Excited Carbon Atoms Research Articles

Theoretical X-ray absorption spectroscopy database analysis for oxidised 2D carbon nanomaterials.

In this work we provide a proof of principle for a theoretical methodology to identify functionalisation patterns in oxidised carbon 2D nanomaterials. The methodology is based on calculating a large number of X-ray absorption spectra of individually excited carbon atoms in different chemical environments using density functional theory. Since each resulting spectrum gives a fingerprint of the local electronic structure surrounding the excited atom, we may relate each spectrum to the functionalisation pattern of that excited atom up to a desired neighbourhood radius. These functionalisation pattern-specific spectra are collected in a database, that allows fast composition of X-ray absorption spectra for arbitrary structures in density functional theory quality. Finally, we present an exemplary application of the database approach to estimate the relative amount of functional groups in two different experimental samples of carbon nanomaterials.

Hybridization and tribomechanical properties of ultrathin amorphous carbon films synthesized by radio-frequency low-pressure plasma discharges

Ultrathin amorphous carbon (a-C) films were deposited on Si(100) substrates by low-pressure radio-frequency plasma discharges of varying substrate bias voltage in pure Ar atmosphere. The surface roughness and tribomechanical properties of the a-C films were measured with an atomic force microscope and a surface force microscope, respectively. Insight into tetrahedral (sp3) and trigonal (sp2) atomic carbon hybridization was obtained from the deconvoluted C1s core level peak of X-ray photoelectron spectroscopy spectra. Energetic particle collision theory was used to correlate hybridization and tribomechanical properties to low-pressure plasma discharge conditions. The results are interpreted in the context of Ar+ ion collisions with carbon atoms on the growing film surface, followed by collision cascades between excited carbon atoms and other surface carbon atoms, resulting in the removal of weakly bonded carbon atoms and dominance of sp3 hybridization or the development of thermal spikes that promote sp2 hybridization. Particle collision analysis shows that the sp3 fraction is a function of Ar+ ion flux, sputtering yield of target carbon atoms, and kinetic energy of surface carbon atoms, in good qualitative agreement with experimental results of the sp3 content of a-C films versus substrate bias voltage.

Study of the cascading effect on photon emission from sputtered excited carbon atoms and ions

A graphite surface was bombarded by 10keV Ar+ ions and intensities of three carbon spectral lines (CI-247.9nm, CI-283.7nm and CII-251.2nm) originating from the sputtered excited carbon atoms and ions were measured as a function of the distance from the target. It is shown that shape of the intensity decay curves was affected by filling up of the upper levels of these transitions via cascade process. In order to identify these cascading transitions the cascade correction was applied to the data. It was found that the prominent cascade transitions to upper levels of three given transitions were respectively from the 4p1S0, 3d2D3/2 and 3p2D5/2 levels.

Probabilistic analysis of tetrahedral carbon hybridization in amorphous carbon films

An energetic particle collision analysis of the effect of Ar+ ion bombardment on tetrahedral carbon hybridization (sp3) in amorphous carbon (a-C) thin films is presented for nonmagnetron radio-frequency sputtering. The analysis is based on sequential surface events involving the collision of Ar+ ions with carbon atoms on the surface of the growing film and subsequent collision cascades between excited carbon atoms and other surface carbon atoms, which promote the formation of sp3 carbon bonding. The model is validated by transmission electron and electron energy loss spectroscopy results that confirm the existence of a two-layer film structure consisting of an ultrathin interface layer and a continuous a-C film. Analytical results for the sp3 carbon content are shown to be in good agreement with similar experimental results obtained from the analysis of the C1s core level spectra of the sputtered a-C films.

Crossed molecular beam reactive scattering: from simple triatomic to multichannel polyatomic reactions

