Core-electron relaxation energy in small metallic particles

A general calculation method for screening in a finite electron gas proposed by Cini, is applied to study of core-hole relaxation energy in small metallic particles. In order to obtain quantitative results, a pseudopotential theory is developed for the core-hole perturbation which provides excellent agreement with the generally accepted excited-atom model in the bulk limit. The transition and noble metals treated by means of a semi-empirical extension of the method. The present calculation method of extra-atomic relaxation energies, involving an electron gas approximation for the conduction electrons allows straightforward applications of the method of Cini in the case of a finite-spherical metal particle. The relaxation energy is found to give an important contribution to core-hole binding energy shifts in small particles.     

Vibrational interference effects in x-ray emission of a model water dimer: implications for the interpretation of the liquid spectrum

We apply the Kramers-Heisenberg formula to a model water dimer to discuss vibrational interference in the x-ray emission spectrum of the donor molecule for which the core-ionized potential energy surface is dissociative but bounded by the accepting molecule. A long core-hole lifetime leads to decay from Zundel-like, fully delocalized vibrational states in the intermediate potential without involvement of a specific dissociated component. Comparison is made to a model with an unbound intermediate state allowing dissociation to infinity which gives a sharp, fully dissociated feature, and a broad molecular peak at long core-hole life time. The implications of the vibrational interference effect on the liquid water spectrum are discussed and it is proposed that this mainly gives rise to an isotope-dependent asymmetrical broadening of the lone pair peak.     

A density functional theory study of shake-up satellites in photoemission of carbon fullerenes and nanotubes

Carbon 1s shake-up spectra of fullerenes C(60), C(70), and C(82) and single-walled carbon nanotubes (SWCNTs) of (5,5), (6,5), and (7,6) have been investigated by using equivalent core hole Kohn-Sham density functional theory approach, in which only one-electron transition between molecular orbitals within core-hole potential is considered. The calculated spectra are generally in good agreement with results of equivalent core-hole time-dependent density functional theory calculations and available experiments, and reliable assignments for the complicated shake-up spectra of such large systems are provided. Calculations have also been performed for endohedral metallofullerene Gd@C(82) to demonstrate the possible use of shake-up processes to identify the charge transfer between the metal ion and the carbon cage. It is found that the exciton binding energy of all systems under investigation is around 0.5 eV.     

Three‐State Single‐Molecule Naphthalenediimide Switch: Integration of a Pendant Redox Unit for Conductance Tuning

We studied charge transport through core‐substituted naphthalenediimide (NDI) single‐molecule junctions using the electrochemical STM‐based break‐junction technique in combination with DFT calculations. Conductance switching among three well‐defined states was demonstrated by electrochemically controlling the redox state of the pendent diimide unit of the molecule in an ionic liquid. The electrical conductances of the dianion and neutral states differ by more than one order of magnitude. The potential‐dependence of the charge‐transport characteristics of the NDI molecules was confirmed by DFT calculations, which account for electrochemical double‐layer effects on the conductance of the NDI junctions. This study suggests that integration of a pendant redox unit with strong coupling to a molecular backbone enables the tuning of charge transport through single‐molecule devices by controlling their redox states.     

DFT approach to calculate electronic transfer through a segment of DNA double helix

The electronic structures of an entire segment of a DNA molecule were calculated in its single‐strand and double‐helix cases using the DFT method with an overlapping dimer approximation and negative factor counting method. The hopping conductivity of the segment was calculated by the random walk theory from the results of energy levels and wave functions obtained. The results of the single‐strand case show that the DFT method is quantitatively in agreement with that of the HF MP2 method. The results for the double helix are in good agreement with that of the experimental data. Therefore, the long‐range electron transfer through the DNA molecule should be caused by hopping of electronic charge carriers among different energy levels whose corresponding wave functions are localized at different bases of the DNA molecule. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 1109–1117, 2000     

The effect of substitution on reorganization energy and charge mobility in metal free phthalocyanine

To gain insight into how the electronic properties of discotic organic materials may be modified through substitution, the reorganization energy and the charge mobility of metal free phthalocyanine, and of several mono-substituted derivatives, are studied by electronic structure methods. It is found that the reorganization energy of phthalocyanine is not significantly changed by substitution on an outer phenyl ring, but is more strongly influenced when the inner crown amine ring is substituted. The relationship between reorganization energy and substituent is studied through the use of; substituent constant, HOMO energy, and geometry relaxation. The computed charge mobility shows stronger relationship to coupling matrix element than reorganization energy. A hybrid computational screening method in which the reorganization energy is calculated at the DFT level and the coupling matrix element is calculated at the AM1 level shows good predicting power for trends in charge mobility at reduced computational expense.     

