State Dipole Moments Research Articles

Electrochemical and optical properties of two dimensional electrostatic assembly of Au nanocrystals.

The spectroscopic and electrochemical properties of two-dimensional electrostatic assembly of Au nanocrystals are examined on poly-L-lysine (pLys) modified gold electrodes. The surface preparation for the nanoparticle deposition involved the self-assembly of a monolayer of 11-mercaptoundecanoic acid on the electrode surface, followed by the electrostatic deposition of pLys from aqueous solution. The polyelectrolyte layer acts as the electrostatic anchor for the Au particles. Electrostatically stabilised Au particles were prepared by homogeneous reduction in the presence of citrate, yielding monodispersed colloidal suspension with an average diameter of 18 +/- 2 nm. After 4 h of deposition, the citrate-stabilised particles reach a maximum surface density of (8.2 +/- 0.1) x 10(10) particles cm(-2), with an average edge-to-edge distance of 25 nm. The particle surface density was estimated from scanning electron micrographs. Kelvin probe measurements were employed for examining changes in surface dipole introduced by the 2D array of nanocrystals. From simple electrostatic arguments, the apparent static dipole moment per particle was estimated of the order of 2700 D. The strong interaction between the nanocrystals and the pLys layer is responsible for the surface charge displacement, leading to changes in the surface dipole of 0.35 eV. These electrostatic interactions also manifest itself by the red shift of the plasmon resonance of the assembly with respect to the aqueous colloidal suspension. Analysis of the spectral broadening was attempted within the framework of the so-called coherent-potential approximation. Finally, electrochemical studies in 1,2-dichloroethane show a large electronic overlap between the nanocrystals and the metal substrate. Results obtained from electrochemical impedance spectroscopy strongly suggest that the electrostatic assembly of nanocrystal behaves like a 2D array of randomly distributed spherical nanoelectrodes.

Design for Charge Transport Property of Positive-Charging -Type Single-Layer OPC Photoreceptor

This report presents a study of design for charge transport property of positive-charging-type single-layer OPC photoreceptor (PSLP) having highly photosensitivity and high photoresponse using an electrophotographic imaging system. Tow design objectives are demonstrated as follows. (1) In the PSLP which has dispersed p-type charge generation material, we confirmed that the existence of ETM is one of dominant factors for photosensitivity. (2) In the design for high-sensitive PSLP, it has revealed it is important to select the combination of HTM and ETM in consideration of dipole moments. In other words, in ternary solid solutions of phenylendiamine (PDA) as a hole transport material (HTM) and phenylnaphthquinone (PNQ) as an electron transport material (ETM) in bisphenol-Z type polycarbonate (PC-Z) as a binder resin, a significant increase in the electron mobility have been observed. The effect is attributed to the elimination of random dipolar field s due to static dipole moments of the carbonyl groups of the polycarbonate.

Electronic and Vibrational Polarizabilities and Hyperpolarizabilities of Azoles: A Comparative Study of the Structure−Polarization Relationship

Density functional theory (DFT) is used to study the static electronic dipole moments, polarizabilities, polarizability anisotropies, and first- and second-order hyperpolarizabilities of azoles. These properties are obtained with a finite field approach implemented in the DFT program ALLCHEM. The calculations were of all-electron type using a local exchange correlation functional. To investigate the dependence of polarizabilities and first- and second-order hyperpolarizabilities on the geometries, all structures were optimized with ALLCHEM and MSINDO. The influence of the substituted atoms on the properties is discussed. The vibrational contributions to the above properties of the considered compounds have also been computed using SCF theory and analytic property derivatives. Several methods (basis sets and approaches to determine the electron correlation contribution) have been employed to confirm the adequacy of the method, which was used. The electronic and vibrational properties are connected with var...

