C-containing Species Research Articles

Numerical study on laminar burning velocity and ignition delay time of ammonia/methanol mixtures

As a carbon-free hydrogen-carrying fuel, ammonia can effectively alleviate the problem of CO2 emissions and has great application potential in marine engines. The low laminar burning velocity and calorific value, and the narrow flammability limit range make the combustion of pure ammonia difficult. Ammonia/methanol mixed combustion is a feasible method to improve ammonia combustion and mitigate CO2 emissions. It is difficult to accurately predict the laminar burning velocity (LBV) and ignition delay time (IDT) of ammonia/methanol combustion simultaneously by the existing chemical kinetic mechanisms. This study aims to develop a detailed chemical kinetic mechanism that can simultaneously predict the LBV and IDT. The new-developed mechanism was fully validated based on experimental data. Furthermore, it was employed to investigate the effects of methanol blending ratios and equivalence ratios on the LBV and IDT in NH3/CH3OH mixed flames. Besides, chemical kinetic analyses were conducted to elucidate the influence of methanol mixing ratios and equivalence ratios on LBV and IDT. Reaction pathway analysis revealed that the NH3 and CH3OH chemistry interacted by the sharing of H, OH and O radicals in fuels’ decomposition, and chemical interactions of N-containing radicals with the C1 species. The reactions involving C-containing species can notably improve the LBV of NH3/CH3OH mixed combustion. At low equivalence ratio, LBV increases in proportion to the fuel proportion. Conversely, at high equivalence ratio, the effect of oxygen concentration on LBV becomes the dominant factor.

Shock tube and comprehensive kinetic modeling study of ammonia/diethyl ether (DEE) mixtures

Ammonia (NH3) is a promising alternative clean fuel due to its carbon-free and high hydrogen content, along with the well-established infrastructure for storage and distribution. To overcome the issue of the low reactivity of NH3, a feasible strategy is co-burning ammonia with highly reactive fuels. Diethyl ether (DEE) is considered a promising alternative and biomass-oxygenated clean diesel substitute. Therefore, the NH3/DEE blend is also a promising carbon neutral alternative fuel. In this work, we measured the ignition delay times (IDTs) of NH3/DEE mixtures with DEE fractions (XDEE) of 0.05, 0.10, 0.30, and 1.00 at equivalence ratios of 0.5, 1.0, and 2.0, pressures of 1.75 and 10 bar, and temperature ranges from 1102 K to 1673 K in a shock tube. The DEE-NH3 model was proposed in this work, which included the DEE sub-model, NH3 sub-model, and some new cross-reactions between N-containing species and C-containing species. The DEE sub-model from Shrestha et al. (Fuel Communications, 2022) was modified in this work, NH3 sub-model was from our previous work (Reaction Chemistry & Engineering, 2023). The DEE-NH3 model was extensively validated by the IDTs, laminar flame speeds (LFSs), and species profiles (SPs) of NH3/DEE mixtures as well as pure DEE and NH3. The comparison of the prediction performance between the DEE-NH3 model and the Shrestha model was conducted for the ignition, flame propagation, and NH3 consumption. The effects of the cross-reactions on the NH3/DEE ignition and combustion were studied in detail.

Dynamics of bulk and surface oxide evolution in copper foams for electrochemical CO2 reduction

Oxide-derived copper (OD-Cu) materials exhibit extraordinary catalytic activities in the electrochemical carbon dioxide reduction reaction (CO2RR), which likely relates to non-metallic material constituents formed in transitions between the oxidized and the reduced material. In time-resolved operando experiment, we track the structural dynamics of copper oxide reduction and its re-formation separately in the bulk of the catalyst material and at its surface using X-ray absorption spectroscopy and surface-enhanced Raman spectroscopy. Surface-species transformations progress within seconds whereas the subsurface (bulk) processes unfold within minutes. Evidence is presented that electroreduction of OD-Cu foams results in kinetic trapping of subsurface (bulk) oxide species, especially for cycling between strongly oxidizing and reducing potentials. Specific reduction-oxidation protocols may optimize formation of bulk-oxide species and thereby catalytic properties. Together with the Raman-detected surface-adsorbed *OH and C-containing species, the oxide species could collectively facilitate *CO adsorption, resulting an enhanced selectivity towards valuable C2+ products during CO2RR.