In our laboratory a recent series of experiments by means of the crossed molecular beam (CMB) scattering technique with mass-spectrometric detection and time-of-flight analysis has been instrumental in fostering progress in the understanding of the dynamics of both simple triatomic insertion reactions and complex polyatomic addition–elimination reactions exhibiting competing channels. In the first part of this review we survey the advances made in the comprehension of the dynamics of the insertion reactions involving excited carbon, nitrogen and oxygen atoms – C(1D), N(2D), O(1D) – with H2(D2), as made possible by synergistic comparisons of experimental reactive differential cross-sections with the results of exact quantum, quasiclassical trajectory and statistical calculations on reliable ab initio potential energy surfaces. Related experimental and theoretical work from other laboratories is noted throughout. In the second part, we review the progress made in the understanding of the dynamics of polyatomic multichannel reactions, such as those of ground state oxygen and carbon atoms, O(3P) and C(3P), with the simplest alkyne, acetylene, and alkene, ethylene, as made possible by the gained capability of identifying virtually all primary reaction channels, characterising their dynamics, and determining their branching ratios. Such a capability is based on an improved crossed molecular beam instrument which features product detection by low-energy electron soft-ionisation for increased sensitivity and universal detection power, and variable beam crossing angle for a larger collision energy range and increased angular and velocity resolution. The scattering results are rationalised with the assistance of theoretical information from other laboratories on the stationary points and product energetics of the relevant ab initio potential energy surfaces. These detailed studies on polyatomic multichannel reactions provide an important bridge between crossed beam dynamics and thermal kinetics research. Contents PAGE 1. Introduction 110 2. Experimental: improved crossed molecular beams apparatus 113 2.1. Soft electron-ionisation detection 114 2.2. Product angular and velocity distributions 116 2.3. Crossed molecular beam experiments with variable beam crossing angle 118 2.4. Supersonic sources of radical beams 120 3. Triatomic insertion reactions 120 3.1. Reaction C(1D) + H2 122 3.2. Reaction N(2D) + H2 126 3.3. Reaction O(1D) + H2 129 4. Polyatomic multichannel reactions 133 4.1. Reaction O(3P) + C2H2 134 4.1.1. Product angular and TOF distributions 135 4.1.2. Determination of branching ratios 138 4.1.3. Determination of product ionisation energies 138 4.2. Reaction C(3P) + C2H2 138 4.2.1. H and H2 elimination channels, and branching ratios 140 4.2.2. Ionisation energy of c/l-C3H radicals 145 4.3. Reaction O(3P) + C2H4 145 4.3.1. Observation of all product channels and branching ratios 146 4.4. Reaction C(3P) + C2H4 151 4.4.1. H-elimination channels 151 4.4.2. C–C bond fission channels 154 5. Summary and outlook 154 Acknowledgements 157 References 158

Radio recombination lines

Over the past forty years, investigations of radio recombination lines have become an effective way of studying the interstellar medium. The electron temperatures, densities, emission measures, kinematics parameters, elemental abundances and other physical characteristics of emission processes in planetary nebulae, distributed ionized gas, stellar winds, the partially-ionized interstellar medium and external galaxies are determined with the lines of hydrogen, helium, carbon, etc. This astrophysical phenomenon is unique because of the large (>1000) number of possible quantum transitions and because of the wide observable frequency range (from ~150 GHz to ~10 MHz). In this short review the main results from recombination radio line investigations are discussed. Special attention is given to the areas which have been given least attention in the literature such as the radio recombination lines of very excited carbon atoms at the extremely low-frequency (v < 30 MHz) end of the decameter wavelength range. This is important to stress in the 70-th jubilee of radio astronomy because cosmic radio emission was discovered by Karl Jansky at decameter wavelengths. Investigations of the extremely low-frequency radio recombination lines are also very interesting for the prospective LOFAR system.

Light Emission from Carbon Nanotubes Induced by Field Electron Emission from Oriented MWCNT Arrays Accompanied by Re-Deposition

AbstractWell-ordered aligned arrays of multiwalled carbon nanotubes were synthesized by the catalytic thermal chemical vapor deposition (CVD) in acetylene gas at the atmospheric pressure. Abrupt spark–type light emission spots and release of the carbon nanotubes from the cathode and succeeding deposition onto the anode without decomposition has been detected above the threshold electron emission current. Spectral analysis of the light showed that the spectra consist of a background similar to blackbody radiation and a set of sharp lines identified with the radiative transitions of excited carbon and iron atoms. Light emission spectra were found to be essentially different depending on the type (AC or DC) of the applied bias voltage and its value. As-grown and re-deposited materials were characterized by Raman spectroscopy and scanning electron microscopy. It was found that the structural properties of the re-deposited carbon nanotubes remained intact, with only negligible amorphization. A mechanism of the spark emission and re-deposition processe is proposed and discussed in terms of fast overheating of CNT, after explosion type melting of Fe-catalyst nanoclusters, followed by CNT transfer from cathode to anode.