Origin of transition dipole-moment polarizability and hyperpolarizability in hydrazones

The origin of transition dipole-moment polarizability and hyperpolarizability in organic materials was investigated using electroabsorption (EA) spectroscopy. It is well established that the following three nonlinear optical properties can be derived from the analysis of EA spectrum; (1) the transition dipole-moment polarizability and hyperpolarizability $(D$ factor), (2) the change of polarizability upon photoexcitation $(\ensuremath{\Delta}\ensuremath{\alpha}),$ and (3) the change of static dipole moment upon photoexcitation $(\ensuremath{\Delta}\ensuremath{\mu}).$ We prepared hydrazones derived from a series of biological polyenes to investigate the physical meaning of the D factor. The hydrazones derived here have two features; (1) they are polar carotenoid analogs with a large third-order nonlinear polarizability, and (2) they have two transition dipoles nearly collinear with the molecular axis. The number of conjugated $\mathrm{C}=\mathrm{C}$ bonds was systematically varied and then the relationship between their structures and nonlinear optical properties of the molecules was investigated. We found that the D factor was the main contributor to the EA spectra of the present set of hydrazones and it had a nearly constant value regardless of the lengths of polyene fragments investigated. Based on the sum rule of the transition dipole moments, the D factor was attributed to charge transfer between the neighboring transition dipoles in the excited states. We conclude that the presence of more than two transition dipole moments in one molecule and the intramolecular charge transfer between them, are necessary for the D factor to be predominant in the nonlinear optical process of organic materials.

Electronic excited states of conjugated cyclic ketones and thioketones: A theoretical study

Absorption spectra of a series of cyclic conjugated ketones and thioketones have been computed at the multiconfigurational second-order multistate perturbation level of theory, the CASSCF/MS-CASPT2 method. Excitation energies, transition dipole moments, oscillator strengths, and static dipole moments are reported and discussed for excited states with energies lower than ≈7–8 eV. The main bands of the spectra have been assigned and characterized in most cases for the first time. The spectroscopy of the different systems is compared in detail. Thioketones in particular have low-energy and intense ππ* transitions which suggest corresponding enhanced nonlinear molecular optical properties. Additionally, some of the methods used to estimate these properties from spectroscopic data have been considered in order to analyze the main contributions to the nonlinear optical properties.

Calculations of the static polarizability and hyperpolarizability of the LiF molecule including vibrational contributions

The static dipole moment, polarizability and first hyperpolarizability of the LiF molecule have been calculated, including vibrational corrections, using Möller—Plesset perturbation theory and coupled cluster methods. The results show that the vibrational corrections, dominated by the nuclear relaxation term, are as important for the calculations of the polarizability and hyperpolarizability as the electronic contribution, and that the electron correlation effects play an important role in the calculations of both electronic and vibrational contributions for these static properties.

Field-dependent electrode-chemisorbate bonding: sensitivity of vibrational stark effect and binding energetics to nature of surface coordination.

Illustrative quantum-chemical calculations for selected atomic and molecular chemisorbates on Pt(111) (modeled as a finite cluster) are undertaken as a function of external field, F, by using Density Functional Theory (DFT) with the aim of ascertaining the sensitivity of the field-dependent metal-adsorbate binding energetics and vibrational frequencies (i.e., the vibrational Stark effect) to the nature of the surface coordination in electrochemical systems. The adsorbates selected--Cl, I, O, N, Na, NH(3), and CO--include chemically important examples featuring both electron-withdrawing and -donating characteristics. The direction of metal-adsorbate charge polarization, characterized by the static dipole moment, mu(S), determines the binding energy-field (E(b-F) slopes, while the corresponding Stark-tuning behavior is controlled primarily by the dynamic dipole moment, mu(D). Significantly, analysis of the F-dependent sensitivity of mu(S) and mu(D) leads to a general adsorbate classification. For electronegative adsorbates, such as O and Cl, both mu(S) and mu(D) are negative, the opposite being the case for electropositive adsorbates. However, for systems forming dative-covalent rather than ionic bonds, as exemplified here by NH(3) and CO, mu(S) and mu(D) have opposite signs. The latter behavior, including electron-donating and -withdrawing categories, arises from diminishing metal-chemisorbate orbital overlap, and hence the extent of charge polarization, as the bond is stretched. A clear-cut distinction between these different types of surface bonding is therefore obtainable by combining vibrational Stark-tuning and E(b)-F slopes, as extracted from experimental data and/or DFT calculations. The former behavior is illustrated by means of potential-dependent Raman spectral data obtained in our laboratory.

