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We report the measurement of a jet-cooled electronic spectrum of the silicon trimer. Si3 was produced in a pulsed discharge of silane in argon, and the excitation spectrum examined in the 18 000-20 800 cm-1 region. A combination of resonant two-color two-photon ionization (R2C2PI) time-of-flight mass spectroscopy, laser-induced fluorescence/dispersed fluorescence, and equation-of-motion coupled-cluster calculations have been used to establish that the observed spectrum is dominated by the 13A1″ - ã 3A2′ transition of the D3h isomer. The spectrum has an origin transition at 18 600± 4 cm-1 and a short progression in the symmetric stretch with a frequency of ∼445 cm-1, in good agreement with a predicted vertical transition energy of 2.34 eV for excitation to the 13A1″ state, which has a calculated symmetric stretching frequency of 480 cm-1. In addition, a ∼505 cm-1 ground state vibrational frequency determined from sequence bands and dispersed fluorescence is in agreement with an earlier zero-electron kinetic energy study of the lowest D3h state and with theory. A weaker, overlapping band system with a ∼360 cm-1 progression, observed in the same mass channel (m/z = 84) by R2C2PI but under different discharge conditions, is thought to be due to transitions from the (more complicated) singlet C2v ground state (1A1) state of Si3. Evidence of emission to this latter state in the triplet dispersed fluorescence spectra suggests extensive mixing in the excited triplet and singlet manifolds. Prospects for further spectroscopic characterization of the singlet system and direct measurement of the energy separation between the lowest singlet and triplet states are discussed. [ABSTRACT FROM AUTHOR]

Gas phase electronic transitions for the C 2B2←X 2A1 and D 2B1←X 2A1 band systems of T-shaped AlC2 (C2v) radical have been measured in the 345–475 nm range. Vibrational analyses of both band systems are reported. Simulation of several rotationally resolved bands confirms previously obtained rotational parameters for the C 2B2 state. The radical is produced by ablating an aluminum rod in the presence of acetylene gas. The resulting supersonic molecular beam is probed using both mass-selective resonant two-color two-photon ionization and laser induced fluorescence. Ab initio calculations and vertical electronic excitation energies help the assignment. Vibrational frequencies for the X 2A1, C 2B2, and D 2B1 states have been determined. Rotational analysis of a number of bands yields spectroscopic constants for one vibronic state in the C 2B2 manifold and the origin band of the D 2B1←X 2A1 system. [ABSTRACT FROM AUTHOR]

The transient part of the ion conductivity enhancement in CaF2/BaF2 heterolayers on annealing at elevated temperatures is investigated. It is well established that annealed heterolayers exhibit a strongly increased F- conductivity parallel to the interfaces and a substantial but less strong enhancement perpendicular to the interfaces [N. Sata et al., Nature 408, 946 (2000) and X. X. Guo et al., Appl. Phys. Lett. 91, 103102 (2007)]. This is explained by a F- redistribution from BaF2 to CaF2 as a consequence of contact equilibrium. As to the behavior during annealing, two remarkable features are observed: (i) freshly prepared films show an even higher conductance enhancement if measured in the parallel direction, which decreases on annealing toward the equilibrium situation, while (ii) in the perpendicular direction the conductance variation is very small and of opposite sign. On the basis of the conductivity experiments as a function of temperature, layer-thickness, and anisotropy, in combination with structural investigations by transmission electron microscopy, we conclude that in agreement with earlier experience on composite materials, a high density of unstable defects is formed close to the heterointerfaces during multilayer preparation, which heals off during the temper process. In the final contact equilibrium a regular array of misfit dislocations is left, enabling the epitaxial contact. By assuming that the electroactive unstable microstructural defects are arranged at the heterointerfaces and are charged by fluoride ion trapping, we can explain the experimental features including the space charge overlap using the combined Mott–Schottky and Gouy–Chapman models used previously. The results indicate that the charge density stays approximately constant during the annealing process. [ABSTRACT FROM AUTHOR]

Results of first-principles simulations on both orthorhombic and monoclinic phases of CaFeO3 crystal are presented. The obtained atomic structures are consistent with x-ray diffraction data. The transition from a metallic orthorhombic to a narrow-gap semiconducting monoclinic phase is ascribed to the larger distortion of the Fe–O–Fe bond angle in the latter case. Calculations of Raman and optic active phonon modes at the Γ point of the Brillouin zone are performed and discussed. The isotopic substitution technique is applied to analyze the vibration modes obtained. The found charge/spin disproportionation is analyzed and compared with available experimental estimates. [ABSTRACT FROM AUTHOR]