The investigation of initial decomposition paths of methyltrichlorosilane on (0001) and (000[formula omitted]) surfaces of 4H-SiC: A DFT study

Methyltrichlorosilane (CH3SiCl3, MTS), a favored precursor for chemical vapor deposition (CVD) process of silicon carbide (SiC) epitaxial layer fabrication in a wide variety of temperatures, has been investigated prevalently. In this work, the investigation focused on the initial decomposition paths of MTS with active surface sites on 4H-SiC surfaces. The surfaces employed in this study include both (0001) face and (0001¯) face, which represent Si face and C face, respectively. The decomposition pathways of MTS decomposed with the active surface sites of 4H-SiC surfaces were researched theoretically with employment of transition state theory (TST). The barriers of Gibbs free energies and the estimated reaction rate constants were acquired to discuss the decomposition reactivity of MTS with the active sites on 4H-SiC surfaces. This work shows that MTS has tendency to decompose on both (0001) face and (0001¯) face of 4H-SiC, the H and Cl atoms can be abstracted by active surface sites on (0001) face and (0001¯) face, and impingement of MTS to active surface sites on (0001) and (0001¯) surfaces can yield C-containing and Si-containing surface species.

Surface intermediates steer the pathways of CO2 hydrogenation on Pt/γ-Al2O3: Importance of the metal-support interface

A variety of adsorbed intermediates have been extensively assigned during CO2 hydrogenation on Pt/γ-Al2O3. However, the surface sites where the surface species reside have not been well resolved, which are extremely important to depict the reaction pathways of CO2 hydrogenation. In the current work, surface species adsorbed on three regions were distinguished by in situ diffuse-reflectance infrared Fourier transform spectroscopy, i.e., contiguous Pt region of Pt nanoparticle, interfacial Ptint-O-Alint region between Pt nanoparticle and γ-Al2O3, and surface region of γ-Al2O3 away from Pt nanoparticle. The dynamics of adsorbed H species, carbonyl species, bicarbonates, formate species, and adsorbed CO2, were identified at different sites, proving that the reaction occurs at the Pt/γ-Al2O3 interface. We found that only the C-containing species at the Pt perimeter are directly active intermediates for the reaction. Such spatial analysis bridges the gap between the hydrogen activation and CO2 activation, providing a deep understanding of CO2 hydrogenation. In addition, the interface structure with oxygen vacancy is important for CO2 activation, driving the reaction through carboxylate (*HOCO) pathway to form CO at the interface rather than formate (*HCOO) pathway and then be hydrogenated at the Pt surface.

Deep understanding of the dependence between Cu surface wettability and C-adsorption/desorption

The dependence between the surface wettability of hydrothermally-treated Cu and C-adsorption/desorption is unclear, which restricts further expanding the practical applications of the superhydrophobic surfaces obtained via hydrocarbon adsorption and developing promising reversible super-wettability transition methods. Herein, spectroscopic (Raman and XPS) and microscopic (FESEM, AFM, SKPM) characterization results reveal that the superhydrophobicity of the hydrothermal Cu foil surfaces with micro-scaled Cu oxides depends on the adsorption of volatile C-containing organic species and is independent of the types of Cu oxides. Theoretical calculation results manifest that the adsorptions of volatile C-containing organic species on Cu2O and CuO both belong to strong chemisorptions. Inspired by the as-established dependence between Cu surface wettability and adsorption/desorption of volatile C-containing organic species, we develop facile, cost-effective, and environmental-friendly methods for reversibly modulating Cu surface super-wettability, which are accomplished by storing the sample with micro-structured Cu oxide(s) in C-rich ambience to realize superhydrophobicity and heating the superhydrophobic sample via short-time IR irradiation to achieve superhydrophilicity. Moreover, these super-wettability reversible transition methods are proved to be effective for other materials.

