Chemical Physics Letters (v.344, #3-4)
Mechanisms and energetics of SiH3 adsorption on the pristine Si(0 0 1)-(2×1) surface by Stephen P. Walch; Shyam Ramalingam; Saravanapriyan Sriraman; Eray S. Aydil; Dimitrios Maroudas (249-255).
A theoretical study is presented of the adsorption mechanisms and energetics of the silyl (SiH3) radical on the pristine Si(0 0 1)-(2×1) surface based on density functional theory and molecular-dynamics (MD) simulations. Adsorption mechanisms include: (i) SiH3 attachment to a surface dangling bond, (ii) dissociative adsorption that involves insertion between dimer atoms and breaking of the dimer bond, (iii) bonding to two surface dimer atoms of neighboring pairs in the same dimer row, and (iv) bridging of neighboring dimer rows. The dissociative adsorption where the Si–Si surface dimer bond is broken is the most exothermic mechanism.
Synthesis of carbon nanotubes from in situ generated cobalt nanoparticles and carbon monoxide by Rohit Kumar Rana; Yuri Koltypin; Aharon Gedanken (256-262).
Multi-walled carbon nanotubes were synthesized via a catalytic process of CO disproportionation over the in situ formed cobalt nanoparticles on an MgO support. For this purpose Co(CO)3NO was used as the source of both cobalt and carbon monoxide. Well-dispersed cobalt nanoparticles supported on MgO gave rise to a narrow size distribution for the generated nanotubes. The carbonaceous materials were separated from the product by a simple acid-treatment method and their structural and morphological characterizations were performed by XRD and TEM. Thermogravimetric analysis of the HCl-treated sample revealed a weight content of about 68% nanotubes and nanoparticles.
Positron lifetime in supramolecular gamma- and delta-cyclodextrin–C60 and –C70 compounds by K. Süvegh; K. Fujiwara; K. Komatsu; T. Marek; T. Ueda; A. Vértes; T. Braun (263-269).
Temperature dependence of positron lifetime parameters have been studied in pure cyclodextrins and their supramolecular complexes with C60 and C70 molecules. It was found that while C60 molecules are localized in molecular cavities in gamma-cyclodextrin, they are too small to be trapped by the larger cavities of delta-cyclodextrin. On the other hand, the larger C70 molecules fill these larger voids completely. In cases where fullerene molecules are localized in molecular cavities, a low-temperature phase transition was observed. The activation energies for the phase transitions were calculated assuming a simple two-state annihilation model.
Impact induced chemisorption of C20 isomers on diamond (0 0 1)–(2×1) surface by A.J Du; Z.Y Pan; Y.K Ho; Z Huang; Z.X Zhang; Y.X Wang (270-276).
The adsorption of low-energy C20 isomers on diamond (0 0 1)–(2×1) surface was investigated by molecular dynamics simulation using the Brenner potential. The energy dependence of chemisorption characteristic was studied. We found that there existed an energy threshold for chemisorption of C20 to occur. Between 10 and 20 eV, the C20 fullerene has high probability of chemisorption and the adsorbed cage retains its original structure, which supports the experimental observations of memory effects. However, the structures of the adsorbed bowl and ring C20 were different from their original ones. In this case, the local order in cluster-assembled films would be different from the free clusters.
Novel fullerene–silver nanocomposite with large optical limiting effect by Na Sun; Yuxiao Wang; Yinglin Song; Zhixin Guo; Liming Dai; Daoben Zhu (277-282).
A novel fullerene–Ag nanocomposite (DTC60–Ag) is prepared by the in situ reduction of silver ions encapsulated in a new mono-functionalized methano-fullerene derivative (DTC60) with reverse micelle-like structure. The formation and size of the nanocomposite has been determined from the UV–Vis plasmon absorption band, FT-IR spectra method and transmission electron microscopic (TEM) analysis. The average size distribution is about 5.2 mm. The optical limiting properties of DTC60 and DTC60–Ag are measured via a nanosecond Nd:YAG pulse laser system at a wavelength of 532 nm. The experimental results demonstrate that the optical limiting behavior of DTC60–Ag is better than that of both C60 and DTC60.
Field emission from carbon nanotubes grown on a tungsten tip by R.B. Sharma; V.N. Tondare; D.S. Joag; A. Govindaraj; C.N.R. Rao (283-286).