Vibrational spectroscopy as a probe of potential-dependent electrode-chemisorbate bonding: an assessment using finite-cluster density functional theory

The application of Density Functional Theory (DFT) using variable-field finite-cluster models to the description of electrode-chemisorbate systems is considered, with the particular objective of assessing the relationships between potential-dependent metal-adsorbate vibrational frequencies and the chemical nature of the surface bond. The validity of employing finite-cluster models in variable homogeneous fields to describe potential-dependent electrode-chemisorbate bonding is discussed, including the choice of bulk-phase adsorbate reference states for ionic systems. The bond length-dependent charge polarization, as reflected in the so-called static ( μ S) and dynamic ( μ D) dipole moments, plays a central role in determining the field ( F)-dependence of the metal-adsorbate binding energies, E b, and vibrational force constants, K MA, respectively. A distinction between ‘ionic’ and ‘dative covalent’ surface bonding, based on whether the signs of μ S and μ D are the same or opposite, respectively, is demonstrated by means of DFT calculations for selected atomic and molecular adsorbates on Pt(111), and interpreted in terms of bond length-dependent charge polarization and orbital overlap. A basic consequence of these behavioral differences between μ S and μ D is to deny the occurrence of any uniform correlation between the E b– F and K MA– F behavior (i.e. between the potential-energy well depth and well ‘stiffness’, respectively). On the other hand, combined potential-dependent bond-energy and bond-frequency data should therefore be invaluable in ascertaining the nature of the surface coordination. A more uniform relationship, however, is observed between F-dependent force constants and equilibrium bond lengths. The applicability of these notions to the interpretation of Stark-tuning (i.e. frequency–potential) data for intramolecular as well as metal-adsorbate vibrations is also briefly discussed.

ELECTRON ENERGIES, DIPOLE ELECTRIC MOMENTS, AND DISTRIBUTION OF THE LAURDAN MOLECULE OVER THE CONFORMATION STATES OF METHYL GROUPS

Using the methods of molecular simulation and HyperChem v.5.0 programs (PM3 method), we carried out calculations of the principal spectroscopic characteristics and of the structure of the laurdan molecule in the ground and the first excited electronic states. The thermal static distribution of molecules over various possible orientations ϕ of the plane of methyl groups relative to the plane of the naphthalene bicycle was taken into account. The energies and dipole moments of these electronic states have been calculated as functions of the torsion angle of methyl groups. The existence of an additional mechanism of electronic spectrum broadening is shown; it is associated with thermal mismatch of the equilibrium orientations of the rotational fragment of the molecule and with the dependence of the electron transition frequency on the degree of deviation of the angle ϕ from the equilibrium value. The dependence of dipole moments on this angle has been found and calculated. This dependence is the strongest for the ground state. The maximum values of dipole moments in the ground and excited states are 4.0 and 7.6 D.

Metal electrode–chemisorbate bonding: General influence of surface bond polarization on field-dependent binding energetics and vibrational frequencies

An analysis is outlined in which the dependence of the binding energy, Eb, stretching frequency, νM–A, and equilibrium bond length, req, of metal–adsorbate bonds on the external interfacial field (F), and hence surface potential of relevance to electrochemical systems, are described in terms of potential-energy surface and bond-polarization parameters. Density Functional Theory (DFT) calculations for finite metal clusters with monoatomic adsorbates forming polar surface bonds are utilized both to examine metal-, adsorbate-, and field-dependent trends in the required parameters and to examine the applicability of the analytic relations in comparison with numerical calculations. To a very good approximation, the Eb–F dependence is described by the field-dependent static dipole moment, μS, of the metal–adsorbate bond. As expected, the field-induced changes in the potential-energy surface (PES) are determined by the r-dependent μS values, i.e., the dynamic dipole moment, μD. The factors determining the νM–A–F dependence, however, are distinctly more intricate, involving the PES anharmonicity coupled with μD, as well as being influenced significantly by the μD–r dependence in some cases. While both μS and μD are influenced importantly by surface bond polarity, the role of bond polarizability appears to be quite different, so that only a crude correlation between μS and μD is evident. These differences further underscore the disparate nature of the Eb–F and νM–A–F behavior. Nevertheless, an approximate inverse correlation between the νM–A–F and req–F dependence is usually anticipated. The broad-based utility of DFT for assessing as well as predicting the role of surface bond polarization in controlling the field-dependent electrode–adsorbate PES is pointed out.