Cavity enhanced wavelength modulation spectrometry for application in chemical analysis

A simple and robust absorption technique is developed which combines cavity enhanced absorption spectroscopy and wavelength modulation spectroscopy and measures the integrated output of unlocked cavities. The detection power of the technique is affected by peculiarities of the laser injection into a cavity causing excess noise which exceeds the shot noise. The noise and ways for its reduction are discussed. The new method is demonstrated by absorption measurements of excited carbon atoms in a microwave induced plasma. Preliminary detection limits equivalent to optical depths below 10-6 were obtained.

Dynamics of the C+C2H2 reaction from differential and integral cross-section measurements in crossed-beam experiments

The reaction between atomic carbon and acetylene has been investigated using complementary crossed molecular beam techniques. Differential cross sections have been obtained for the reactions of both ground and excited carbon atoms, C(3PJ, 1D2)+C2H2(X 1Σg+), in experiments conducted with continuous supersonic beams, mass spectrometric detection, and time-of-flight analysis at a relative translational energy of 29.3 kJ mol−1. The reaction C(3PJ)+C2H2(X 1Σg+) has been found to lead to C3H+H and C3+H2 products in comparable amounts. Both H and H2 elimination pathways are found to proceed through the formation of a C3H2 long-lived intermediate complex whose lifetime may be comparable to its rotational period. The spin-forbidden H2 elimination channel is attributed to the occurrence of intersystem-crossing between the triplet and singlet manifolds of the C3H2 potential-energy surfaces. The reaction C(1D2)+C2H2(X 1Σg+) has been found to lead to formation of C3H+H, with a C3H center-of-mass angular distribution strongly forward peaked, indicating a short-lived intermediate complex. Integral cross sections have been obtained for the C(3PJ)+C2H2(X 1Σg+)→C3H+H(2S1/2) reaction in experiments conducted with pulsed, supersonic molecular beams in the range of relative translational energies ET=0.38−25.5 kJ mol−1, the H(2S1/2) product being detected by laser-induced fluorescence. The reaction has been found to be without a barrier, relative integral cross sections being proportional to (ET)−0.80±0.03 below ET=10 kJ mol−1. These findings provide direct evidence that the C(3PJ)+C2H2 reaction can occur under the physical conditions prevailing in dense interstellar clouds and, in particular, that it may be the source of both C3H and C3 species in these extreme environments.

Stripping dynamics in the reactions of electronically excited carbon atoms, C(1D), with ethylene and propylene—production of propargyl and methylpropargyl radicals

The reactions of electronically excited carbon atoms, C(1D), with ethylene and propylene were studied at three collision energies between 48 and 104 kJmol−1 employing the crossed molecular beam technique. Forward-convolution fitting of our data combined with electronic structure calculations suggests that the reactions proceed via stripping dynamics. Extremely short-lived allene and 1,2-butadiene intermediates decompose via atomic hydrogen emission to yield propargyl and methylpropargyl radicals, respectively. These production routes are of potential importance to form benzene, toluene, and o-/p-xylenes in circumstellar envelopes of carbon stars and combustion flames.

Soot detection using excimer laser fragmentation fluorescence spectroscopy

Excimer laser fragmentation fluorescence spectroscopy (ELFFS) fragments soot particles with 193 nm photons to produce excited carbon atoms that emit at 248 nm. The ELFFS detection limit for soot is 40 ppt in cooled combustion exhaust. We quantify and correct for interferences from carbon dioxide (248 nm emission) and oxygen (250 nm); unburned hydrocarbons are removed with a diffusion denuder. We find that the ELFFS signal is more closely proportional to the surface area concentration of the particle than the volume fraction over a large range of concentrations. The technique is suitable for measuring soot from spark ignition and diesel engine exhaust. The soot particles are produced using a methane/air/nitrogen diffusion flame in an inverted flow reactor, designed to generate soot particles with independently variable and repeatable concentrations and size distributions. By varying the methane, nitrogen, and air flow rates, we independently vary the mean particle diameter from 60 to 250 nm and the volume concentration from 40 ppt to 9 ppb.

Chemical dynamics of cyclopropynylidyne (c-C3H; X2B2) formation from the reaction of C(1D) with acetylene, C2H2(X 1Σg+)

The reaction between electronically excited carbon atoms, C(1D), and acetylene was studied at two average collision energies of 45 kJ mol−1 and 109 kJ mol−1 employing the crossed molecular beam technique. The time-of-flight spectra recorded at mass to charge m/e=37(C3H+) and m/e=36(C3+) show identical patterns indicating the existence of a carbon versus atomic hydrogen exchange pathway to form C3H isomer(s); no H2 elimination to the thermodynamically favorable tricarbon channel was observed. Forward-convolution fitting of our data shows that the reaction proceeds via direct stripping dynamics on the A′1 surface via an addition of the carbon atom to the π-orbital of acetylene to form a highly rovibrationally, short lived cyclopropenylidene intermediate which decomposes by atomic hydrogen emission to c-C3H(X 2B2). The dynamics of this reaction have important impact on modeling of chemical processes in atmospheres of comets approaching the perihelon as photolytically generated C(1D) atoms are present.

