Carbon K-shell excitation of C2H2, C2H4, C2H6 and C6H6 by 2.5 keV electron impact

Energy loss spectra of 2.5 keV electrons in the region of the carbon K-edge in C₂H₂, C₂H₄, C₂H₆ and C₆H₆ are report     

Buffer gas pressure shift in the mF = 0 → mF = 0 ground state hyperfine line in Cs

The pressure shift of the m_F:0 → 0 microwave transition of the cesium ground state has been measured for seven buffer gases (He, Ne, N₂, CH₄, C₂H₆, C₃H₈, C₄H₁₀). The high resolution of the experimental apparatus (about one part in 10¹¹ also allows an accurate measurement of the temperature dependence of the shift.     

J-aggregates of pseudoisocyanine with long alkyl substituents in thin films: Synthesis, spectral properties, and thermal stability

The formation and properties of J-aggregates in thin solid films of pseudoisocyanines with long N-alkyl groups, obtained by centrifuging from solutions in organic solvents, were studied. It is shown for the first time that nonsymmetric cyanine dyes, containing a C₂H₅ group at one nitrogen atom and a C₁₀H₂₁, C₁₅H₃₁, or C₁₈H₃₇ group at another nitrogen atom, spontaneously form J-aggregates stable at room temperature and pressing a narrow absorption band with a half-width at half maximum of 200 cm^?1. The thermal stability of J-aggregates in thin films of pseudoisocyanines with alkyl substituents decreases in the following order: C₂H₅-C₂H₅> C₂H₅-C₆H₁₃>C₂H₅-C₁₈H₃₇>C₂H₅-C₁₀H₂₁>C₂H₅-C₁₅H₃₁. By introducing 1-octadecyl-2-methylquinolinium iodide in the film, it was found that the J-aggregates studied consist of a small number (2–4) of dye molecules.     

High-efficiency,high-energy performance of a pulsed HF laser pumped by phototriggered discharge

A comparison of the performance of pulsed infrared HF lasers pumped by phototriggered discharges using either Ne/SF₆/H₂ or Ne/SF₆/C₂H₆ mixtures are presented. For an active volume of 50 cm³, a specific output energy as high as 11 J/ has been achieved with an efficiency higher than 3% when C₂H₆ is used as H atom fuel. The replacement of ethane by molecular hydrogen reduces the laser performance by 40%. The investigation of the temporal evolution of the laser intensity shows that this dramatic decrease results from a shortening of the laser pulse duration rather than from a decrease of the peak power. Indications are given that this behavior is correlated to a very different temporal evolution of the discharge parameters, especially at low reduced electric field E/N.     

Die Feldemission des wolframs nach der adsorption von Kohlenwasserstoffen

Field emission currents of electrons were measured for different planes of tungsten after the interaction of CH₄, C₂H₆, C₃H₈, and C₂H₄ at 78°K and 298°K and used to obtain work function and pre-exponential values from Fowler-Nordheim plots. A critical analysis of these data yields a conclusive result on surface potentials only if emitting areas and pre-exponentials are comparable.     

Geometric shielding corrections for total cross section calculations of electron scattering by CH4, C2H6, C2H3F3, C2H4, C2F4, C2Cl4 and C2Cl2F2 from 30–5000?eV

To quantify the changes in the geometric shielding effect in a molecule as the incident electron energy varies, an empirical fraction, which represents the total cross section contributions of shielded atoms in a molecule at different energies, is presented. Using this empirical fraction, the total cross sections for electron scattering by CH₄, C₂H₆, C₂H₃F₃, C₂H₄, C₂F₄, C₂Cl₄ and C₂Cl₂F₂ are calculated over a wide energy range from 30 to 5000 eV by the additivity rule model at the Hartree-Fock level. The quantitative total cross sections are compared with those obtained by experiment and other theories where available. Good agreement is attained above 100 eV.     

