C. B. Duke

Localized molecular excitons in polyaniline

Abstract An analysis of the UV absorption spectra of polyemeraldine and nigrosine reveals that the absorption band which appears at about 2.2 eV in both compounds is due to the creation of a localized molecular exciton with the electron on a quinoid moiety and the hole on the neighboring two benzoid moieties. The quinoid moiety is rotated approximately 90° relative to its ground-state conformation. Higher-energy absorption bands may be associated with molecular excitons localized on individual phenyl moieties.

Dependence of oxide surface structure on surface topology and local chemical bonding

The atomic geometries of the charge neutral surfaces of several oxides exhibiting different crystal structures and varying participation of O(2p) electrons in the chemical bonding have been calculated using tight‐binding total energy models. Surface structures have been computed for exemplary cubic (MgO), wurtzite (ZnO), β‐tridymite, and ideal β‐cristobalite (SiO2) oxides. The cubic oxide exhibits a minimum energy structure involving small outward relaxations of the oxygens and inward relaxations of the cations. For the cleavage faces of wurtzite ZnO, large bond‐length‐conserving relaxations occur because the surface atoms can relax without appreciable distortion of the local bond lengths. The charge neutral faces of β‐tridymite and ideal β‐cristobalite SiO2 also undergo bond‐length‐conserving relaxations. Thus the mechanism for the surface relaxation of tetrahedrally coordinated oxides is significantly different from that of the cubic oxides as is the role of the oxygen p electrons in the surface chemica...

Electronic structure and twisted intramolecular charge transfer in dimethylanilines

The spectroscopically parameterized CNDO/S3 model is used to provide a quantitative description of the valence electron photoemission, ultraviolet absorption, and twisted intramolecular charge transfer fluorescence in N,N‐dimethylaniline, p‐cyano‐N,N‐dimethylaniline, and 3‐methyl‐4‐cyano‐N,N‐dimethylaniline. Additionally, the CNDO/S3 density of valence states was used to determine a twist angle of 30° for both N,N‐dimethylaniline and p‐cyano‐N,N‐dimethylaniline via the analysis of gas phase photoelectron spectra.

Electronic structure of emeraldine and related molecules

The CNDO/S3 molecular orbital model is applied to describe the electronic structure of aniline, (C6H5)NH2, emeraldine, (C6H5)N(C6H4)N(C6H4)NH(C6H4)NH2, its hydrogenated and dehydrogenated forms, and its singly and doubly protonated forms. The cation eigenvalue spectrum, i.e., density of valence states (DOVS), is found to be sensitive to molecular conformation which, in turn, can be inferred from comparison of the calculated DOVS with suitable measured photoemission spectra. For fixed molecular conformation, the major effect of hydrogenation of emeraldine is the occupation of its lowest empty molecular orbital with accompanying electronic and atomic relaxation. Dehydrogenation leads to the emptying of its highest filled molecular orbital. Protonation leads to rearrangements of the valence electron charge among the filled orbitals but not to deviations from a closed shell system. Protonation‐induced atomic relaxation can create major changes in the character of the highest filled or lowest empty orbitals. C...

Surface structure determination of the cleavage faces of CdSe via low‐energy positron diffraction

Low‐energy positron diffraction intensities were measured for 10 beams from CdSe(1010) and 14 beams from CdSe(1120). These were compared with calculated intensities for bond‐length‐conserving relaxed surface structures for both surfaces. Using an R‐factor methodology the best‐fit structures were obtained for these surfaces corresponding to local planar tilt angles of ω=15°±5° for CdSe(1010) and ω=27±7° for CdSe(1120). Both results are in excellent correspondence with structures predicted by tight‐binding total‐energy minimization calculations.

Electronic structure and photoelectron spectra of substituted anilines

The spectroscopically parametrized CNDO/S3 model is used to provide a quantitative description of the valence electron photoemission spectra of a series of substituted anilines. The CNDO/S3 density of valence states was used to determine the twist angles in these molecules via analysis of gas‐phase photoemission spectra. These twist angles are compared with prior estimates. Finally, a discussion of the use of photoelectron spectroscopy in the determination of molecular geometries is presented.

Low‐energy positron diffraction from GaAs(110)

Intensities of 16 beams of near normal incidence positrons have been measured at T=120 K and analyzed using a multiple scattering model of the low‐energy positron diffraction (LEPD) process. Excellent correspondence between the measured and calculated intensities is obtained for a reconstruction that is primarily a bond‐length‐conserving rotation of the top layer, with As relaxed outward and Ga inward with a tilt angle ω1 = 28.6 ± 3°, confirming the results of previous structure analyses for this surface. The quality of the description of the measured intensities, as measured by the x‐ray R factor, is significantly better for LEPD than for low‐energy electron diffraction. This result is attributed to the repulsive character of the positron‐ion core potential and a resulting more surface sensitive diffraction process for LEPD.

Surface structure, bonding, and dynamics: Universality of zinc blende (110) potential energy surfaces

Using a tight‐binding, total energy (TBTE) model we examine the hypothesis that the potential energy surfaces (PES) describing the (110) cleavage faces of the tetrahedrally coordinated zinc blende structure compound semiconductors exhibit a common ‘‘universal’’ form if expressed in terms of suitably scaled parameters. TBTE calculations on both III–V and II–VI compounds reveal a linear scaling with bulk lattice constant of the geometric parameters of the reconstructed surfaces. This scaling is analogous to that found using low‐energy, electron‐diffraction surface‐structure determination. The surface atomic force constants (found from a TBTE calculation) also scale monotonically with the lattice constant. Using TBTE models proposed previously for GaP, GaAs, GaSb, InP, InSb, and ZnSe, we find that the force constants scale as the inverse square of the bulk lattice constant. These results suggest that if distances are measured in units of the bulk lattice constant, the PES may be a universal function for the ...

Surface atomic structure and bonding of GaAs(110)‐p(1×1)–Bi (1 ML)

The surface atomic and electronic structure of GaAs(110)‐p(1×1)–Bi (1 ML) has been examined using a tight‐binding total‐energy (TBTE) model. Two structural candidates were considered; the epitaxial continued‐layer structure (ECLS) and the epitaxial on‐top structure (EOTS). The TBTE computations indicate that the ECLS is more stable than the EOTS by ∼0.06 eV/surface atom/unit cell in agreement with a recent low‐energy electron diffraction (LEED) intensity analysis which selected the ECLS as the best‐fit structure. The predicted TBTE structural parameters for the ECLS are in good agreement with the best‐fit LEED results. The energy difference between the ECLS and EOTS is much smaller for the Bi/GaAs interface than for the Sb/GaAs interface (∼0.11 eV/surface atom/unit cell) in agreement with the proposition that the relative stability of the ECLS and EOTS is a function of the overlayer‐substrate mismatch; with the EOTS being favored for larger overlayer‐substrate mismatch. The primary difference between the ...

Molecular conformations from ultraviolet photoelectron spectra: Substituted anilines

Abstract The determination of the twist angles for the radical cations of thesaturated ring N-substituted aminobenzonitriles p -cyanojulolidine, piperidine benzonitrile, and pyrrolidine benzonitrile via analysis of valence-level photoemission spectra is re-examined. Three different procedures for determining the twist angles are compared and shown to give twist angles to within Δφ≈°. Reanalysis of the photoemission spectrum of p -cyanojulolidine confirms the initial assignment of φ = 10° for the twist angle. This result is supported by a new analysis of its ultraviolet absorption spectrum.