Production of Vanadium–Carbon Clusters Using DC Arc Discharges with Pulsed Helium/Methane Gas Injection

Experimental studies of the synthesis of new types of vanadium–carbon clusters have been carried out using dc arc discharges, where a pair of graphite rods mixed with the vanadium carbide (VC) powders were used as the electrodes. A high-pressure pulsed gas injection technique, where pulsed He gases were periodically injected into the dc arc plasma to cool the excited carbon and vanadium atoms, was used. The effect of an addition of carbon atoms on cluster production was also examined by injecting pulsed CH4 gases in addition to high-pressure He gas pulses. The soots produced in the dc arc discharges were analyzed using a laser desorption reflectron type time-of-flight (TOF) mass spectrometer. Experimental results show that when CH4 gas was added to He gas with a partial pressure of 15%, enhanced VnC2n clusters with a VC2-mass spacing of Δm/z=75 were detected in the mass range of 225<m/z<675. Experiments show that these vanadium–carbon clusters are formed by the vanadium atoms evaporated from the electrodes and the carbon atoms supplied from the injected CH4 gas.

Characteristics of transverse volume discharge in Cf2Cl2 and in Ar/Cf2Cl2 mixture

The conditions of ignition and the characteristics are given for a nanosecond transverse volume discharge in Freon-12 (CF2C12) and in an Ar/CF2C12 mixture. It is shown that, in the ultraviolet and visible ultraviolet spectral region, a plasma based on CF2C12 molecules is a source of radiation of excited carbon atoms and Cl2 molecules. The volumetric stage of the discharge exists atU charge ≤ 12 kV and [CF2C12] = 0.1–3 kPa, and the current and voltage oscillograms are similar to the respective characteristics of a transverse discharge with prior-to-prebreakdown electron multiplication in He(Ne)/SF6(F2) mixtures. A plasma of an Ar/CF2C12 mixture is a source of radiation of the λ = 258 nm bands for Cl 2 * , 193 nm for ArF, and 175 nm for ArCl. Such sources may be used in short-wave pulse photometry, microelectronics, and photo- and biochemistry.

Optimization and control of a plasma carburizing process by means of optical emission spectroscopy

In this work optical emission spectroscopy (OES) is used to characterize the dissociation process of methane in an argon/hydrogen/methane discharge for plasma carburizing. The optical emission spectra of the discharge have been measured as a function of process parameters: discharge voltage, pulse duration and pulse pause time. A correlation between the intensities of hydrocarbon molecules, carbon atoms and ions, and the carbon mass flow model of the carburizing process has been confirmed. The dominant species identified in the spectra used for correlation are excited and ionized carbon atoms, as well as excited carbon and hydrocarbon molecules such as excited CH with a molecular band at 431.42 nm and 314.41 nm, and excited C 2 with a molecular band at 501.50 nm. Excited carbon atoms at 493.21 nm and excited carbon ions at 387.17 nm and 426.70 nm are also detected. It can be concluded that the intensity of excited CH molecules at a molecular band at 431.42 nm is a function of pulse duration time, voltage and pulse pause time, respectively, indicating that carbon mass flow ( ṁ c ) is directly proportional to the intensities of emission. This means that OES allows in situ, real-time control of the plasma carburizing process.

A high-intensity, pulsed supersonic carbon source with C(3P j) kinetic energies of 0.08–0.7 eV for crossed beam experiments

An enhanced supersonic carbon source produces carbon atoms in their C(3Pj) electronic ground states via laser ablation of graphite at 266 nm. The 30 Hz (40±2) mJ output of a Nd-YAG laser is focused onto a rotating graphite rod with a 1000 mm focal length UV-grade fused silica plano-convex lens to a spot of (0.5±0.05) mm diameter. Ablated carbon atoms are subsequently seeded into helium or neon carrier gas yielding intensities up to 1013 C atoms cm−3 in the interaction region of a universal crossed beam apparatus. The greatly enhanced number density and duty cycle shift the limit of feasible crossed beam experiments down to rate constants as low as 10−11–10−12 cm3 s−1. Carbon beam velocities between 3300 and 1100 m s−1, with speed ratios ranging from 2.8 to 7.2, are continuously tunable on-line and in situ without changing carrier gases by varying the time delay between the laser pulse, the pulsed valve, and a chopper wheel located 40 mm after the laser ablation. Neither electronically excited carbon atoms nor ions could be detected within the error limits of a quadrupole-mass spectrometric detector. Carbon clusters are restricted to ∼10% C2 and C3 in helium, minimized by multiphoton dissociation, and eliminating the postablation nozzle region.

