Showing papers on "Electronic structure published in 1986"

Density Functional Calculations of Molecular Bond Energies

Abstract: The calculation of molecular spectroscopic properties is an interesting application of the local density approximation (LDA) for the exchange-correlation energy of many-electron systems. LDA bond lengths and vibrational frequencies agree remarkably well with experiment. Dissociation energies are also reasonably good, but tend to overestimate the experimental results. Therefore, we consider in this work the effect of non-local gradient-type correction terms on LDA bond energies. In particular, we consider the gradient-corrected exchange-correlation functional of Langreth and Mehl, and also a semi-empirical exchange approximation recently developed by the present author.

Electronic Structure of the Si(111)2 × 1 Surface by Scanning-Tunneling Microscopy

Abstract: The tunneling current is measured as a function of voltage, lateral position, and vertical separation between a tungsten probe tip and a Si(111)2 \ifmmode\times\else\texttimes\fi{} 1 surface. A rich spectrum is obtained in the ratio of differential to total conductivity, revealing the structure of the surface-state bands. The magnitude of the parallel wave vector for certain surface states is determined from the decay length of the tunneling current. Real-space images of the surface states reveal a phase reversal between those states on either side of the surface-state band gap.

Theoretical study of small silicon clusters: Equilibrium geometries and electronic structures of Sin (n=2–7,10)

Abstract: The geometries and energies of small silicon clusters have been investigated in a systematic manner by means of accurate ab initio calculations. The effects of polarization functions and electron correlation have been included in these calculations. Several geometrical arrangements and electronic states have been considered for each cluster. All the geometries considered have been completely optimized within the given symmetry constraints with several basis sets at the Hartree–Fock level of theory. Single point calculations have been performed at these geometries using complete fourth‐order perturbation theory with the polarized 6‐31G* basis set. The effects of larger basis sets including multiple sets of polarization functions have been considered for Si2 and Si3. Singlet ground states are found for Si3–Si7 with the associated geometries corresponding to a triangle, a planar rhombus, a trigonal bipyramid, an edge‐capped trigonal bipyramid, and a tricapped tetrahedron, respectively. The best calculated structure for Si10 corresponds to a tetracapped octahedral arrangement where alternate faces of the octahedron have been capped to yield a structure with overall tetrahedral symmetry. All the geometries are considerably different from those derived from microcrystal fragments. Binding energies have been computed for all clusters and used to interpret the distribution and fragmentation patterns of small silicon cluster ions observed recently.

The low‐lying electronic excitations in long polyenes: A PPP‐MRD‐CI study

Abstract: A correct description of the electronic excitations in polyenes demands that electron correlation is accounted for correctly. Very large expansions are necessary including many‐electron configurations with at least one, two, three, and four electrons promoted from the Hartree–Fock ground state. The enormous size of such expansions had prohibited accurate computations of the spectra for polyenes with more than ten π electrons. We present a multireference double excitation configuration interaction method (MRD‐CI) which allows such computations for polyenes with up to 16 π electrons. We employ a Pariser–Parr–Pople (PPP) model Hamiltonian. For short polyenes with up to ten π electrons our calculations reproduce the excitation energies resulting from full‐CI calculations. We extend our calculations to study the low‐lying electronic excitations of the longer polyenes, in particular, the gap between the first optically forbidden and the first optically allowed excited singlet state. The size of this gap is shown to depend strongly on the degree of bond alternation and on the dielectric shielding of the Coulomb repulsion between the π electrons.