Combustion chemistry of ammonia/C1 fuels: A comprehensive kinetic modeling study

The application of ammonia (NH3) as a fuel could contribute to mitigating global warming and achieving carbon neutrality by mid-century. To enhance the reactivity of NH3 combustion, cofiring NH3 with hydrogen or C1 fuels like syngas, methanol and methane is proposed. In previous work, we studied the combustion chemistry of NH3/H2 mixtures using a comprehensive kinetic model. In this work, we expanded our comprehensive kinetic model to cover NH3/syngas, NH3/methanol and NH3/methane mixtures. Rate constants of critical cross reactions between C- and N-containing species were evaluated based on previous experimental and theoretical studies for selection and incorporation in the present model. Experimental data for both NOx/C1 and NH3/C1 fuels were selected from literature to test the present model, including speciation data measured in shock tubes, flow reactors, jet-stirred reactors, burner-stabilized flames, and the global parameters like ignition delay times and laminar flame speeds. The kinetic model validation conditions cover temperatures of 473–2000 K, pressures of 0.04–100 atm, and equivalence ratios of 0.04–116. In general, the present model adequately captures the selected experimental data. The present model can not only be used to predict the combustion of NH3/C1 fuel mixtures, but is also capable to predict the mutual interaction of NOx/C1 fuels. It is found that the rate constants between C-containing species and H2-related radicals are generally faster than (or close to) those between C- and N-containing species. At high temperatures, H2-related radicals (i.e. H, O, OH) have higher concentrations than reactive N-related radicals (NH, NH2, NO2). Therefore, under these conditions, C-containing species are more likely to react with H2-related radicals rather than N-related ones, making the C/N cross reactions less prominent. However, under low- and intermediate-temperature conditions, the concentrations of H and O radicals become lower, while those of NH2 and NO2 become higher, making the cross reactions between C- and N-containing species possible to compete with C/H cross reactions. The present model can be used as the base chemistry model for NH3/larger hydrocarbon fuels.

Conversion of CO2 to Methanol and Ethanol on Pt/CeOx/TiO2(110): Enabling Role of Water in C–C Bond Formation

The surface chemistry of alcohol synthesis from CO2 hydrogenation has been investigated using kinetic testing, ambient pressure X-ray photoelectron spectroscopy (AP-XPS), and DFT calculations over a multicomponent system, where Pt and ceria nanoparticles coexisted on a titania template, Pt/CeOx/TiO2(110). Due to its high ability to bind and activate CO2, not seen for typical Cu–ZnO catalysts, the Pt–CeOx–TiO2 interface is excellent for the hydrogenation of CO2 to methanol, with some ethanol also being produced (21% selectivity). The results of AP-XPS and DFT calculations indicate that the active state involves a mixture of Ti4+/Ti3+, Ce3+, and Pt0/Pt+. A fast pathway for the formation of CH3O species is only plausible when Ce3+ and Pt are present. The addition of water to the reaction feed facilitates the first hydrogenation of CO2 and substantially enhances the surface coverage of C-containing species (CH3O, HCOO, CO3, CHx), facilitating the formation of C–C bonds and the production of ethanol (38% selectivity).

Facilely tuning the surface wettability of Cu mesh for multi-functional applications

The Cu meshes with reversible super-wettability have multifunctions of self-cleaning, oil–water separation, floating-oil collection and underwater oil lossless-transportation. However, the facile method to tune the surface wettability and prepare reversibly super-wetting Cu meshes remains a great challenge. In this work, one-step chemical-etching method was used to construct the hierarchically-structured CuO of nano-sheets and micro-clusters on Cu meshes to provide suitable surface roughness essential for super-wettability. The surface wettability of Cu mesh was then facilely tuned by controlling the adsorption and desorption of C-containing species at the Cu mesh surface through C-rich ambience storage and IR irradiation for short duration. The super-wetting transition of Cu meshes between superhydrophobicity/superoleophilicity and superhydrophilicity/underwater-superoleophobicity could repeat for many cycles. Moreover, the super-wetting Cu meshes demonstrated excellent mechanical, physical and chemical stabilities. The switchable super-wettability endowed the Cu meshes with integrated multi-functions, which demonstrated remarkable performance of self-cleaning, oil–water separation, floating-oil collection and underwater oil lossless-transportation. This study suggests that utilizing C-rich ambience storage and IR irradiation to control the content of C-containing species at the Cu mesh surface is a facile and effective way to tune the surface wettability of Cu meshes, which can be easily extended to realize super-wettability transition of various materials.