Dynamics and correlated performance of a photo-triggered discharge-pumped HF laser using SF6 with hydrogen or ethane

3 , has been performed in Ne/SF₆/H₂ and Ne/SF₆/C₂H₆ mixtures. Parameters involved have been the storage line capacitance and the circuit inductance, the capacitors charging voltage, the RH-molecule type and partial pressure, and the X-ray dose for the preionization. High laser performance has been achieved with C₂H₆: an output energy up to 3 J corresponding to a specific energy of 9.6 J/l at an efficiency of 4.7%, which strengthens the advantage of the photo-triggering technique to energize high-power HF lasers. However the optimum performance achieved with H₂, 5.75 J/l and 3.5%, are lower. It is shown, through a time-resolved study of the electrical discharge and spatial dynamics correlated to laser power and energy measurements, that discharge instabilities are responsible for the poor laser performance of the mixture with H₂. These instabilities, which lead to arc development, are characteristics of the discharge in Ne/SF₆. It is demonstrated for the first time that addition of a heavy hydrocarbon, such as C₂H₆, to that mixture induces the discharge stabilization so that the laser emission arises in a homogeneous active medium. This effect allows us to achieve better laser performance than with H₂. Received: 17 March 1998/Revised version: 13 July 1998     

Energy and angular distributions of electrons ejected from CH<Subscript>4</Subscript> and C<Subscript>3</Subscript>H<Subscript>8</Subscript> under 16.0 keV electron impact

Relative cross sections, differential in energy and angle, for electrons ejected from CH₄ and C₃H₈ molecules under 16.0 keV electron impact have been measured. Electrons were analyzed by a 45° parallel plate electrostatic analyzer at emission angles varying from 60° to 135° with energies from 50 eV to 1000 eV. The angular distributions of electrons exhibit structures which are found to arise from Coulomb and non-Coulomb interactions. Furthermore, the double differential cross sections of electrons ejected from C₃H₈ molecule are found to be higher in magnitude than those from CH₄. This result supports the fact that the number of ejected electrons participating in collisions with C₃H₈ molecules is more than that in CH₄. Also, the angular distributions of C-K-shell Auger electrons emitted from the target molecules have been studied and shown to be isotropic within the experimental uncertainty     

The scattering of He from adsorbed layers of gases on Ag(110)

The adsorption of gases on Ag(110) has been studied using inelastic He atom scattering. Vibrational spectra have been obtained for Kr, Xe, C₂H₆, C₂H₄, CH₄, CF₄, CHF₃, CO₂ and H₂O. Spectra have also been obtained for multilayers of Xe (2 layers) and C₂H₆ (3 and 4 layers) where the energy changes move to lower values. The scattering from Kr and Xe can be shown to be dispersionless as has been previously found for these adsorbates on Cu(100) and Cu(110). The energy changes for Kr and Xe are smaller than on Cu surfaces and attempts were made to account for this based on an Einstein model of the adsorbed atoms in the surface holding potential.     

Molecular dynamics simulation for sodium atom in and on the two layers of C150H 30 graphite plane

For inspection of thermal behaviors of sodium (Na) atom in the bulk and on the surface of two layered hydrogen terminated cluster model, 2C₁₅₀H₃₀, the molecular dynamics calculation was taken place at molecular mechanics 2 level. From the requirement of structural optimization, interlayer distance of 2C₁₅₀H₃₀ is 3.38 Å which is consistent with the observed value. In the cluster models intercalated and adsorbed by one Na atom, C₁₅₀H₃₀·Na·C₁₅₀H₃₀ and Na·2C₁₅₀H₃₀, respectively, the Na atom is stabilized beneath and above the nearest central carbon atom, C₀, in the upper layer where the distances, Na-C₀, are 2.76 and 3.16 Å, respectively. Adsorption of the Na atom to the surface has no influence on the geometrical structure of cluster model, whereas, intercalation to two layers expands the interlayer distance maximally to 5.01 Å which will be responsible for the carbon expansion of graphite electrode in cryolite melt-alumina slurries. Diffusion processes are observed above 200 K for the Na atoms stabilized in both sites. Although the Na atom migrates parallel to the layers in the range between 200 and 300 K in C₁₅₀H₃₀·Na·C₁₅₀H₃₀, it moves above the carbon layer from the center to the circumference periodically below 250 K and gets out at 300 K for Na·2C₁₅₀H₃₀. The migration rates of Na atom are almost the same irrespective of the diffusion areas.     

Structure and electronic properties of Hn@C20 molecule

Density functional theory has been employed to optimize the structure of endohedral doped C₂₀ fullerene. We have also investigated electronic properties. We have found that C₂₀ cage can accommodate up to 8 hydrogen atoms. Some hydrogen atoms get chemisorbed on the inner surface of C₂₀ cage and form C-H bond. Structural deformation is found to increase with increase in H-atoms. From the analysis of electronic properties, we observe that due to endohedral doping of hydrogen atoms inside C₂₀, H-atoms acquire net negative charge by accepting electrons and fullerene molecules acquire positive charge by donating electrons to H-atoms. For endohedral complexes where H₃ triangular molecule formation takes place, the nature of net charge transfer changes, i.e. fractional electronic charge is transferred from H-atoms to fullerene. C₂₀ doped with odd number of H-atoms should be more reactive compared to the even number case. Most of the present results are similar to those of endohedral C₆₀.     