An atomic linear Stark shift violating P but not T arising from the electroweak nuclear anapole moment

We propose a direct method of detection of the nuclear anapole moment. It is based on the existence of a linear Stark shift for alkali atoms in their ground state perturbed by a quadrupolar interaction potential and a magnetic field. This shift is proportional to the T-even pseudoscalar built from the quadrupolar potential symmetry axis, the directions of the applied electric and magnetic fields.It involves on the one hand the anisotropy of the hyperfine interaction induced by the quadrupolar interaction and, on the other,the static electric dipole moment arising from electroweak interactions inside the nucleus. The case of ground state cesium atoms trapped in a uniaxial (hcp) phase of solid helium-4 is examined. From an explicit evaluation of both the hyperfine structure anisotropy and the static P-odd T-even dipole deduced from recent empirical data about the cesium nuclear anapole moment, we predict the Stark shift. It is three times the experimental upper bound to be set on the T-odd Stark shift of free cesium atoms in order to improve the present limit on the electron EDM.

Neutral Fermion with a Magnetic Moment in External Electromagnetic Fields

The interaction between a massive neutral fermion with a static (spin) magnetic dipole moment μ and an external electromagnetic field is described by the Dirac–Pauli equation. Exact solutions of this equation are obtained along with the corresponding energy spectrum for an axially symmetric external magnetic field and for some centrally symmetric electric fields. It is shown that the spin–orbital interaction of a neutral fermion with a magnetic moment determines both the characteristic properties of the quantum states and the fermion energy spectrum. It is found that (1) the discrete energy spectrum of a neutral fermion depends on the projection of the fermion spin on a certain quantization axis, (2) the ground energy level of a fermion in these electric fields as well as the energy levels of all bound states with a fixed value of the quantum number characterizing the projection of the fermion spin in the electric field E = er is degenerate and the degeneration order is countably infinite, and (3) the energy spectra of neutral fermions and antifermions with spin magnetic moments are symmetric in centrally symmetric fields. Bound states of a neutral fermion with a magnetic moment in an external electric field do exist even if the Dirac–Pauli equation does not explicitly contain the term with the fermion mass. In addition, in centrally symmetric electric fields, there exist a countably infinite set of pairs of isolated charge-conjugate zero-energy solutions of the Dirac–Pauli equation.

Nonlinear electrodynamics of electrons in a quasi-one-dimensional ballistic ring

We consider ballistic electron motion in a quasi-one-dimensional ring under the uniform high-frequency electric field induced by an electromagnetic field. The electron satisfies a nonlinear equation of motion which is formally identical to that for a pendulum with a vibrating suspension point. The averaging method of Kapitza is used. The electromagnetic emission spectrum is calculated. The spectrum consists of low-frequency radiation, scattered radiation at the incident radiation frequency and combination scattered radiation; the intensities and frequencies of all components depend nonlinearly on the incident radiation frequency. At a certain value of that intensity the spontaneous symmetry breakdown occurs. As a result, the system acquires some static electric dipole moment.

Large static dipole moment in substituted polyacetylenes obtained by electroabsorption

Changes in static dipole moment and polarizability upon electronic excitation are determined from electroabsorption spectra of unoriented substituted polyacetylenes: poly[( o-isopropylphenyl)acetylene] (PPPA) and poly[[( o-trimethylsilyl)phenyl]acetylene] (PMSPA). The electroabsorption spectrum agrees well with the sum of the first and second derivatives of the absorption spectrum. The dominant contribution of the second derivative is interpreted by the linear Stark effect induced by the permanent dipole moments, which are associated with the charge-transfer transition from the side groups of the main chain. Large static dipole moment differences were obtained to be |Δ μ|=74±10 and 57±10 D for PPPA and PMSPA, respectively.

Anharmonic effects on monolayer phonons

The anharmonic frequency shifts of the phonons in monolayer solids of Ar/Pt(111) and commensurate Xe/Pt(111) are calculated with quasiharmonic perturbation theory and with self-consistent-phonon theory. Results from the two methods are in good agreement for Ar/Pt(111). Uncertainties in the frequency shifts for the Ar/Pt(111) modes are small compared to present uncertainties in the interaction model and in the experimental data. Anharmonic effects in the commensurate Xe/Pt(111) solid at 80 K are large, both because of the small adatom-adatom force constants in this dilated lattice and because of the large degree of thermal excitation. A version of the improved self-consistent-phonon approximation is used for comparison to experimental data, and it is found that the anharmonic terms cause changes on the scale of adjustments in Xe-Pt forces previously found necessary to achieve fits of the harmonic theory to the data. The contribution of static adsorption-induced dipole moments to the monolayer lattice dynamics is analyzed and is found to be negligible for Xe/Pt(111).