Catalytic Oxidation of Methane on IrO2(110) Films Investigated Using Ambient-Pressure X-ray Photoelectron Spectroscopy

The catalytic oxidation of CH4 over IrO2(110) films grown on Ir(100) was investigated using ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) at total pressures near 1 Torr. The IrO2(110) films undergo negligible reduction during catalytic CH4 oxidation in reactant mixtures with as much as 95% CH4 and temperatures from ca. 500–650 K, demonstrating that IrO2(110) can catalyze the oxidation of CH4 over a wide range of temperatures and mixture compositions. High coverages of OH groups and oxidized C-containing species formed on the IrO2(110) surfaces during CH4 oxidation, including excess OH groups bound directly to the initially, coordinatively unsaturated Ir atoms. The formation of excess OH groups demonstrates that O-rich IrO2(110) surfaces were maintained even under highly CH4-rich conditions and provides evidence that the dissociative adsorption of O2 is more facile than CH4 activation and conversion to adsorbed intermediates on IrO2(110). Extensively oxidized surface species with a CHyO2 stoichiometry preferentially formed under all reaction conditions studied. The conversion of CH4 to the CHyO2 surface species became optimal at an intermediate composition of the reactant mixture (∼90% CH4), consistent with a site competition between CH4 and O2 during their initial adsorption as well as a high oxidation activity of chemisorbed O atoms on IrO2(110). These results provide quantitative information about the identities and coverages of adsorbed species that form during the catalytic oxidation of CH4 on IrO2(110). Such knowledge is essential for validating first-principles models of the reaction kinetics for this system and ultimately gaining insights needed to optimize the performance of IrO2 catalysts for the oxidation of light alkanes.

Structure-sensitive scaling relations among carbon-containing species and their possible impact on CO2 electroreduction

The arduous modelling of reactions at heterogeneous catalysts is greatly simplified when adsorption-energy scaling relations between intermediates exist. Generally, the offset of these linear relations is structure-independent when the slope is unity and otherwise depends on the coordination number of the active sites. Here we examine the adsorption of *C, *CH, *CH2, *CH3 and *COH on five different surface sites of nine transition metals to establish their structure-sensitive scaling relations. Interestingly, we show that the scaling relations of *C (valency 4) and C-containing species with valency 3 (*CH, *COH) have peculiar structure-independent offsets. These offsets stem from the analogous bonding of those species to the adsorption sites, in spite of their dissimilar valency. We show how this result implies that reaction pathways in catalysis involving *C, *CH and *COH, for instance CO2 electroreduction to CH4, will usually have sizable thermodynamic limits imposed to their optimization.

Mechanism of Graphene Formation via Detonation Synthesis: ADFTB Nanoreactor Approach.

With the development of theoretical and computational chemistry, as well as high-performance computing, molecular simulation can now be used not only as a tool to explain the experimental results but also as a means for discovery or prediction. Quantum chemical nanoreactor is such a method which can automatically explore the chemical process based only on the basic mechanics without prior knowledge of the reactions. Here, we present a new method which combines the semiempirical quantum mechanical density functional tight-binding (DFTB) method with the nanoreactor molecular dynamic (NMD) method, and we simulated the reaction process of graphene synthesis via detonation at different oxygen/acetylene mole ratios. The formation of graphene is initiated by the breaking of acetylene (C2H2) molecules by collision into pieces such as H atoms, ethynyl (HC≡C•), and vinylidene (H2C═C:) radicals. It is followed by the formation of long straight carbon chains coupled with a few branched carbon chains, which then turned intoa2-D framework made of carbon rings. Trace oxygen could modulate the size of the rings during graphene formation and promote the formation of regular graphene with fused six-membered rings as we see, but the addition of high oxygen content makes more C-containing species oxidized to small oxide molecules instead of polymerization. The calculation speed of the DFTB nanoreactor is greatly improved compared to the ab initio nanoreactor, which makes it a valuable option to simulate chemical processes of large sizes and long time scales and to help us uncover the "unknown unknowns".

Pre-coalescence scaling of graphene island sizes

Graphene grown using cold-wall chemical vapor deposition on Cu surfaces follows a classical nucleation and growth mechanism. Following nucleation at the earliest growth stages, isolated crystallites grow, impinge, and coalesce to form a continuous layer. During the pre-coalescence growth regime, the size distributions of graphene crystallites exhibit scaling of the form N(s) = θ/⟨s⟩2 g(s/⟨s⟩), where s is the island area, θ is the graphene coverage, ⟨s⟩ is the average island area, N is the areal density, and g(x) is a scaling function. For graphene grown on Cu surfaces that have been annealed in a reducing Ar + H2 ambient, excellent data collapse onto a universal Avrami scaling function is observed irrespective of graphene coverage, surface roughness, or Cu grain size. This result is interpreted to indicate attachment-limited growth and desorption of diffusing C-containing species. Graphene grown on Cu surfaces that were annealed in a non-reducing environment exhibits a qualitatively different scaling function, indicating diffusion-limited growth with a lower attachment barrier combined with C detachment from the graphene edges.