Field emission microscopy studies of carbon nanotubes grown by the pyrolysis of ferrocene on a tungsten field emitter are reported. A field emission current density of 1.5 A cm −2 has been drawn from such an emitter at a field of 290 V/μm , and this density is considerably higher than that found with planar cathodes. Accordingly, the field enhancement factor, calculated from the slope of the F–N plot in the low field region is also large. Field emission micrographs reveal the lobe structure symmetries typical of carbon nanotube bundles. The emission current is found to be remarkably stable over an operating period of more than 3 h for current values of up to 500 μA .
Iron–hydrocarbon cluster Fe13(C2H2)6 by L.C. Cune; M. Apostol (287-291).
The stability of the iron–hydrocarbon cluster Fe13(C2H2)6 is studied within a metallic-bond-type theory. The theoretical model for the cluster structure indicates a centered Fe 13 -icosahedron with C2H2-radicals arranged symmetrically on the icosahedron sides.
ArF laser photodissociation dynamics of furfuryl alcohol: LIF observation of OH state distribution by Pradyot K Chowdhury; Hari P Upadhyaya; Prakash D Naik (292-298).
The dynamics of furfuryl alcohol (FURFUROL) photodissociation at 193 nm is reported, using laser-induced fluorescence (LIF) of the nascent OH radical and RRKM theory. The nascent OH fragments are probed by LIF under collisionless conditions, to determine the initial product state distributions. There is no significant population (<2%) in excited vibrational levels of OH (X2 Π). However, the initial rotational state distribution is Boltzmann-like, characterized by a single rotational temperature T rot of 780±40 K . The average relative translational energy of the photofragments is determined to be 26±4 kJ mol −1 . The measured rate constant for the FURFUROL dissociation vis-a-vis statistical RRKM theory, suggests a threshold dissociation energy of 357±20 kJ mol −1 .
In situ visible Raman spectroscopic study of phase change in LiCoO2 film by laser irradiation by Seung-Wan Song; Kyoo-Seung Han; Hirofumi Fujita; Masahiro Yoshimura (299-304).
A phase change in LiCoO2/Co films in a spot of 1 μm diameter has been investigated under ambient conditions using 514.5 nm radiation by laser Raman spectroscopy. Laser irradiation resulted in a remarkable phase change of the film from hexagonal to cubic spinel, forming a lithium deficient phase Li1−x CoO2, which was enhanced with laser power. Based on comparative Raman studies, incorporation of cobalt atoms from the Co substrate by the local heating effect of laser irradiation has been suggested for the phase change. The in situ tool is expected to be useful for applications in control of micro-structure of cathode films for micro-batteries and micro-structural analysis.
Magnetic exchange coupling in tetranuclear copper clusters of the type [Cu4(μ 4O)Cl6−n Br n L4] by Roman Boča; L'ubor Dlháň; Darina Makáňová; Jerzi Mrozinski; Gregor Ondrejovič; Miroslav Tatarko (305-309).
The magnetic susceptibility of [Cu4OCl6−n Br n L4] type tetranuclear complexes (L=morpholine) has been recorded and analysed using models that account for a decreased symmetry. Although the structural distortions are rather low, a symmetry descent to S4 is ultimate for a successful fitting of the magnetic data. On a systematic substitution of Cl for Br atoms a decrease of the `intradimer' coupling constants is registered.
Low temperature rate coefficients for the reactions of CN and C2H radicals with allene (CH2CCH2) and methyl acetylene (CH3CCH) by David Carty; Valery Le Page; Ian R. Sims; Ian W.M. Smith (310-316).
Using a continuous flow CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme or Reaction Kinetics in Uniform Supersonic Flow) apparatus, rate coefficients have been measured for the reactions of the cyanogen (CN) and ethynyl (C2H) radicals with allene (CH2CCH2) and methyl acetylene (CH3CCH) at temperatures from 295 down to 15 K for the reactions of CN and down to 63 K for those of C2H. All four reactions occur at rates close to the collision-determined limit. The results are compared with those obtained earlier for the reactions of other alkenes and alkynes, and, in the accompanying Letter by Vakhtin et al., with results for C2H+CH2CCH2 and C2H+CH3CCH obtained at 103 K using a pulsed Laval apparatus. The implications of these latest results for the chemistry of interstellar clouds and planetary atmospheres are discussed.
