compounds with lithium, calcium and magnesium atoms at 77 K

Calcium and magnesium atoms were cocondensed with aromatic heterocycles containing five- and six-membered rings in the presence of THF at 77 K. In the case of calcium the cocondensation with five-membered heterocyclic compounds resulted in C-H bond activations and led to the corresponding aryl calcium compounds, while magnesium did not show comparable reactions. When six-membered heterocyclic compounds, e.g., pyridine and 4-methylpyridine (4-picoline) were cocondensed with calcium, magnesium and lithium atoms, all reactions led to the formation of non-metallated aromatic products and the formation of metal hydride. DFT calculations at the B3LYP/6-31G** level of theory were carried out in order to interpret the pathways of the cocondensation reactions using calcium atoms and identify the possible intermediates involved. In all reactions π- and σ-complexes between calcium atoms and the heterocyclic aromatic reactant were found as stable intermediates on the energy hypersurface.     

Charge and radial correlations in helium and helium-like atoms

Summary For two-electron atoms, the method of a variable exponent, which treats the orbital exponent (or effective nuclear charge) of an electron as an explicit function of the radial coordinate of the other electron, is studied. The method is shown to improve the energy and other electronic properties remarkably. An incorporation of the variable exponent into the Kellner approximation for He, for example, gives the energy –2.872 606 1 a.u., which is lower than the original Kellner energy by 0.024 949 8 a.u. and exceeds the Hartree-Fock limit energy by 0.010 926 1 a.u. The improvement due to the variable exponent originates from the inclusion of the charge and radial correlations. Applications of the method to the Eckart and Hylleraas approximations are also presented.     

Structure,dynamics and quantum chaos in atoms and molecules under strong magnetic fields

Although studies on the interaction of atoms and molecules with external magnetic fields are more than 100 years old, beginning from the pre-quantum-mechanical days to the era of quantum mechanics, interest in the application of strong, static magnetic fields on atoms and molecules is only about three decades old. Although a great deal of insight has been obtained on the consequent changes in electronic structure and chemical bonds by such strong fields, the more realistic, dynamic, strong magnetic fields were not studied. Based on our own works in the last decade, we will discuss in this article how strong static as well as oscillating, strong magnetic fields on atoms and molecules affect electron density and chemical bonds. The dynamic fields generate completely new, hitherto unknown exciting phenomena. Our discussions will be based on three inter-linked, fundamental aspects of matter-external-magnetic-field interactions, viz., (1) action of static, strong magnetic fields as well as associated changes in electronic structure and the chemical bond, (2) Dynamics of electron density, and (3) Non-linear effects inherent in these interactions. Each aspect, although discussed separately for clarity, is a part of a larger intertwining picture.     

Kinetics of heterogeneous recombination of hydrogen atoms on solid surfaces

Heterogeneous recombination rates of hydrogen atoms on solid surfaces were measured for the first time. An anomalous form of these curves testifies that the reaction on surfaces of metals, semiconductors and dielectrics follows a universal mechanism with participation of preadsorbed atoms.     

Isoelectronic changes in total Hartree-Fock energy of atoms

It is shown numerically that to a reasonably good accuracy, the isolectronic changes in total Hartree-Fock energy of atomic systems can be predicted, in terms of the energy ratio between the adjacent members in a given isoelectronic series, simply as a function of atomic numbers.Alexander von Humboldt Fellow, on leave from School of Chemistry, University of Hyderabad, India.     

Orbital electronegativity and electron affinity of rare earth atoms using Xα-theory

Slater's transition state concept and the relativistic numerical Hartree-Fock-Slater theory has been used to calculate the electronegativity and first ionization potential for the rare earth atoms. Based on these results the electron affinity has also been estimated. The theory predicts almost constant values of the electronegativity as 2 eV, first ionization potential as 8 eV and the electron affinity of -(4–5) eV respectively.This work is dedicated to the memory of Prof. J. C. Slater.Alexander von Humboldt fellow, on leave from school of chemistry, University of Hyderabad, India.     

Effects of static electric fields on the photoionization spectra of two-electron atoms and ions

The simultaneous photoexcitation of two electrons is a highly correlated process, producing doubly excited states, observed as resonant structure in the ionization cross-section. This review paper explores the ability of a static electric field to affect these correlations, as evidenced by changes in the shape and energy of resonances. A comparison of field effects on the spectra of H⁻, Ps⁻, He, Ba and Cs⁻ summarizes current understanding of the processes and the capability of theory to predict the cross-sectional features.     

Subshell-pair interelectronic angles of atoms in momentum space

Average angles between linear momenta of an electron in a subshell nl and another electron in a subshell nl are examined for the 102 atoms He through Lr in their ground states, where n and l are the principal and azimuthal quantum numbers, respectively. Congruency in the mathematical structures of the average interelectronic angles in position and momentum spaces leads to the theoretical results that with even |l–l| are exactly equal to 90°, while with odd |l–l| are always larger than 90°. Numerical analyses of 3,275 subshell-pair angles with odd |l–l| in the 102 atoms clarify that deviations of the total average interelectronic angles from 90° are mainly governed by subshell pairs with |n–n|1 and |l–l|=1, in contrast to the position-space results where only subshell pairs with n=n and |l–l|=1 are important.Acknowledgments. We thank Mr. T. Shimazaki for his assistance in the compilation of data. This work was supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education of Japan.     