The Temperature Dependence of Static Dielectric Permittivities and Dipole Moments of Two Mesogens and their Mixture

The compounds p-pentylphenyl-2-chloro-4-(p-pentylbenzoyloxy)-benzoate and p-octyl-phenyl-2-chloro-4-(p-heptylbenzoyloxy)-benzoate and their mixture (1:1 mole fraction) show nematic phases with supercooling effect below room temperature. Molecular dipole moments μ of the compounds have been determined by solution method. The static dielectric constants ϵ and ϵ⊥ have been measured as functions of temperature at a frequency of 10 kHz. Mesogenic molecular dipole moments μ and their inclination angles β with the molecular long axis are determined following Maier and Meier equations. Short range antiferroelectric order is found to exist in the nematic phase. Kirkwood correlation factors have also been calculated.

Optical elements for slowly moving neutral atoms based on magnetic fields

Magnetostatic and quasi-magnetostatic devices can act as optical elements for the de Broglie waves of magnetic atoms, i.e. atoms with a net static magnetic dipole moment in the ground (or isomeric) state. Such magnetic atom optical elements complement, and are alternatives to, resonantly tuned laser atom optical elements which rely on dynamically generated electric dipole moments. The physical principles behind such magnetic devices are discussed briefly, as well as the potential problems of decoherence and its management. The way that these optical elements may be realized is also discussed, with special reference to mirrors and static and quasi-static gratings. Gratings are important to atom interferometry as they can act as beam-splitters.

Magnetic Moment of the1−Ground State inN18Measured with a NewβLevel Mixing Nuclear Magnetic Resonance Technique

The ${}^{18}\mathrm{N}$ ground state magnetic moment $|\ensuremath{\mu}|\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.135(15){\ensuremath{\mu}}_{N}$ has been measured using a modified $\ensuremath{\beta}$ nuclear magnetic resonance technique. The value is compared to shell-model calculations. Spin-aligned ${}^{18}\mathrm{N}$ projectile fragments were produced in the fragmentation of ${}^{22}\mathrm{Ne}$ at 60.3 MeV/nucleon. Polarization of the nuclear spins was resonantly induced by a combined magnetic dipole, electric quadrupole, and radio frequency interaction. This is the first application of a new method that allows production of polarized nuclei from spin-aligned projectile fragments, allowing one to measure $\ensuremath{\beta}$ asymmetries. The method opens a new range of applications to study static dipole and quadrupole moments of exotic nuclei near the drip lines.

Interacting boson approximation-2 analysis of the Pd and Ru chains. I. Mixed symmetry states ofFmax−1character in even palladium isotopes

An analysis of positive parity levels in the even palladium isotopes ${}^{100\ensuremath{-}116}$Pd has been carried out in the framework of the interacting boson approximation-2 model to identify states having large mixed-symmetry components. By means of numerical calculations, performed in the U(5) limit of the model, it has been possible to find simple relations obeyed by the eigenvalues of the generalized Majorana operator for the mixed-symmetry states having ${F=F}_{\mathrm{max}}\ensuremath{-}1.$ These results have been utilized as a starting point for our analysis. Experimental energies, static and transition electric quadrupole and magnetic dipole moments as well as mixing ratios and intensity ratios have been compared to the values calculated by using a realistic Hamiltonian. In the analysis only six out of the twelve model parameters have been varied as a function of the neutron number. A good overall agreement has been found. A detailed investigation of the level structure made it possible to identify whole groups of states of predominant mixed-symmetry character and to establish, in most cases, a close correspondence with states of the U(5) limit of the model.

Modification of the electron spectrum and vibrational properties of amorphous carbon by copper doping

The near-gap electron spectrum and the effective charge distribution in graphite-like carbon nanoclusters of simple geometry in a-C: H containing a single Cu atom are calculated in the tight-binding approximation. Only the coupling between π electrons of the constituent C atoms and one valence s electron of the Cu atom is taken into account. The binding energy of the Cu atom in the clusters and the static dipole moment of the clusters are calculated. The results are invoked to interpret the experimentally observed activation of the Raman G band in the IR spectrum of a-C: H: Cu as a consequence of a lowering of the symmetry of the graphite-like clusters due to copper intercalation. Experimental data on the time dependence of the G band intensity during isothermal annealing of a-C: H: Cu are presented. The data suggest the possibility of reversible transfer of Cu atoms between the impurity states in the copper-carbon clusters and the impurity states in the purely copper clusters. The average activation energies of direct and reverse transfer are estimated from the experiment.