Analytic Raman intensities from molecular electronic wave functions

Abstract: An analytic method for the evaluation of Ramanintensities from closed−shell self‐consistent‐field wave functions is presented. Predictioinsf or ethylenemolecule are also reported. (AIP)

On the large‐gradient behavior of the density functional exchange energy

Abstract: A coordinate‐space model of the exchange hole density, previously introduced by the author, is extrapolated to the case of very strongly inhomogeneous systems (ie, large density gradients) As a result, we propose a new gradient‐corrected exchange energy functional for application to atomic and molecular problems The model provides theoretical estimates of the parameters in the functional, and these compare well with empirical values deduced from a least squares fit to exact atomic data

Forbidden Lines in ns2npk Ground Configurations and nsnp Excited Configurations of Beryllium through Molybdenum Atoms and Ions

Abstract: Observed and predicted wavelengths of magnetic dipole lines arising within ground configurations of the type ns2npk(n=2 and 3, k=1 to 5) are compiled. For n=2 the compilation includes the elements B through Kr, and for k=5 it extends to Mo. For n=3 Al through Mo are included. In addition the 2s2p excited configuration of the Be i isoelectronic sequence for Be through Kr and 3s3p of the Mg sequence for Mg through Mo are included. For each line we give a calculated value for the transition probability obtained mainly from the Dirac‐Fock method or from the use of scaled radial integrals. The calculated wavelengths are obtained from known energy levels or from levels derived from scaled radial integrals. A small group of electric quadrupole lines seen in astronomical sources are included. The list contains 1660 predicted wavelengths in the range 100 A to 25.9 mm and 406 observed wavelengths in the range 325 A to 609 μm.

Higher excited electronic states in clusters of ZnSe, CdSe, and ZnS: Spin‐orbit, vibronic, and relaxation phenomena

Abstract: Metal selenide clusters have been made and characterized, using the arrested precipitation colloidal technique. A comparison of sulfide and selenide spectra enables observation of the effect of changes in the highest occupied molecular orbitals upon cluster electronic properties. The first and second excited electronic states are both observed as a function of size in ZnSe clusters. The systematic dependence of the spectra lead to assignment of the higher state to a 1S‐type hole based upon the split‐off valence band. It is shown that the energy spectrum of discrete hole states is controlled by the spin‐orbit energy and the isotropic hole mass in small, highly symmetrical clusters. This result contrasts with the heavy hole and light hole states observed for planar confinement. In ≂ 20 A diameter ZnS clusters, there is a strong vibronic temperature dependence in the excited state spectra, while in clusters of smaller gap materials such vibronic effects are very minor. We conjecture that lifetime broadening ...

Determination of the electronic-structure of transition-metal compounds - 2p x-ray photoemission spectroscopy of the nickel dihalides

Abstract: A recently proposed impuritylike many-body theory for the electronic structure of transition-metal compounds is extended to core photoemission. The theory is worked out in detail for the nickel dihalides, and we compare the results with experimental 2p spectra. We show that these spectra can be used for a quantitative determination of the parameters in the theory. We furthermore show that the easy-to-handle cluster model is a fair approximation to the full theory as far as core x-ray photoemission is concerned. We find that the Coulomb interaction energies are the largest energies in the system, and we show that the order of magnitude of these can be estimated from an ionic screening model. Furthermore we show that the charge-transfer energies strongly vary along the series. A comparison with estimates for the parameters derived from other experiments show systematic discrepancies. We argue that these are due to the neglect of higher-order interactions in the model Hamiltonian used.

New Born–Oppenheimer potential energy curve and vibrational energies for the electronic ground state of the hydrogen molecule

Abstract: The Born–Oppenheimer potential energy curve for the electronic ground state of the hydrogen molecule has been computed for internuclear distances 0.2≤R≤12.0 bohr. At all R the energies are lower than any previously reported values. At the equilibrium distance the improvement is negligible; its largest value, 0.47 cm−1, has been obtained for R=2.4 bohr. Vibrational energies have been computed for all isotopic species using the new potential energy curve. For H2, HD, and D2 the recent nonadiabatic corrections of Wolniewicz have been included. The resulting dissociation energies of H2 and HD agree with the experimental values within the experimental error limits; for D2 the discrepancy amounts to 0.6±0.3 cm−1.

Electronic and structural properties of BN and BP.