Subshell-pair correlation coefficients of atoms in momentum space

Angular correlation coefficients τ ^{nl,n^′ l^′} [p] between linear momenta of an electron in a subshell nl and another electron in a subshell n′ l′ are studied for the 102 neutral atoms He through Lr in their ground states, where n and l are the principal and azimuthal quantum numbers, respectively. We theoretically find that electron momenta are negatively correlated or uncorrelated; τ ^{nl,n^′ l^′} [p] < 0 when |l − l′|=1, while τ ^{nl,n^′ l^′} [p]=0 when |l − l′| ≠ 1. Numerical examinations of the atoms show that except for the He–B atoms, negative correlations are largest between 1s and 2p subshells, which have the most diffuse electron distributions in momentum space.     

Even-tempered Roothaan-Hartree-Fock wave functions for the third- and fourth-row atoms

Summary Roothaan-Hartree-Fock wave functions composed of 12s8p6d, 12s10p6d, and 12s10p8d even-tempered (ET) Slater-type functions (STFs), respectively, are reported for the atoms K-Zn, Ga-Kr, and Rb-Xe in their ground state. Despite the limited variational freedom in the Et method, the resultant atomic energies are found to compare well with fully-optimized wave functions of similar sizes. In particular, the present ET results reproduce almost completely the fully-optimized Sekiya-Tatewaki energies with the same basis set size for the atoms K-Zn. All the present energies are also lower than the Clementi-Roetti ones with slightly smaller but fully-optimized basis sets. A generalized even-tempered scheme is suggested and shown to give good results for Xe.     

Subshell-pair interelectronic angles of atoms

For the 102 atoms from He to Lr in their ground states, the average interelectronic angles <₁₂> _{nl, n'l'} between an electron in a subshellnl and another electron in a subshell n'l' are examined, where n and l are the principal and azimuthal quantum numbers, respectively. Theoretical study clarifies that <₁₂> _nl,n'l' are 90° precisely if l–l' are even, while they are larger than 90° if l–l' are odd. Numerical analysis of 3,275 subshell pairs with odd l–l' of the 102 atoms shows that the increases in the total average interelectronic angles <₁₂> from 90° are attributed predominantly to subshell pairs with n=n' and l–l'=1.     

Core electrons and hydrogen atoms in chemical graph theory

General and complete graphs have recently been used to free chemical graph theory, and especially molecular connectivity theory, from spurious concepts, which belonged to quantum chemistry with no direct counterpart in graph theory. Both types of graph concepts allow the encoding of multiple bonds, non-bonding electrons, and core electrons. Furthermore, they allow the encoding of the bonded hydrogen atoms, which are normally suppressed in chemical graphs. This suppression could sometimes have nasty consequences, like the impossibility to differentiate between compounds, whose hydrogen-suppressed chemical graphs are completely equivalent, like for the CH₂F₂ and BHF₂ compounds. At the computational level the new graph concepts do not introduce any dramatic changes relatively to previous QSPR/QSAR studies. These concepts can nevertheless help in encoding the many electronic features of a molecule, achieving, as a bonus, an improved quality of the modeled properties, as it is here exemplified with a set of properties of different classes of compounds.     

Electronic structure of first and second row atoms under harmonic confinement

Atoms under pressure undergo a number of modifications of their electronic structure. Good examples are the spontaneous ionization, stabilization of excited-state configurations, and contraction of atomic-shells. In this work, we study the effects of confinement with harmonic potentials on the electronic structure of atoms from H to Ne. Dynamic and static correlation is taken into account with coupled cluster with single and double excitations and CASSCF calculations. Because the strength of harmonic confinement cannot be translated into pressure, we envisioned a “calibration” method to transform confinement into pressure. We focused on the effect of confinement on: (a) changes of electron distribution and localization within the K and L shells, (b) confinement-induced ionization pressure, (c) level crossing of electronic states, and (d) correlation energy. We found that contraction of valence and core-shells are not negligible and that the use of standard pseudopotentials might be not adequate to study solids under extreme pressures. The critical pressure at which atoms ionize follows a periodic trend, and it ranges from 28 GPa for Li to 10.8 TPa for Ne. In Li and Be, pressure induces mixing of the ground state configuration with excited states. At high pressure, the ground states of Li and Be become a doublet and a triplet with configurations 1s²2p and 1s²2s2p, respectively, which could change the chemistry of Be. Finally, it is observed that atoms with fewer electrons correlation increases, but for atoms with more electrons, the increasing of kinetic energy dominates over electron correlation.