Kinetics of reactions of C2H radical with acetylene, O2, methylacetylene, and allene in a pulsed Laval nozzle apparatus at T=103 K by Andrei B. Vakhtin; Dwayne E. Heard; Ian W.M. Smith; Stephen R. Leone (317-324).
The rate constants of the reactions of C2H radical with HCCH (k1), O 2 (k2), CH 3 CCH (k3), and H 2 CCCH 2 (k4) are measured at 103 K using a pulsed Laval nozzle supersonic expansion and the CH(A2 Δ) chemiluminescence monitor method: k 1=(1.3±0.3)×10−10, k 2=(3.8±0.9)×10−11, k 3=(2.7±0.6)×10−10, and k4=(2.5±0.6)×10−10 cm 3 molecule −1 s −1 . These values are compared to previous results and measurements in a continuous Laval nozzle apparatus (accompanying paper by Carty et al.). The non-Arrhenius behavior and possible implications of the results for atmospheric chemistry of Titan are discussed.
Comparison of FAM mixing to single-pulse mixing in 17O 3Q- and 5Q-MAS NMR of oxygen sites in zeolites by Peidong Zhao; Philip S Neuhoff; Jonathan F Stebbins (325-332).
Fast amplitude modulation (FAM) shifted-echo pulse sequences for 3Q- and 5Q-MAS NMR experiments were compared for resolution of 17O on chemically and/or crystallographically distinct framework oxygen sites (i.e., Si–O–Si, Si–O–Al) in zeolites 13X, 4A, and a natural analcime (Na1.01Al1.01Si1.99O6·H2O). Significantly better sensitivity is afforded by fast amplitude modulation sequences. Resolution among different sites is also better in 5Q-MAS than in 3Q-MAS spectra, although sites with large quadrupolar coupling constants are more efficiently excited and reconverted in the latter experiments. No Al–O–Al linkages were detected in these zeolites.
Coherent learning control of vibrational motion in room temperature molecular gases by T.C. Weinacht; R. Bartels; S. Backus; P.H. Bucksbaum; B. Pearson; J.M. Geremia; H. Rabitz; H.C. Kapteyn; M.M. Murnane (333-338).
An evolutionary learning algorithm in conjunction with an ultrafast optical pulse shaper was used to control vibrational motion in molecular gases at room temperature and high pressures. We demonstrate mode suppression and enhancement in sulfur hexafluoride and mode selective excitation in carbon dioxide. Analysis of optimized pulses discovered by the algorithm has allowed for an understanding of the control mechanism.
A new type of water splitting system composed of two different TiO2 photocatalysts (anatase, rutile) and a IO3 −/I− shuttle redox mediator by Ryu Abe; Kazuhiro Sayama; Kazunari Domen; Hironori Arakawa (339-344).
A new photocatalytic reaction that splits water into H2 and O2 was designed by a two-step photoexcitation system composed of a IO3 −/I− shuttle redox mediator and two different TiO2 photocatalysts, Pt-loaded TiO2-anatase for H2 evolution and TiO2-rutile for O2 evolution. Simultaneous gas evolution of H 2(180 μ mol/h) and O 2(90 μ mol/h) was observed from a basic (pH=11) NaI aqueous suspension of two different TiO2 photocatalysts under UV irradiation (λ>300 nm, 400 W high-pressure Hg lamp).
Continuous changes of the Jahn–Teller deformation of Cu(H2O)6 complex in ferroelastic Cs2Cu(ZrF6)2·6H2O crystal by M.A. Augustyniak-Jabłokow; Yu.V. Yablokov; K. Łukaszewicz; A. Pietraszko; V.E. Petrashen; V.A. Ulanov (345-351).
Q-band EPR spectra of Cs2Cu(ZrF6)2·6H2O measured in temperature range 4.2–350 K and supported by the X-ray crystal structure analysis, show a gradual transformation of an adiabatic potential surface of the Jahn–Teller (JT) Cu(H2O)6 2+ complex leading to the switch of its elongation direction. This effect is joined with a ferroelastic phase transition at 320 K. It is shown that gradual movement of minima of the adiabatic potential surface along the φ coordinate for ρ≅ρ 0, describing this phenomenon, is a result of a competition between a small lattice deformation at T>T c and the new deformation appearing at T c and increasing with decreasing temperature.