Abstract: We present a pseudopotential study within the local-density formalism of the structural and electronic properties of zinc-blende BN and BP. The ground-state properties of these systems such as bulk moduli, lattice constants, cohesive energies, and frequencies of the TO phonon mode are in good agreement with experimental results. The valence charge density of BP shows two local maxima along the bond, which is similar to the case of diamond. In contrast, the charge density of BN is similar to that of a typical III-V compound semiconductor. The resulting band structures have some important features which are in disagreement with previously published work. Like most III-V compound semiconductors, the fundamental gap in BP decreases with decreasing volume. The corresponding gap in BN, however, increases with decreasing volume as was also found in diamond.

Molecular and electronic structure of penta- and hexacoordinate silicon compounds

Abstract: Formation, structure (stereochimie et stereodynamique), et donnees de spectroscopie RMN des composes du titre

Ab initio calculations of electronic and vibrational energies of HCO and HOC

Abstract: The ab initio calculation of electronic energies for numerous configurations of HCO and HOC, and a novel method for fitting the energies to a global surface are reported This surface is used to calculate all the bound vibrational states of nonrotating HCO and DCO using the Watson Hamiltonian Some quasibound vibrational states are also reported for nonrotating HOC for energies below the HOC saddle point energy Comparisons of the HCO and DCO vibrational energies are made with recent experimental results

The electronic structure of the benzene molecule

Abstract: The Kekule description of benzene as a mixture of the two structures and was given a firm foundation in quantum theory as a ‘resonance hybrid’ (see, for example, ref. 1). As molecular orbital (MO) theory developed, it was felt that the aromatic character of benzene was explained more naturally in terms of delocalized orbitais. The view that delocalized electrons provide essentially the correct description for this type of system appears to be accepted at all levels2; however, we present here theoretical evidence which challenges this view. We show that the π-electrons in benzene are almost certainly localized and that the characteristic properties of such a system arise from the mode of spin coupling.

Electronic structure and stability of different crystal phases of magnesium oxide.

Abstract: Three different crystal phases of magnesium oxide [B1 (NaCl), B2 (CsCl), and B${8}_{1}$ (inverse NiAs)] are investigated theoretically. An ab initio all-electron linear combination of atomic orbitals Hartree-Fock approximation is adopted; extended basis sets are used which have been variationally optimized in each case. In ordinary conditions, the B${8}_{1}$ and B2 structures result less stable with respect to the rocksalt structure by 0.44 and 1.77 eV, respectively. With increasing pressure, the transition B1\ensuremath{\rightarrow}B2 is estimated to occur around 2.2 Mbar, while it is not excluded that a pressure interval exists around 2 Mbar where the B${8}_{1}$ phase is the most stable of the three structures. Data on the electronic properties of the three phases are provided and discussed: Mulliken populations, charge-density distribution, electron momentum distribution and anisotropy, magnesium core deformation, and band structure.

Origin of surface states on Si(111)(7×7)

Abstract: First-principles pseudopotential total-energy and electronic-structure calculations are reported for two Si(111)(\ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 ):Si adatom geometries. Based on these calculations, a theory of the electronic structure of Si(111)(7\ifmmode\times\else\texttimes\fi{}7) is formulated, and the binding energy of Si adatoms on Si(111) is determined.

A study of the low‐lying electronic states of Fe2 and Co2 by negative ion photoelectron spectroscopy

Abstract: The anions Fe−2 and Co−2 were prepared and cooled to 300 K in a flowing afterglow ion source, and the low‐lying electronic states of the neutral dimers were probed by negative ion photoelectron spectroscopy. Previous ab initio studies of Fe2 and Co2 have predicted single 4s–4s bonds, and extremely high densities of low‐lying states due to the small energy cost in transferring electrons among nonbonding 3d orbitals. In contrast to the complex photoelectron spectra implied by these calculations, the observed spectra are remarkably simple. It is argued that this spectral simplicity implies a greater role for the 3d electrons in the iron and cobalt dimer bonds. These data also provide values for the electron affinities of the neutral dimers (0.902±0.008 eV Fe2, 1.110±0.008 eV Co2), the bond elongation on electron attachment (0.08±0.02 A Fe2, Co2), and the vibrational frequencies of the anions (250±20 cm−1 Fe−2, 240±15 cm−1 Co−2). Related studies of the atomic anions yield improved values for the electron affi...