The role of metal cation in electron-induced dissociation of tryptophan

The fragmentation of tryptophan (Trp) – metal complexes [Trp+M]⁺, where M = Cs, K, Na, Li and Ag, induced by 22 eV energy electrons was compared to [Trp+H]⁺. Additional insights were obtained through the study of collision-induced dissociation (CID) of [Trp+M]⁺ and through deuterium labelling. The electron-induced dissociation (EID) of [Trp+M]⁺ resulted in the formation of radical cations via the following pathways: (i) loss of M to form Trp^+?, (ii) loss of an H atom to form [(Trp-H)+M]^+?, and (iii) bond homolysis to form C₂H₄NO₂M^+?. Deuterium labelling suggests that H atom loss can occur from heteroatom and/or C–H positions. Other types of fragment ions observed include: C₉H₇NM⁺, C₉H₈N⁺, M⁺, C₂H₃NO₂M⁺, CO₂M⁺, C₁₀H₁₁N₂M⁺, C₁₀H₉NOM⁺. Formation of C₂H₄NO₂M^+? and C₉H₇NM⁺ cations suggests that the metal interacts with both the backbone and aromatic side chain, thus implicating π-interactions for all M. CID of [Trp+M]⁺ resulted in: loss of metal cation (for M = Cs and K); successive loss of NH₃ and CO as the dominant channel for M = Na, Li and Ag; formation of C₂H₃NO₂M⁺. Preliminary DFT calculations were carried out on [Trp+Na]⁺ and [(Trp-H)+Na]^+? which reveal that: the most stable conformation involves chelation by the backbone together with a $\pi $ -interaction with the indole side chain; loss of H atom from $\alpha $ -CH of the side chain is thermodynamically favoured over losses from other positions, with the resultant radical cation maintaining a (N, O, ring) chelated structure which is stabilized by conjugation.     

Theoretical and experimental studies on the adsorption behavior of thiophenol on gold nanoparticles

FT‐Raman spectra were obtained for thiophenol (TP) and TP on gold nanoparticles. All vibrational fundamentals for the TP molecule are assigned on the basis of the scaled quantum force field procedure. Three model systems are studied and compared for the interactions of TP with the Au atom: (1) TP with a Au atom, C₆H₅SH Au; (2) TP anion with a Au atom, C₆H₅S⁻ Au; and (3) TP with a Au atom and subsequent formation of thiophenylate, C₆H₅SAu. The equilibrium structures and Raman spectra were calculated for the model systems using density functional theory (DFT) with the B3LYP functionals and the mixed basis set 6‐311 + G** (for C, S, H) and LANL2DZ (for Au), and theoretical Raman wavenumbers of C₆H₅SAu and C₆H₅S⁻ Au were assigned according to potential energy distributions. The third model system is shown to be preferred over the other two. The calculated binding energies are also shown to support the third model system. It is suggested that a simple model, such as the one used in the present study, is reasonable to describe surface‐enhanced Raman spectroscopy of thiophenol adsorbed on gold nanoparticles. Copyright © 2007 John Wiley & Sons, Ltd.     

Mechanisms of inelastic scattering of low-energy protons by C6H6, C60, C6F12, and C60F48 molecules

The mechanisms of inelastic scattering of low-energy protons with a kinetic energy of 2–7 eV by C₆H₆, C₆F₁₂, C₆₀, and C₆₀F₄₈ molecules are studied using the methods of quantum chemistry and nonempirical molecular dynamics. It is shown that, for the C₆H₆ + proton and C₆₀ + proton systems, starting from a distance of 6 Å from the carbon skeleton, the electronic charge transfer from the aromatic molecule to H⁺ occurs with a probability close to unity and transforms the H⁺ ion into a hydrogen atom and the neutral C₆H₆ and C₆₀ molecules into cation radicals. The mechanism of interaction of low-energy protons with C₆F₁₂ and C₆₀F₄₈ molecules has a substantially different character and can be considered qualitatively as the interaction between a neutral molecule and a point charge. The Coulomb perturbation of the system arising from the interaction of the noncompensated proton charge with the Mulliken charges of fluorine atoms results in an inversion of the energies of the electronic states localized, on the one hand, on the positively charged hydrogen ion and, on the other hand, on the C₆F₁₂ and C₆₀F₄₈ molecules. As a result, the neutral molecule + proton state becomes the ground state. In turn, this inversion makes the electronic charge transfer energetically unfavorable. Quantum-chemical and molecular-dynamics calculations on different levels of theory showed that, for fluorine derivatives of some aromatic structures (C₆F₁₂, C₆₀F₄₈), the barriers to proton penetration through carbon hexagons are two to four times lower than for the corresponding parent systems (C₆H₆, C₆₀). This effect is explained by the absence of active π-electrons in the case of fluorinated molecules.     

