Electronic and structural properties of Si10 cluster

Possible structures for Si₁₀ cluster are considered using a tight-binding model and drawing on significant work done in the past. It is shown that the tight-binding parametrization, fitted to the bulk, is also valid for smaller systems. This model is found to essentially reproduce other published results, but requires much less effort thanab initio techniques—thus, allowing the study of a wide variety of structures and their ions. However, unlike classical force-field calculations, it yields information about the electronic structure of clusters. A new geometric structure for Si₁₀ is found, which is not only of lowest energy, but which also matches the experimental photoelectron band gap and explains the experimental reactivity data. Because of the Jahn-Teller effect, the photoelectron spectrum is very sensitive to geometry. Also, ionization of the cluster alters the geometry slightly.     

Standardization of silver-silver chloride electrodes from 0 to 60°C

Reference values of the activity coefficient of hydrochloric acid at intervals of 5°C from 0 to 60°C are proposed. With their use, interlaboratory differences among silver—silver chloride electrodes can be eliminated.     

Electronic spectra and photodissociation of vinyl chloride: a symmetry-adapted cluster configuration interaction study

The vertical absorption spectrum and photodissociation mechanism of vinyl chloride (VC) were studied by using symmetry-adapted cluster configuration interaction theory. The important vertical pi --> pi* excitation was intensively examined with various basis sets up to aug-cc-pVTZ augmented with appropriate Rydberg functions. The excitation energy for pi --> pi* transition obtained in the present study, 6.96 eV, agrees well with the experimental value, 6.7-6.9 eV. Calculated excitation energies along with the oscillator strengths clarify that the main excitation in VC is the pi --> pi* excitation. Contrary to the earlier theoretical reports, the results obtained here support that the C-Cl bond dissociation takes place through the n(Cl-)sigma(C-Cl)* state.     

Adsorption of tetraphenylphosphonium chloride at the mercury-water interface

Interfacial tersion measurements of mercury in contact with aqueous solutions of tetraphenylphosphonium chloride indicate the existence, at negative rational potentials, of an ordered monolayer of tetraphenylphosphonium ions, either as such or as its neutral salt. The analysis clearly illustrates the limits to the molecular information obtainable from electrical measurements.     

Electronic states at the silicon-silicon dioxide interface

Localized electronic states at the silicon/silicon dioxide interface are reviewed, and the current knowledge of interface states and fixed oxide charges, and how they influence the characteristics of the MOS structure, is summarized. Experimental results on the properties of these two classes of localized states are presented and discussed. General trends of properties over a very extensive amount of data are noted and an overall picture of these states is given. Various phenomenological models for these electronic states are reviewed and critically discussed. It is pointed out that the current understanding of localized states is, in general, far from satisfactory, and a complete picture is lacking. Theoretical efforts on computing the energy levels of these electronic states are summarized. Most of these approaches are based on highly simplified structures, such as a simple unit cell with an atom missing. While they have been moderately successful, more realistic approaches are still needed. Finally, some theoretical techniques are introduced, in the hope that they may have applications to these problems, and the need for more measurements of non-electrical properties of these states is emphasized.     

The standard electrode potential of the silver-silver chloride electrode in 5 wt. % tetramethylurea-water mixture at 30°C

Electromotive force measurements were made at 30°C with the cell $$H_2 (g)|Pt(s)|HCl(m){\text{ }}TMU(x){\text{ }}H_2 O(100 - x)|AgCl(s)|Ag(s)$$ wherex=5 wt. % tetramethylurea (TMU). The standard electrode potential of the silver-silver chloride electrode, the mean molal activity coefficient of hydrogen chloride, the primary medium effect, and the free energy of transfer of hydrogen chloride from the aqueous standard state to the standard state in the mixed solvent were derived from the measurements. The acquisition of data was limited to this single composition and temperature because of the difficulty of preparing hydrogen electrodes for this solvent medium. The results obtained for hydrogen chloride in 5 wt. % tetramethylurea-water mixture are discussed, relative to other organic-aqueous mixtures of the same composition, in terms of structural effects and hydrogen bonding.     