Comparison study on surface depth distribution of chemical species of different nanocomposite Ti–Si binary oxides

Two depths of surface of three kinds of 10 % SiO2 Ti–Si binary oxide powder, prepared by three kinds of sol–gel processes and annealed at higher temperature, are comparatively studied by ARXPS, SEM and HRTEM. ARXPS shows that the two depths of surface for all samples enrich in Si about 38.2–63.2 % and are the mixture of TiO x (x ≤ 2), SiOy (y ≤ 2), Ti–O–Si, adsorbed O-containing species and C-containing species. With increasing depth, the change of Si/Ti ratio closely relates to the original structure formed in sol–gel. The surface contents of SiO2 and Ti–O–Si linkages increase in depth for all samples. The surface of silica-supported titania and intimately mixed silica–titania has more sub-valence Ti and Si than that of silica-coated titania, but the surface of silica-coated titania has higher concentration of Ti, O and C than that of the other two. ARXPS and SEM images consistently confirm that the surface of silica-coated titania is a porous structure. HRTEM verifies that nanocrystalline anatase TiO2 with many crystal defects has been produced in silica-coated titania. Three nanocomposite silica–titania binary oxide powders with three different structures were synthesized by three different sol–gel processes and analyzed by ARXPS, SEM and HRTEM. As it shows that the surface of silica-coated titania is a porous structure with more O and C, the surfaces of intimately mixed silica–titania and silica-supported titania, respectively, conglomerate and sinter with more suboxide Ti and Si.

In Situ 13C and 23Na Magic Angle Spinning NMR Investigation of Supercritical CO2 Incorporation in Smectite–Natural Organic Matter Composites

This Article presents an in situ NMR study of clay–natural organic polymer systems (a hectorite–humic acid [HA] composite) under CO2 storage reservoir conditions (90 bar CO2 pressure, 50 °C). The 13C and 23Na NMR data show that supercritical CO2 interacts more strongly with the composite than with the base clay and does not react to form other C-containing species over several days at elevated CO2. With and without organic matter, the data suggest that CO2 enters the interlayer space of Na–hectorite equilibrated at 43% relative humidity. The presence of supercritical CO2 also leads to increased 23Na signal intensity, reduced line width at half height, increased basal width, more rapid 23Na T1 relaxation rates, and a shift to more positive resonance frequencies. Larger changes are observed for the hectorite–HA composite than for the base clay. In light of recently reported MD simulations of other polymer–Na–smectite composites, we interpret the observed changes to be due to an increase in the rate of Na+ s...

Rotational spectra of rare isotopic species of fluoroiodomethane: Determination of the equilibrium structure from rotational spectroscopy and quantum-chemical calculations

Supported by accurate quantum-chemical calculations, the rotational spectra of the mono- and bi-deuterated species of fluoroiodomethane, CHDFI and CD(2)FI, as well as of the (13)C-containing species, (13)CH(2)FI, were recorded for the first time. Three different spectrometers were employed, a Fourier-transform microwave spectrometer, a millimeter/submillimter-wave spectrometer, and a THz spectrometer, thus allowing to record a huge portion of the rotational spectrum, from 5 GHz up to 1.05 THz, and to accurately determine the ground-state rotational and centrifugal-distortion constants. Sub-Doppler measurements allowed to resolve the hyperfine structure of the rotational spectrum and to determine the complete iodine quadrupole-coupling tensor as well as the diagonal elements of the iodine spin-rotation tensor. The present investigation of rare isotopic species of CH(2)FI together with the results previously obtained for the main isotopologue [C. Puzzarini, G. Cazzoli, J. C. López, J. L. Alonso, A. Baldacci, A. Baldan, S. Stopkowicz, L. Cheng, and J. Gauss, J. Chem. Phys. 134, 174312 (2011); G. Cazzoli, A. Baldacci, A. Baldan, and C. Puzzarini, Mol. Phys. 109, 2245 (2011)] enabled us to derive a semi-experimental equilibrium structure for fluoroiodomethane by means of a least-squares fit procedure using the available experimental ground-state rotational constants together with computed vibrational corrections. Problems related to the missing isotopic substitution of fluorine and iodine were overcome thanks to the availability of an accurate theoretical equilibrium geometry (computed at the coupled-cluster singles and doubles level augmented by a perturbative treatment of triple excitations).