The correlation of vibrational broadening of core lines in x-ray photoelectron spectra with valence bond resonance structures

When core ionization of an atom in a molecule causes significant changes in bond orders, the core-hole ion is formed in a strained configuration. This strain causes vibrational broadening of the core line. The core-hole ion can be represented as an ordinary chemical species by applying the equivalent cores approximation. Then simple rules of classical valence bond theory can be used to predict changes in the weighting of resonance structures and corresponding changes in bond orders. Thus qualitative changes in relative linewidths can be predicted.     

Relativistic contributions to single and double core electron ionization energies of noble gases

We have performed relativistic calculations of single and double core 1s hole states of the noble gas atoms in order to explore the relativistic corrections and their additivity to the ionization potentials. Our study unravels the interplay of progression of relaxation, dominating in the single and double ionization potentials of the light elements, versus relativistic one-electron effects and quantum electrodynamic effects, which dominate toward the heavy end. The degree of direct relative additivity of the relativistic corrections for the single electron ionization potentials to the double electron ionization potentials is found to gradually improve toward the heavy elements. The Dirac-Coulomb Hamiltonian is found to predict a scaling ratio of ～4 for the relaxation induced relativistic energies between double and single ionization. Z-scaling of the computed quantities were obtained by fitting to power law. The effects of nuclear size and form were also investigated and found to be small. The results indicate that accurate predictions of double core hole ionization potentials can now be made for elements across the full periodic table.     

Theoretical study of the He-HF+ complex. I. The two asymptotically degenerate ground state potential energy surfaces

Two three-dimensional potential energy surfaces (PESs) are reported for the cationic complex He-HF+; they are degenerate for linear geometries of the complex and correlate with the doubly degenerate X2Pi ground state of the HF+monomer. The PESs are computed from the interaction energies of the neutral dimer and the ionization potentials of the He-HF complex and the HF molecule. Ionization potentials are obtained from the outer valence Green's function (OVGF) method, while the energies of the neutral species are computed by means of the single and double coupled-cluster method with perturbative triples [CCSD(T)]. For comparison, interaction energies of the ionic complex were computed also by the use of the partially spin-restricted variant of the CCSD(T) method. After asymptotic scaling of the OVGF results, good agreement is found between the two methods. A single global minimum is found in the PES, for the linear He-HF+ geometry. The well depth and equilibrium separation are 2.240 A and 1631.3 cm(-1), respectively, at an HF+ bond length r=1.0012 A, in rather good agreement with results of Schmelz and Rosmus [Chem. Phys. Lett. 220, 117 (1994)]. The well depth depends much more strongly on the internuclear H-F separation than in the neutral He-HF complex and the global minimum in a full three-dimensional PES occurs at r=1.0273 A.     

Ab initio adiabatic and quasidiabatic potential energy surfaces of lowest four electronic states of the H+ + O2 system

Ab initio global adiabatic and quasidiabatic potential energy surfaces of lowest four electronic (1-4 (3)A(")) states of the H(+)+O(2) system have been computed in the Jacobi coordinates (R,r,γ) using Dunning's cc-pVTZ basis set at the internally contracted multireference (single and double) configuration interaction level of accuracy, which are relevant to the dynamics studies of inelastic vibrational and charge transfer processes observed in the scattering experiments. The computed equilibrium geometry parameters of the bound [HO(2)](+) ion in the ground electronic state and other parameters for the transition state for the isomerization process, HOO(+)?OOH(+) are in good quantitative agreement with those available from the high level ab initio calculations, thus lending credence to the accuracy of the potential energy surfaces. The nonadiabatic couplings between the electronic states have been analyzed in both the adiabatic and quasidiabatic frameworks by computing the nonadiabatic coupling matrix elements and the coupling potentials, respectively. It is inferred that the dynamics of energy transfer processes in the scattering experiments carried out in the range of 9.5-23 eV would involve all the four electronic states.     