A Novel Detector for Gas Chromatography Based on UV Laser-Produced Microplasmas

The use of UV laser-produced microplasmas for the detection of analyte molecules in the effluent gases of a gas chromatograph is described. The microplasmas are formed when a carbon-containing analyte is present in the carrier gas flow but not in the carrier gas alone. The microplasmas are produced by using the 193-nm output of the ArF excimer laser, with only modest pulse energies (10 mJ) required. Three means for detecting the presence of the microplasmas have been effected and are compared: optogalvanic (plasma electron) detection, photoacoustic (blast wave) detection, and photometric (plasma emission) detection, particularly of electronically excited carbon atoms at 248 nm. The relative responses of these three techniques have been determined for microliter injection of acetylene into the helium flow. Present limits for acetylene with the use of the optogalvanic, photoacoustic, and photometric techniques are 500, 500, and 10 ng, respectively. Optimization of these techniques is expected to improve the detection limits by 2–4 orders of magnitude. The virtues of this detector include the fact that it requires no flame and that it is sensitive to carbon-containing species such as CO and CO2.

Collisional quenching of electronically excited carbon atoms, C[2p 2(1 S 0)

Electronically excited carbon atoms in the 2p2(1S0) state, 2.684 eV above the 2p2(3P0) ground state, have been monitored in the time-resolved mode by resonance line absorption at λ= 247.9 nm [3s(1P01)â†� 2p2(1S0)] following the irradiation of CCl4 in the gas phase. The experimental arrangement, including low-energy repetitive pulsing on a flow system kinetically equivalent to a static system, pre-trigger photomultiplier gating, signal averaging and computerised analysis of the photoelectric decay signals, has been employed to study the removal of this optically metastable species by a range of added gases. Absolute rate data for the collisional quenching of C(21S0) are reported for the species He, Xe, N2, Cl2, CO, CO2, H2O, CH4, CCl4, C2H2 and C3H6. The resulting second-order rate constants obtained by this technique are compared with those reported hitherto, principally from “single-shot” mode high-energy photolysis pulse experiments. Rate data for all the atomic states of carbon in the 2p2 configuration (3PJ, 1D2, 1S0) are discussed, where appropriate, within the context of the symmetry of the potential surfaces involved in collision on the basis of the weak spin–orbit coupling approximation. The rate data for C[2p2(1S0)] are also compared with the analogous data for Si[3p2(1S0)].

Formation of C 2(e 3π g, C 1π g) in the photolysis of CO(X 1ϵ g+) at 193 nm with an ArF laser

CO(X 1ϵ g +) was irradiated with an ArF laser (193 nm) at room temperature at pressures of 2–20 mbar. C(2 1D) atoms were formed by two-photon absorption and observed by the 247.8 nm C(3 1P o→2 1S) fluorescence. Fluorescence of C 2 (e 3π g → a 3π u, C 1π g → A 1π g and d 3π g → a 3π g) was detected. The emission time dependence led to the conclusion that C 2 ∗ is formed by recombination of electronically excited carbon atoms.

Isotope effect in multiphoton ultraviolet photolysis of CO

Two-photon dissociation of carbon monoxide near 193 nm with a tunable ArF* laser to yield excited C(2 1D) atoms is reported. The atoms are detected by subsequent absorption of a third laser photon to C(3 1P°) followed by emission of 248 nm fluorescence to C(2 1S). The process is found to be isotopically sensitive; the C(2 1D) yield from 13C 16O and 12C 18O is a factor of 6 greater than from 12C 16O. The isotope effect is attributable to an enhancement in the two-photon matrix element due to the shift in the near resonant a 2Π, v=2 intermediate state. C2 Swan band emission is also observed, arising from association of the free carbon atoms, and exhibits a corresponding isotope effect. Finally, collisional processes are observed involving excited carbon atoms and electrons, using two-photon ionization of a small quantity of added xenon atoms as a new technique for producing a controlled density of free electrons.