Low temperature decomposition of ethylene over Ni(100): Evidence for vinyl formation

Thermal programmed desorption (TPD), high resolution electron energy loss spectroscopy (HREELS), and time-resolved laser-induced desorption (LID) have been used to study the chemisorption and decomposition of ethylene over Ni(100). Ethylene adsorbs molecularly on this surface at temperatures below 150 K. The molecule is π bonded in this state, showing very little rehybridization. At coverages below half saturation, decomposition to vinyl plus a hydrogen atom occurs unimolecularly with a rate constant of (8.0 ± 2.0) × 10⁻² s⁻¹ at 170 K. A strong kinetic isotope effect was observed; vinyl formation from C₂D₄ does not occur until about 200 K. The proposal of vinyl as the intermediate is supported by studies with C₂H₄, 1,1− and 1,2−C₂D₂H₂, and C₂D₄. The reaction is slower at saturation coverages, where molecular desorption is still seen above 200 K. Vinyl decomposes further at 230 K to form an acetylenic fragment.     

Inelastic atom scattering from layers of atoms and molecules on Cu(110)

The vibrational properties of adsorbed layers have been studied using inelastic He atom scattering. The substrate was Cu(110) viewed along the [001] and [11&#x0304;0] azimuth. The adsorbates included, Kr, Xe, CH₄, CD₄, C₂H₆, CO, CO₂ and O₂ and covered the region of both physisorption and chemisorption. Despite a range of binding energies and mass ratios the vibrational frequencies showed no dependence on the momentum change, i.e. scattering was dispersionless, although intensity changes occurred in the inelastic peaks. There seemed to be no dependence of the adsorbate spectra on coverage below a monolayer. The peaks of CH₄ and C₂H₆ appeared broader than those of Kr and Xe suggesting molecular motions. Further, CH₄ and C₂H₆ gave very similar spectra. Both energy gain and loss events were observed in the inelastic spectrum. For those adsorbates which gave multilayer adsorption (Xe, C₂H₆) changes in the spectra were observed as the second layer developed. In the latter category also, mixed layers (Xe on C₂H₆, Xe on CO and C₂H₆ on Xe) were studied.     

Flame structure studies of rich ethylene-oxygen-argon mixtures doped with CO2, or with NH3, or with H2O

Structures of several premixed ethylene-oxygen-argon rich flat flames burning at 50 mbar have been established by using molecular beam mass spectrometry in order to investigate the effect of CO₂, or NH₃, or H₂O addition on species concentration profiles. The aim of this study is to examine the eventual changes of profiles of detected hydrocarbon intermediates which could be considered as soot precursors (C₂H₂, C₄H₂, C₅H₄, C₅H₆, C₆H₂, C₆H₄, C₆H₆, C₇H₈, C₆H₆O, C₈H₆, C₈H₈, C₉H₈ and C₁₀H₈). The comparative study has been achieved on four flames with an equivalence ratio (f) of 2.50: one without any additive (F2.50), one with 15% of CO₂ replacing the same quantity of argon (F2.50C), one with 3.3% of NH₃ in partial replacement of argon (F2.50N) and one with 13% of H₂O in replacement of the same quantity of argon (F2.50H). The four flat flames have similar final flame temperatures (1800 K).CO₂, or NH₃, or H₂O addition to the fresh gas inlet causes a shift downstream of the flame front and thus flame inhibition. Endothermic processes CO₂ + H = CO + OH and H₂O + H = H₂ + OH are responsible of the reduction of the hydrocarbon intermediates in the CO₂ and H₂O added flames through the supplementary formation of hydroxyl radicals. It has been demonstrated that such processes begin to play at the end of the flame front and becomes more efficient in the burnt gases region.The replacement of some Ar by NH₃ is responsible only for a slight decrease of the maximum mole fraction of C₂H₂, but NH₃ becomes much more efficient for C₄H₂ and C₅ to C₁₀ species. Moreover, the efficiency of NH₃ as a reducing agent of C₅ to C₁₀ intermediates is larger than that of CO₂ and H₂O for equal quantities added.     