Looking at orbitals in the laboratory: The experimental investigation of molecular wavefunctions and binding energies by electron momentum spectroscopy

Abstract: The experimental technique of electron momentum spectroscopy (EMS) (i.e., binary (e, 2e) spectroscopy) is discussed together with typical examples of its applications over the past decade in the area of experimental quantum chemistry. Results interpreted within the framework of the plane wave impulse and the target Hartree—Fock approximations provide direct measurements of, spherically averaged, orbital electron momentum distributions. Results for a variety of atoms and small molecules are compared with calculations using a range of Fourier transformed SCF position space wavefunctions of varying sophistication. Measured momentum distributions (MD) provide a “direct” view of orbitals. In addition to offering a sensitive experimental diagnostic for semiempirical molecular wavefunctions, the MD's provide a chemically significant, additional experimental constraint to the usual variational optimization of wavefunctions. The measured MD's clearly reflect well known characteristics of various chemical and physical properties. It appears that EMS and momentum space chemistry offer the promise of supplementary perspectives and new vistas in quantum chemistry, as suggested by Coulson more than 40 years ago. Binding energy spectra in the inner valence region reveal, in many cases, a major breakdown of the simple MO model for ionization in accord with the predictions of many-body calculations. Results are considered for atomic targets, including H and the noble gases. The measured momentum distribution for H2 is also compared with results from Compton scattering. Results for H2 and H are combined to provide a direct experimental assessment of the bond density in H2, which is compared with calculations. The behavior of the outer valence MD''s for small row two and row three hydride molecules such as H2O and H2S, NH3, HF, and HCl are consistent with well known differences in chemical and physical behavior such as ligand-donor activity and hydrogen bonding. MD measurements for the outermost valence orbitals of HF, H2O and NH3 show significant differences from those calculated using even very high-quality wavefunctions. Measurements of MD's for outer σg orbitals of small polyatomic molecules such as CO2, COS, CS2, and CF4 show clear evidence of mixed s and p character. It is apparent that EMS is a sensitive probe of details of electronic structure and electron motion in atoms and molecules.

Symmetry through the eyes of a chemist

Abstract: Simple and Combined Symmetries.- Molecular Shape and Geometry.- Helpful Mathematical Tools.- Molecular Vibrations.- Electronic Structure of Atoms and Molecules.- Chemical Reactions.- Space-Group Symmetries.- Crystals.

Spin-polarized photoemission study of epitaxial Fe(001) films on Ag(001).

Abstract: The electronic and magnetic character of epitaxial Fe films on Ag(001) has been studied as a function of Fe coverage by spin- and angle-resolved photoemission with synchrotron radiation. At coverages well below a monolayer, the spectra exhibit a local spin-split electronic state. Although spectra for films in the monolayer coverage range display electronic structure in close agreement with calculated monolayer film critical-point energies, no spin polarization is observed up to 2.5 monolayers coverage. Thicker films approach the spin-split electronic structure and spin polarization of bulk Fe(001).

Silicon and germanium clusters. A theoretical study of their electronic structures and properties

Abstract: Silicon and germanium clusters containing three to seven atoms have been studied with the pseudopotential MO‐LCAO method followed by configuration interaction procedure. Si and Ge clusters have very similar electronic structures and consequently analogous physico‐chemical properties but differ substantially from small carbon clusters. Linear structures are clearly less favorable than more compact structures. On the other hand, some planar geometries possess considerable stability. The Si and Ge clusters which are sections of the diamond‐type crystal lattice are less stable than clusters which can be considered as segments of closed‐packed lattices or as steps in pentagonal crystal growth. The reason is that the majority of atoms in small clusters are surface atoms which cannot assume the tetrahedral coordination characteristic of Si and Ge bulk atoms. The appearance of typical bulk properties is expected only for very large Si and Ge clusters with small surface atoms/bulk atoms ratio.