Electronic spectra of metal cluster molecules

The electronic absorption spectra (UV-visible-NIR) of a range of molecular metal cluster compounds, including new spectra of Pt₃₀₉(phen*)₃₆O₂₈ in solution and Au₅₅(PPh₃)₁₂Cl₆ in the solid state, are discussed and compared with the spectra of colloidal particles of the corresponding metals. We consider frontier orbital separations, the development of interband absorptions, the possible appearance of molecular plasma resonances, and charge-transfer in the solid state.     

Electronic structure calculations on the C4 cluster

The ground and the electronically excited states of the C4 radical are studied using interaction configuration methods and large basis sets. Apart from the known isomers [l-C4(X(3)Sigmag (-)) and r-C4(X(1)Ag)], it is found that the ground singlet surface has two other stationary points: s-C4(X(1)Ag) and d-C4(X(1)A1). The d-C4 form is the third isomer of this cluster. The isomerization pathways from one form to the other show that deep potential wells are separating each minimum. Multireference configuration interaction studies of the electronic excited states reveal a high density of electronic states of these species in the 0-2 eV energy ranges. The high rovibrational levels of l-C4((3)Sigmau (-)) undergo predissociation processes via spin-orbit interactions with the neighboring (5)Sigmag + state.     

Electronic and geometric properties of ETS-10: QM/MM studies of cluster models

Hybrid DFT/MM methods have been used to investigate the electronic and geometric properties of the microporous titanosilicate ETS-10. A comparison of finite length and periodic models demonstrates that band gap energies for ETS-10 can be well represented with relatively small cluster models. Optimization of finite clusters leads to different local geometries for bulk and end sites, where the local bulk TiO6 geometry is in good agreement with recent experimental results. Geometry optimizations reveal that any asymmetry within the axial O-Ti-O chain is negligible. The band gap in the optimized model corresponds to a O(2p) --> Tibulk(3d) transition. The results suggest that the three Ti atom, single chain, symmetric, finite cluster is an effective model for the geometric and electronic properties of bulk and end TiO6 groups in ETS-10.     

Electronic relaxation dynamics of water cluster anions

The electronic relaxation dynamics of water cluster anions, (H(2)O)(n)(-), have been studied with time-resolved photoelectron imaging. In this investigation, the excess electron was excited through the p<--s transition with an ultrafast laser pulse, with subsequent electronic evolution monitored by photodetachment. All excited-state lifetimes exhibit a significant isotope effect (tau(D)2(O)/tau(H)2(O) approximately 2). Additionally, marked dynamical differences are found for two classes of water cluster anions, isomers I and II, previously assigned as clusters with internally solvated and surface-bound electrons, respectively. Isomer I clusters with n > or = 25 decay exclusively by internal conversion, with relaxation times that extrapolate linearly with 1/n toward an internal conversion lifetime of 50 fs in bulk water. Smaller isomer I clusters (13 < or = n < or = 25) decay through a combination of excited-state autodetachment and internal conversion. The relaxation of isomer II clusters shows no significant size dependence over the range of n = 60-100, with autodetachment an important decay channel following excitation of these clusters. Photoelectron angular distributions (PADs) were measured for isomer I and isomer II clusters. The large differences in dynamical trends, relaxation mechanisms, and PADs between large isomer I and isomer II clusters are consistent with their assignment to very different electron binding motifs.     

Electronic states at the water/air interface

Using combined path integral-molecular dynamics simulation techniques, we analyze electronic solvation at the water/air interface. Superficial electrons present a considerable extent of spatial confinement, somewhat less marked but still comparable to that found in bulk. The characteristics of the interfacial polarization promote an overall structure for the solvated electron-polymer which looks flatter along the direction perpendicular to the interface. Spatial and orientational responses of different slabs in the close vicinity of the interface were also investigated. Solvent configurations obtained from the simulations have been used to analyze electronic excited states and the optical absorption spectrum of superficial electrons. Compared to bulk results, the distribution of bound electronic states at the surface presents similar characteristics, that is, a ground s-state and three, quasi-degenerate, p-like excited states. The reduction of the energy gap between the ground state and the rest of excited states leads to a approximately 0.52 eV red-shift in the position of the absorption maximum.     