Observation of strong exciton–photon coupling in an organic microcavity by P Schouwink; H.V Berlepsch; L Dähne; R.F Mahrt (352-356).
An optical organic semiconductor microcavity showing the strong coupling regime was prepared. The dye 1,1′-diethyl-2,2′-cyanine (PIC) forming J-aggregates was used as optically active material in a planar λ/2-microcavity configuration. Stable thin layers of homogeneously distributed aggregates without any matrix were prepared by spin-coating of a specific salt of the dye [(PIC+)2B10H10 2−]. Strong exciton–photon coupling was demonstrated by the anticrossing of exciton and photon modes. A Rabi-splitting of more than 50 meV was observed in transmission measurements. The asymmetric normal mode splitting could be explained by the asymmetric absorption spectrum of the J-aggregate.
Electron transfer and solvation in 9,9′-bianthryl and derivatives: a sub-ps fluorescence upconversion study by Martin Jurczok; Thomas Gustavsson; Jean-Claude Mialocq; Wolfgang Rettig (357-366).
The time-evolution of the fluorescence spectra of 9,9′-bianthryl and 9-carbazolyl-anthracene in ethanol and butanol have been studied with subpicosecond time-resolution by the fluorescence upconversion technique. For both compounds, an initial phase (<1 ps) with a structured anthracene-type fluorescence spectrum could be observed which transforms into an unstructured redshifted emission spectrum further shifting to the red on the time scale of the slow component of solvent relaxation. The results are compared to recent transient absorption studies and are interpreted as evidence for a subpicosecond relaxation process (ca 0.4 ps) occurring prior to electron transfer in these compounds.
Two-dimensional MAS–NMR spectra which correlate fast and slow magic angle spinning sideband patterns by Charles Crockford; Helen Geen; Jeremy J. Titman (367-373).
A new NMR experiment which allows a measurement of the chemical shift anisotropy (CSA) tensor under magic angle spinning (MAS) is described. This correlates a fast MAS spectrum in the ω 2 dimension with a sideband pattern in ω 1 in which the intensities mimic those for a sample spinning at a fraction of the rate ω r/N. This method is particularly useful for accurately measuring narrow shift anisotropies. Since the sidebands intensities in ω 1 are identical to those expected at ω r/N, standard methods can be used to extract the principal tensor components. The nature of the experiment is such that a minimal number of t 1 increments is required.
A first-principles method to model perturbed electronic wavefunctions: the effect of an external homogeneous electric field by M. Aschi; R. Spezia; A. Di Nola; A. Amadei (374-380).
In this Letter, we show that with the use of matrix notation to express the time-independent Schroedinger equation, it is possible to model perturbed electronic wavefunctions. Such a method makes use of first principles of the quantum mechanical theory and hence is rigorous within the only approximation due to the truncation of the perturbed Hamiltonian matrix used. Results show that for three different molecules in vacuo under an electric field, the proposed method provides reliable perturbed electronic wavefunctions at a low computational costs.
Analysis of the concentration dependence of KCl:Ag optical absorption by random-walk method by Sergey Berdnikov; Irina Bobkova (381-386).
The absorption spectra of KCl:Ag single crystals with varied concentration of Ag+ ions are analyzed by the multivariable random-walk method. It is shown that intracenter absorption and charge-transfer with formation of perturbed-impurity centers of different coordination are responsible for the absorption event within the spectral range 6.35–5.3 eV.
Photoinduced alkaline pH-jump on the nanosecond time scale by Stefania Abbruzzetti; Mauro Carcelli; Paolo Pelagatti; Dominga Rogolino; Cristiano Viappiani (387-394).
The triphenylmethane leucohydroxide salt 4-(dimethylamino)-4′-(trimethylammonium) triphenylmethanol iodide has been used as a photoactivatable caged hydroxide to rapidly increase the pH of a neutral aqueous solution on the nanosecond time scale. Photoexcitation of the compound with a nanosecond ultraviolet laser pulse results in the formation of a colored carbocation with a rate greater than 108 s −1 ; recombination occurs with a multiexponential kinetics. The dissociation is completely reversible on time scales of 102 s and offers a large time window after the pH-jump, in which the proton transfer reactions can be studied. The effectiveness of the photodissociation process to increase the pH of the solution has been demonstrated by means of the pH indicator bromoxylenol blue.