Valence electronic structure of AuZn and AuMg alloys derived from a new way of analyzing Auger-parameter shifts.

Abstract: A new model is presented that makes it possible to determine the degree of electron transfer in alloys from measurements of the Auger parameter This approach is superior to the use of core-ionization-energy shifts in conjunction with a potential model The Auger parameter does not depend on any reference level, whereas ionization energies measured with respect to the Fermi level must be corrected to the vacuum level before they can be used in the potential model Furthermore, the new model does not require inclusion of contributions from Madelung, surface-dipole, or other bulk contributions, which must be considered in the potential model This new technique is applied to the alloys AuZn and AuMg to show that approximately 01e is transferred to the gold in the first case and about 02e in the second The analysis shows that there is a small increase in the gold 5d population through the series Au, AuZn, AuMg The results are combined with data on M\"ossbauer isomer shifts and theoretical band-structure calculations for Au to give a description of the valence electronic configuration in these alloys The substantial transfer of electrons to the gold 6s orbital, inferred from the M\"ossbauer results, is partially offset by back transfer of 6p electrons from gold to the partner atom This conclusion is supported by band-structure calculations for AuCs

On the manifestation of electronic structure effects in metal clusters

Abstract: Potasium (K x x<40) and sodium(Na x , x<66) clusters were generated in molecular beams and probed by photoionizationmass spectroscopy. Results obtained include measurements of neutral cluster abundances and determinations of ionization potentials. The I. P. values can be rationalized in terms of a global electrostatic model which extrapolates to the bulk work function. This model also applies to transition metals as can be shown by comparng it to the limited experimental data sets available. The I. P.’s of small clusters exhibit ‘‘quantum size’’ effects which can be understood by individual quantum chemical calculations. As previously found for sodium clusters, photoionization mass spectra obtained for potassium, mixed potassium/sodium and potassium/lithium exhibit abundance maxima at M+ 8 and M+ 20, where M is an alkali metal. This has been interpreted in terms of increased thermodynamic stability of the corresponding neutrals relative to neighboring clusters. We present data which show that a spherical jellium model, while providing a set of numbers correlating well with those of preferred stability in alkali clusters, is less successful in explaining other properties.

Atomic and electronic contributions to Si(111)-(7×7) scanning-tunneling-microscopy images

Abstract: Bias-dependent scanning tunneling microscopy (STM), used in conjunction with atomic-charge superposition calculations, is shown to provide information on both atomic and electronic surface structure. Si(111)-(7\ifmmode\times\else\texttimes\fi{}7) STM images measured with different bias voltages are compared with a number of recent structural models. Striking agreement is found with the stacking-fault-adatom model of Takayanagi. The unit-cell asymmetry found at negative sample bias is attributed to a stacking fault in one half of the unit cell, locally modifying the surface electronic structure.

Molecular beam photoelectron spectroscopy and femtosecond intramolecular dynamics of H2O+ and D2O+

Abstract: The 584 A photoelectron spectra of supersonic molecular beams of H2O and D2O have been obtained with improved resolution. The spectroscopic constants of the X 2B1 and A 2A1 state ions, including ω01, x011, ω02, x022, and x012, are reported. For the first two electronic states of the ion, precise line splittings were evaluated with a least squares fitting procedure, employing sums of empirical instrument response functions and a linear background. A simulation of the vibrational manifolds of the B 2B2 state ions with combination progressions in the symmetry‐allowed modes ν1 and ν2 failed to reproduce the diffuse photoelectron bands observed for both H2O and D2O. Autocorrelation functions were calculated from the photoelectron bands of all three electronic states. The B 2B2 state correlation functions exhibit ultrafast decay, occurring on a 10−14 s time scale. The ν2 motion appears to define the decay in the correlation function. This behavior supports a previously proposed B 2B2–A 2A1 curve‐crossing model for the nonradiative relaxation of the B 2B2 state ions.