Formation of ultracold SrYb molecules in an optical lattice by photoassociation spectroscopy: theoretical prospects

State-of-the-art ab initio techniques have been applied to compute the potential energy curves for the SrYb molecule in the Born-Oppenheimer approximation for the electronic ground state and the first fifteen excited singlet and triplet states. All the excited state potential energy curves were computed using the equation of motion approach within the coupled-cluster singles and doubles framework and large basis-sets, while the ground state potential was computed using the coupled cluster method with single, double, and noniterative triple excitations. The leading long-range coefficients describing the dispersion interactions at large interatomic distances are also reported. The electric transition dipole moments have been obtained as the first residue of the polarization propagator computed with the linear response coupled-cluster method restricted to single and double excitations. Spin-orbit coupling matrix elements have been evaluated using the multireference configuration interaction method restricted to single and double excitations with a large active space. The electronic structure data were employed to investigate the possibility of forming deeply bound ultracold SrYb molecules in an optical lattice in a photoassociation experiment using continuous-wave lasers. Photoassociation near the intercombination line transition of atomic strontium into the vibrational levels of the strongly spin-orbit mixed b(3)Σ(+), a(3)Π, A(1)Π, and C(1)Π states with subsequent efficient stabilization into the v' = 1 vibrational level of the electronic ground state is proposed. Ground state SrYb molecules can be accumulated by making use of collisional decay from v' = 1 to v' = 0. Alternatively, photoassociation and stabilization to v' = 0 can proceed via stimulated Raman adiabatic passage provided that the trapping frequency of the optical lattice is large enough and phase coherence between the pulses can be maintained over at least tens of microseconds.     

Core localization and sigma* delocalization in the O 1s core-excited sulfur dioxide molecule

Electron-ion-ion coincidence measurements of sulfur dioxide at discrete resonances near the O 1s ionization edge are reported. The spectra are analyzed using a model based upon molecular symmetry and on the geometry of the molecule. We find clear evidence for molecular alignment that can be ascribed to symmetry properties of the ground and core-excited states. Configuration interaction (CI) calculations indicate geometry changes in accord with the measured spectra. For the SO(2) molecule, however, we find that the localized core hole does not produce measurable evidence for valence localization, since the transition dipole moment is not parallel to a breaking sigma* O-S bond, in contrast to the case of ozone. The dissociation behavior based upon the CI calculations using symmetry-broken orbitals while fixing a localized core-hole site is found to be nearly equivalent to that using symmetry-adapted orbitals. This implies that the core-localization effect is not strong enough to localize the sigma* valence orbital.     

A theoretical investigation of the ground and core-hole states of the secondary butyl cation

Non-empirical LCAO MO SCF computations have been carried out on the ground and core-hole states of various structures for the 2-butyl cation. The enhancement of weak interactions on going to the core-hole state manifold potentially provides a straightforward means of distinguishing between isomeric structures since the computed ESCA spectra are so distinctive.     

First-principles Periodic Density Functional Study of CO Adsorption on Spinel-type CuCr2O4 (100) Surface

The catalytic properties of CuCr2O4 with the cubic normal spinel-type structure were discussed by means of studying CO adsorption on the CuCr2O4 (100) surface in the framework of density functional theory. The results of geometry optimization show that CO prefers to adsorb at a Cu site with the adsorption energy of 133.2 kJ/mol. The adsorptions at all sites lead to a decrease in C-O stretching frequency, an increase in C-O bond length and a net positive Mulliken charge for the CO molecule. Population analysis indicates that the charges transfer from the CO molecule to substrate. The density of states for CO molecule before and after adsorption are also computed to discuss the bonding mechanism of CO.     