Cubic Si6−z Al z O z N8−z (z=1.8 and 2.8) spinels formed by shock compression by T. Sekine; H. He; T. Kobayashi; M. Tansho; K. Kimoto (395-399).
We present the formation of cubic Si6−z Al z O z N8−z (z=1.8 and 2.8) spinels by shock compressions from the corresponding β-sialon. Formed sialon spinels coexist with considerable amounts of the amorphous phase. The amorphization is considered to be a solid–solid reaction due to the sluggish phase transition under shock compression. The lattice parameter of the spinel increases and the density decreases with increasing z parameter. Shock-synthesized sialon spinels are nanocrystals with most grain sizes less than 30 nm and consist of main units of SiN4, SiN6 and AlO6.
NO x reduction by reburning: theoretical study of the branching ratio of the HCCO+NO reaction by L. Vereecken; R. Sumathy; S.A. Carl; J. Peeters (400-406).
The product distribution of the HCCO + NO reaction is examined í theoretically over an extended temperature range, using RRKM-master equation analyses incorporating the effect of internal rotations on the state densities. The potential energy surface was characterized at the B3LYP-DFT/6-311++G(d,p) and CCSD(T)/6-31G(d,p) levels of theory, combined with high-level single point CCSD(T)/6-311++G(2d,p) calculations. The only reaction products of importance were found to be HCNO+CO and HCN+CO2, where the predicted CO2 yield can be expressed as α(T)=0.0652+0.220exp(−T/917.9 K), for T=300–2500 K. The predicted product distribution is in agreement with the available experimental values.
Flame thermometry by femtosecond CARS by P. Beaud; H.-M. Frey; T. Lang; M. Motzkus (407-412).
The potential of time-resolved coherent anti-Stokes Raman scattering (CARS) for thermometry studies in combustion processes using the isotropic Q-branch of nitrogen is discussed. Despite the increasing complexity of the femtosecond (fs)-CARS transients with rising temperature, due to the high number of contributing levels, the time-resolved CARS signal can be simulated with excellent correspondence to the experiment. The feasibility of flame thermometry with fs-CARS is demonstrated for an atmospheric pressure methane/air flame.
Two-electron relativistic corrections to the potential energy surface and vibration–rotation levels of water by Harry M. Quiney; Paolo Barletta; György Tarczay; Attila G. Császár; Oleg L. Polyansky; Jonathan Tennyson (413-420).
Two-electron relativistic corrections to the ground-state electronic energy of water are determined as a function of geometry at over 300 points. The corrections include the two-electron Darwin term (D2) of the Coulomb–Pauli Hamiltonian, obtained at the cc-pVQZ CCSD(T) level of theory, as well as the Gaunt and Breit corrections, calculated perturbationally using four-component fully variational Dirac–Hartree–Fock (DHF) wavefunctions and two different basis sets. Based on the calculated energy points, fitted relativistic correction surfaces are constructed. These surfaces are used with a high-accuracy ab initio nonrelativistic Born–Oppenheimer (BO) potential energy hypersurface to calculate vibrational band origins and rotational term values for H 2 16 O . The calculations suggest that these two-electron relativistic corrections, which go beyond the usual kinetic relativistic effects and which have so far been neglected in rovibrational calculations on light many-electron molecular systems, have a substantial influence on the rotation–vibration levels of water. The three effects considered have markedly different characteristics for the stretching and bending levels, which often leads to fortuitous cancellation of errors. The effect of the Breit interaction on the rovibrational levels is intermediate between the effect of the kinetic relativistic correction and that of the one-electron Lamb-shift effect.
Algebraic calculation of vibrational energy levels for polyatomic molecules XH3 and XH4: application to ammonia and silane by Xi-Wen Hou; F. Borondo; R.M. Benito (421-428).
The vibrational energy levels of coupled stretching and bending overtone transitions in polyatomic molecules XH 3 and XH 4 are calculated using an algebraic method, in which couplings between stretching and bending are described by Fermi resonance terms. The algebraic Hamiltonian, established in terms of local creation (annihilation) operators, is shown to give results in good agreement with the experimental data available for ammonia and silane.