The B 2Σu+←X 2Πg absorption spectrum of NCCN+ in the gas-phase was observed using a two-color, two-photon photodissociation technique. This was measured at ∼20 K in a 22-pole ion trap with laser bandwidths of less than a cm-1. The spectrum shows distinct vibrational structure, with the origin band near 11 253 cm-1, and the excitation of four normal modes in the excited state. The rotational structure of the 000 band in the gas phase could not be resolved, indicating that the B 2Σu+ state has a lifetime of a few picoseconds because of a fast intramolecular process. [ABSTRACT FROM AUTHOR]

Whipple’s solution of the problem of grain boundary diffusion and Le Claire’s relation, which is often used to determine grain boundary diffusion coefficients, are examined for a broad range of ratios of grain boundary to bulk diffusivities Δ and diffusion times t. Different reasons leading to errors in determining the grain boundary diffusivity (DGB) when using Le Claire’s relation are discussed. It is shown that nonlinearities of the diffusion profiles in ln Cav-y6/5 plots and deviations from “Le Claire’s constant” (-0.78) are the major error sources (Cav=averaged concentration, y=coordinate in diffusion direction). An improved relation (replacing Le Claire’s constant) is suggested for analyzing diffusion profiles particularly suited for small diffusion lengths (short times) as often required in diffusion experiments on nanocrystalline materials. [ABSTRACT FROM AUTHOR]

Three electronic absorption systems for C5 at 511, 445, and 232 nm and one for C6, C8, and C9 centered at 228, 259, and 288 nm have been observed in the gas phase. The C5 chain was produced in both discharge and ablation sources and detected using resonant two-color two-photon ionization spectroscopy involving 10.5 eV photons. The decay of the excited singlet electronic states indicates fast intramolecular processes on a subpicosecond time scale. The internal energy is assumed to be trapped in a triplet state for at least 15 μs. Hole-burning experiments on the 2 ² Σ u-← X ³ Σg- transition of C6, C8, and ¹Σu+←X ¹Σg+ of C9 confirm the predissociative nature of the excited electronic states. [ABSTRACT FROM AUTHOR]

Three electronic absorption systems for C5 at 511, 445, and 232 nm and one for C6, C8, and C9 centered at 228, 259, and 288 nm have been observed in the gas phase. The C5 chain was produced in both discharge and ablation sources and detected using resonant two-color two-photon ionization spectroscopy involving 10.5 eV photons. The decay of the excited singlet electronic states indicates fast intramolecular processes on a subpicosecond time scale. The internal energy is assumed to be trapped in a triplet state for at least 15 μs. Hole-burning experiments on the 2 ² Σ u-← X ³ Σg- transition of C6, C8, and ¹Σu+←X ¹Σg+ of C9 confirm the predissociative nature of the excited electronic states. [ABSTRACT FROM AUTHOR]

The electronic spectra of C18 and C22 in the 15 150–36 900 cm-1 range have been detected in the gas phase by a mass-selective resonant two-color two-photon ionization technique coupled to a laser ablation source. The spectra were assigned to several electronic systems of monocyclic cumulenic isomers with a D9h symmetry for C18 and D11h for C22, based on time-dependent-density-functional calculations and reactivity with respect to H2. The best cooling conditions were achieved with Kr as the buffer gas, and the origin of the Ã1A2″←X1A1′ transition of C18 at 592.89 nm shows a pair of 1 cm-1 broadbands spaced by 1.5 cm-1. The next electronic transitions exhibited much broader, ∼30 (in the visible) to 200 cm-1 (in ultraviolet range), features. The spectrum of C22 exhibits an absorption pattern similar to C18, except that the narrow features to the red are missing; the oscillator strength of the Ã←X transition is predicted to be low. [ABSTRACT FROM AUTHOR]

Recently, artificial ion conductors have been prepared by growing epitaxial heterolayers consisting of BaF[sub 2]–CaF[sub 2] using molecular beam epitaxy. The ionic conductivity of these heterolayers shows a strong dependence on the layer thickness [N. Sata, S. Eberman, K. Eberl, and J. Maier, Nature 408, 996 (2000)]. In this paper three such heterolayers with different spacings (sample A: 80 nm, sample B: 10 nm, sample C: 1 nm) are investigated by conventional transmission electron microscopy and high-resolution transmission electron microscopy. The spacings are chosen such that they fall into the three conductivity regimes observed in N. Sata et al. (l>50 nm; 8