Electric properties of the chloride ion

The dipole (), quadrupole (C), and dipole-quadrupole (B) polarizabilities and the dipole hyperpolarizability () of the chloride ion have been calculated by using the many-body perturbation theory approach and a series of large polarized GTO/CGTO basis sets. The complete fourth-order treatment of the electron correlation effects with a basis set comprising the s, p, d, f, and g functions gives: =38.01 a.u., C=211.5 a.u., B=–5.14×10³ a.u., and =128. 5×10³ a.u. as compared to the corresponding SCF values (=31.49 a.u., C=158.9 a.u., B=–2.92×10³ a.u., =57.7×10³ a.u.). The quenching of polarizabilities of the Cl^– ion in solutions and ionic crystals is discussed.     

Adsorption of dioctyldimethylammonium chloride at water/hexane interface

The adsorption behavior of dioctyldimethylammonium chloride at water/ hexane interface has been studied by measuring the interfacial tension as a function of temperature and pressure at various bulk concentrations. By applying the thermodynamics of adsorption at interfaces to the experimental results, the thermodynamic quantity changes associated with adsorption and the interfacial density of dioctyldimethylammonium chloride have been evaluated.The interfacial tension vs temperature and concentration curves have shown the breaks and it has been concluded that the first order phase transition takes place between a gaseous and an expanded state. The entropy and volume changes associated with adsorption have shown the remarkable dependence on temperature and pressure and have been found to decrease with increasing the molality. Also the energy change associated with adsorption has been evaluated and it has been concluded that the adsorption of dioctyldimethylammonium chloride at water/hexane interface is enhanced by negative values of the partial molar energy change. Further, all the thermodynamic quantities have been characterized by the discontinuous change attributable to the phase transition.     

Electronic structure of the truncated-icosahedral C60 cluster

The electronic structure of the truncated-icosahedral C₆₀ cluster (“footballene”) is theoretically examined by performing a linear muffin-tin orbitais (LMTO) calculation.     

Electronic structure of NaCl crystal in crystal cluster model

Conclusions By the use of the crystal cluster method, it becomes possible (1) to obtain good agreement with band calculations of charge distribution, practically beginning with minimal clusters and, (2) to reproduce well the energy position of the VB; however (3), in order to analyze the detailed structure of density of states of the VB for the NaCl crystal, it is necessary to account for five or six spheres of the environment in order to provide convergence of this most sensitive characteristic. In this case, (4) the calculated model density of states of the atom-representatives of the bases converges with that obtained in the band calculation. In calculations of states of the CB, replacement of the operator of projection on the space of Wannier functions by an external localizing potential leads to a considerable distortion of the spectrum. The states obtained in this region (5) are determined by this external potential; and (6) they should be interpreted as lower states of the CB. (7) The use of a correction for the localizing potential improves the agreement with respect to the width of the optical gap in the series of calculations.Ural Polytechnic Institute. Translated from Zhurnal Strukturnoi Khimii, Vol. 31, No. 2, pp. 17–23, March–April, 1990.     

Electronic properties of the interface between p-CuI and anatase-phase n-TiO2 single crystal and nanoparticulate surfaces: a photoemission study

We present a study of the growth of the p-type inorganic semiconductor CuI on n-type TiO2 anatase single crystal (101) surfaces and on nanoparticulate anatase surfaces using synchrotron radiation photoemission spectroscopy. Core level photoemission data obtained using synchrotron radiation reveal that both the substrate (TiO2) and the overlayer (CuI) core levels shift to a lower binding energy to different degrees following the growth of CuI on TiO2. Valence band photoemission data show that the valence band maximum of the clean substrate differs from that of the dosed surface which may be interpreted qualitatively as due to the introduction of a new density of states within the band gap of TiO2 as a result of the growth of CuI. The valence band offset for the heterojunction n-TiO2p-CuI has been measured using photoemission for both nanoparticulate and single crystal TiO2 surfaces, and the band energy alignment for these heterojunction interfaces is presented. With the information obtained here, it is suggested that the interface between p-CuI and single crystal anatase-phase n-TiO2 is a type-II heterojunction interface, with significant band bending. The measured total band bending matches the work function change at the interface, i.e., there is no interface dipole. In the case of the nanoparticulate interface, an interface dipole is found, but band bending within the anatase nanoparticles remains quite significant. We show that the corresponding depletion layer may be accommodated within the dimension of the nanoparticles. The results are discussed in the context of the functional properties of dye-sensitized solid state solar cells.