An experimental study of the premixed benzene/oxygen/argon flame with tunable synchrotron photoionization

A comprehensive experimental study of the premixed benzene/oxygen/argon flame at 4.0 kPa with a fuel equivalence ratio (?) of 1.78 has been performed with the tunable synchrotron photoionization and molecular-beam sampling mass spectrometry. Isomers of most observed species in the flame have been unambiguously identified by measurements of the photoionization efficiency spectra. Mole fraction profiles of species up to C₁₆H₁₀ have been measured at the selective photon energies near ionization thresholds, and the flame temperature profile is obtained using Pt/Pt-13%Rh thermocouple. Compared with previous studies on benzene flames by Bittner and Howard, and by Defoeux et al., a number of new species are observed in the present work. These new combustion intermediates should be included in the kinetic models of the growth of polycyclic aromatic hydrocarbons (PAHs) and benzene oxidation. Free radicals detected in the flame include CH₃, C₂H, C₂H₃, C₂H₅, C₃H, C₃H₃, C₃H₅, C₄H, C₄H₃, C₄H₅, C₄H₇, C₅H₃, C₅H₅, C₅H₇, C₆H₅, C₆H₅O, C₇H₇, and C₉H₇. More significantly, isomers of some PAHs have been identified, which should be of importance in understanding the mechanism of soot formation.     

富勒烯C20分子器件的电子结构和传导特性

运用基于密度泛函理论和基于非平衡格林函数的第一性原理方法研究了富勒烯C₂₀分子及连接电极构成的C₂₀分子器件的电子结构及电子输运性质.构建了三个基于C₂₀分子的嵌入K和Si原子的电子输运系统,并得到了电子透射谱和分子轨道分布.分析了三种器件的电子结构和输运性质的产生原因,说明C₂₀分子器件的电子传导主要集中在外壳.在C₂₀分子空笼中嵌入K和Si原子后,其电子输运仍然主要集中于富勒烯C₂₀的外壳. 关键词： 20分子')" href="#">富勒烯C₂₀分子 电子结构 电子传导     

Flame structure of laminar premixed anisole flames investigated by photoionization mass spectrometry and photoelectron spectroscopy

Two laminar, premixed, fuel-rich flames fueled by anisole-oxygen-argon mixtures with the same cold gas velocity and pressure were investigated by molecular-beam mass spectrometry at two synchrotron sources where tunable vacuum-ultraviolet radiation enables isomer-resolved photoionization. Decomposition of the very weak O–CH₃ bond in anisole (C₆H₅OCH₃) by unimolecular decomposition yields the resonantly-stabilized phenoxy radical (C₆H₅O). This key intermediate species opens reaction routes to five-membered ring species, such as cyclopentadiene (C₅H₆) and cyclopentadienyl radicals (C₅H₅). Anisole is often discussed as model compound for lignin to study the phenolic-carbon structure in this natural polymer. Measured temperature profiles and mole fractions of many combustion intermediates give detailed information on the flame structure. A very comprehensive reaction mechanism from the literature which includes a sub-scheme for anisole combustion is used for species modeling. Species with the highest measured mole fractions (on the order of 10^?3–10^?2) are CH₃, CH₄, C₂H₂, C₂H₄, C₂H₆, CH₂O, C₅H₅ (cyclopentadienyl radical), C₅H₆ (cyclopentadiene), C₆H₆ (benzene), C₆H₅OH (phenol), and C₆H₅CHO (benzaldehyde). Some are formed in the first destruction steps of anisole, e.g., phenol and benzaldehyde, and their formation will be discussed and with regard to the modeling results. There are three major routes for the fuel destruction: (1) formation of benzaldehyde (C₆H₅CHO), (2) formation of phenol (C₆H₅OH), and (3) unimolecular decomposition of anisole to phenoxy (C₆H₅O) and CH₃ radicals. In the experiment, the phenoxy radical could be measured directly. The phenoxy radical decomposes via a bicyclic structure into the soot precursor C₅H₅ and CO. Formation of larger oxygenated species was observed in both flames. One of them is guaiacol (2-methoxyphenol), which decomposes into fulvenone. The presented speciation data, which contain more than 60 species mole fraction profiles of each flame, give insights into the combustion kinetics of anisole.