Quantum Monte Carlo study of ground and first excited state of C,N, O,F, and Ne atoms using Slater-Jastrow-Backflow wave function

All-electron fixed-node diffusion quantum Monte Carlo energies of the two lowest-lying states of C, N, O, F, and Ne atoms are reported. The Slater-Jastrow form is used as the trial wave function. We will use single- and multideterminant wave functions as the Slater part. The single-determinant wave function has been computed by the Hartree-Fock method and the multideterminant wave functions have been computed by the complete active space self-consistent field, configuration interaction with single and double excitation, configuration interaction with single, double, triple, and quadruple excitation and second-order configuration interaction. For the ground- and first excited states, the multideterminant wave functions have computed more than 99% of the correlation energy. Significant improvements have been achieved using the backflow transformations and up to 99.8% of the correlation energy has been recovered. A very good agreement with the experimental data has been obtained for the excitation energies.     

Microscopic description of elementary growth processes and classification of structural defects in pentacene thin films

Elementary growth processes such as kink initiation, adding a molecule to a kink, and adding a molecule between two neighboring kinks and between two grains are theoretically studied in pentacene films by adding one molecule at a time to a predefined aggregate. For each molecule, the potential energy surface is calculated using the MM3 molecular mechanics force field, which allowed one to identify useful parameters like the energy barrier for diffusion and the energy to create kinks, as well as defect configurations. Depending on the properties of the potential energy surface and the resulting growth-condition-dependent probabilities of initiating defect configurations in the film, three types of pentacene defects are identified: a thermally activated defect, an intrinsic defect, and a kinetic defect. Upon film growth, most defects relax into the ideal crystal configuration. Bulk defects that resist relaxation have densities lower than 10(16) defects/cm3 at typical growth conditions. Grain boundary defects, on the other hand, are very stable. Moreover, interstitial molecules at grain boundaries are identified as a source of compressive stress.     

Adsorption of a single molecule of a diarylethene photochromic dye on Cu(111)

On the route to single (large) molecule unimolecular chemistry, the adsorption of a photochromic dithienylethene dye on Cu(111) at a submonolayer level has been studied by Ultra High Vacuum-Scanning Tunneling Microscopy at Low Temperature. This technique has shown that the observed adsorbed molecule's shape is compatible with an helical conformation but has also revealed a surrounding electronic corrugation due to the perturbed surface states. Careful examination of the standing wave pattern indicated that only a part of the molecule is indeed interacting with the metallic substrate. Geometric considerations were used to infer that the bridging ethene moiety could be responsible for the electronic scattering. Scanning Tunneling Spectroscopy has shown a substantial amount of charge transfer from the surface to the adsorbate. The hypothesis that this precise double bond is a reactive locus toward charge transfer processes is confirmed by the electrochemical results: this double bond is indeed reduced upon coulometric reduction on glassy carbon. Furthermore, the use of a copper cathode strongly facilitates the reduction since a +0.6 V shift was recorded.     

Solvated ensemble averaging in the calculation of partial atomic charges

In the calculation of partial atomic charges, for use in molecular mechanics or dynamics simulations, it is common practice to select only a single conformation for the molecule of interest. For molecules that contain rotatable bonds, it is preferable to compute the charges from several relevant conformations. We present here results from a charge derivation protocol that determines the partial charges by averaging charges computed for conformations selected from explicitly solvated MD simulations, performed under periodic boundary conditions. This approach leads to partial charges that are weighted by a realistic population of conformations and that are suitable for condensed phase simulations. This protocol can, in principle, be applied to any class of molecule and to nonaqueous solvation. Carbohydrates contain numerous hydroxyl groups that exist in an ensemble of orientations in solution, and in this report we apply ensemble averaging to a series of methyl glycosides. We report the extent to which ensemble averaging leads to charge convergence among the various monosaccharides and among the constituent atoms within a given monosaccharide. Due to the large number of conformations (200) in our ensembles, we are able to compute statistically relevant standard deviations for the partial charges. An analysis of the standard deviations allows us to assess the extent to which equivalent atom types may, nevertheless, require unique partial charges. The configurations of the hydroxyl groups exert considerable influence on internal energies, and the limits of ensemble averaged charges are discussed in terms of these properties.