The B [SUP3]Σ[SUP-,SUBg] ← X[SUP3]Σ[SUP-,SUBg] transitions of HC[SUB13]H and HC[SUB19]H have been measured in the gas phase, exhibiting broad, Lorentzian shaped bands. More extensive A [SUP3]Σ[SUP-,SUBu] ← X[SUP3]Σ[SUP-,SUBg] spectra have been observed for HC[SUB2n+1]H (n = 3 - 6) than before with many new vibronic bands identified. The spectra were obtained by means of a mass selective resonant two-color two-photon ionization technique coupled to a supersonic plasma source. The electronic structures of this series of molecules (n = 2 - 9) in both the ground and excited states have been investigated using DFT, MP2, and state-averaged CASSCF theories. The three lowest dipole allowed electronic transition systems are A [SUP3]Σ[SUP-,SUBu] ← X[SUP3]Σ[SUP-,SUBg], B[SUP3]Σ[SUP-,SUBu] ← X[SUP3]Σ[SUP-,SUBg], and C [SUP3]Γ[SUBu] ← X[SUP3]Σ[SUP-,SUBg], located, for the smaller members of the series, in the visible, UV and VUV range, respectively. The A [SUP3]Σ[SUP-,SUBu] ← X[SUP3]Σ[SUP-,SUBg] system is found to be of medium intensity and the B[SUP3]Σ[SUP-,SUBu] ← X[SUP3]Σ[SUP-,SUBg] transition is predicted to be very strong. This is a result of configuration mixing in the excited states. The oscillator strength of the lowest energy transition is not strongly dependent on the length of the chain, but that of the B[SUP3]Σ[SUP-,SUBu] ← X[SUP3]Σ[SUP-,SUBg] system increases monotonically with size. The C [SUP3]Γ[SUBu] state is Rydberg in character. The astrophysical implications are considered and an upper limit of the column densities of these carbon chains in diffuse clouds has been estimated as 10[SUP13] cm[SUP-2], based on calculated oscillator strengths. [ABSTRACT FROM AUTHOR]

The visible electronic spectra of the linear l-C[SUB2n+1] H (n=2-4) radicals have been measured in the gas phase. These have been obtained by means of a mass-selective resonant two-color two-photon ionization technique coupled to a supersonic plasma source. The observed spectra are assigned to the A [SUP2]Δ←X[SUP2]Π, B [SUP2]Σ[SUP-]←X[SUP2]Π, and C [SUP2]Σ[SUP+]←X [SUP2]Π electronic transitions arising from σ→π electron promotion. The assignments are based on ab initio calculations, wavelength dependence of the 0[SUP0,SUB0] transition on size, and isotopic substitution. The lifetime broadening of the bands and effects due to vibronic coupling are associated with the carbon skeleton bending modes. The detection of these carbon chains in the diffuse interstellar medium appears to be more favorable by radio astronomy rather than by electronic spectroscopy. [ABSTRACT FROM AUTHOR]

The electronic spectra of the C[sub 2n]H(D)[sup -], n = 2-4, anions have been observed in the gas phase using photodetachment spectroscopy. These are assigned to ¹Π ← X¹Σ[sup +] electronic transitions. The ¹Π excited states possess a dipole bound character indicated by the energetic proximity between the origin of the transitions and electron affinities. The dipole bound states are related to the X²Π ground states of C[sub 6]H and C[sub 8]H whereas for C[sub 4]H, which has an X²Σ[sup +] ground state, the ²Π is an excited state. Vibronic coupling through a bending motion of the carbon skeleton is inferred to be the reason of the stabilization of this state. [ABSTRACT FROM AUTHOR]

The rotationally resolved A²II[sub u]+←Χ²II[sub g] electronic origin band spectrum of dicyanodiacetylene cation, NC[sub 6]N[sup +], has been recorded in the gas phase using frequency-production double modulation spectroscopy in a liquid nitrogen cooled hollow cathode discharge and cavity ring down spectroscopy in a supersonic plasma. The analysis of the complementary results provides accurate molecular parameters for the two spin-orbit components in both electronic states. [ABSTRACT FROM AUTHOR]