Low-temperature thermal removal of template from high silica ZSM-5. Catalytic effect of zeolitic framework

Topochemical and catalytic effects of ZSM-5 zeolite framework in the course of thermal degradation of tetrapropyl ammonium (TPA +) template upon calcination under oxidative (air) and inert (nitrogen, N 2) atmosphere was analyzed using elemental analysis, X-ray Photoelectron Spectroscopy (XPS) and absorption UV–Vis–NIR spectroscopy. It has been demonstrated that template residues were selectively removed from the interior of the crystals. The concentration of (N (C)-containing template species was ⩽1 atom per unit cell (u.c.)) by prolonged (24 h) calcination in air already at temperature T max = 310 °C (N-containing template species) and T max = 330 °C (C-containing template species). Calcination in N 2 requires temperatures higher by about 20 °C. The surface region of ZSM-5 crystals contained, however, a substantial amount of template residua (20–30% of N-containing and ∼60% of C-containing species) even at T max = 430 °C. This effect is associated with conversion of propylene, originating from TPA + degradation, into low volatile products by their dehydrogenation on acidic centers and defect sites of ZSM-5. The dehydrogenation reaction was enhanced under oxidative conditions. The volatility of carbonaceous and N-containing residues was further reduced by their strong interaction with defect sites of ZSM-5.

Understanding the Ag/Al2O3 hydrocarbon-SCR catalyst deactivation through TG/DT analyses of different configurations

Understanding the parameters affecting the Ag/Al2O3 hydrocarbon SCR catalyst activity or the procedure by which the C-containing species are deposited on the catalyst surface, under actual diesel engine operation, can lead to design a low temperature active Ag/Al2O3 SCR catalyst. In this work we investigated the role of the Ag/Al2O3 HC-SCR catalyst configuration for NOx reduction activity and its deactivation by coking under passive and active operation. This was done by separating the powdered catalyst into two layers/partitions (front and rear). The single and double-layered catalysts were exposed to real diesel engine exhaust gas, and then thermo gravimetric (TG) and differential thermal (DT) analyses were carried out. The analysis showed that the retention of carbon-rich species (hydrocarbons and soot) in the double-layered configuration occurs mainly in the first catalyst layer (front layer), therefore resulting in a cleaner second layer (rear layer). The results confirm that the catalyst deactivation initiated at the front part of the catalyst and progressively spreads towards the back. TG/DT analyses also showed that the retention of carbon-rich species over the catalysts was strongly reduced at temperatures higher than 350°C or when hydrogen was introduced, but the deactivation pattern was similar. The reactor design (e.g. catalyst configuration) in HC-SCR with silver is an important factor, and in the presented work the space in between the beds shown to be a critical parameter as it enhanced the low temperature catalytic activity.

The Influences of Hydrology on the Radiogenic and Stable Carbon Isotope Composition of Cave Drip Water, Grotta di Ernesto (Italy)

14C and δ13C values of C-containing species in cave drip waters are mainly controlled by the C isotope composition of karst rock and soil air, as well as by soil carbon dynamics, in particular the amount of soil CO2 in the unsaturated soil zone and the process of calcite dissolution. Here, we investigate soil carbon dynamics by analyzing the 14C activity and δ13C values of C dissolved in cave drip water. Monthly over a 2-yr period, we collected drip water from 2 drip sites, one fast and one relatively slow, within the shallow Grotta di Ernesto Cave (NE Italy). The 14C data reveal a pronounced annual cycle. In contrast, the δ13C values do not show an annual pattern and only small interannual variability compared to the δ13C values of soil waters. The annual 14C drip-water cycle is a function of drip-rate variability, soil moisture, and ultimately hydrology.

C2H in prestellar cores

We study the abundance of CCH in prestellar cores both because of its role in the chemistry and because it is a potential probe of the magnetic field. We also consider the non-LTE behaviour of the N=1-0 and N=2-1 transitions of CCH and improve current estimates of the spectroscopic constants of CCH. We used the IRAM 30m radiotelescope to map the N=1-0 and N=2-1 transitions of CCH towards the prestellar cores L1498 and CB246. Towards CB246, we also mapped the 1.3 mm dust emission, the J=1-0 transition of N2H+ and the J=2-1 transition of C18O. We used a Monte Carlo radiative transfer program to analyse the CCH observations of L1498. We derived the distribution of CCH column densities and compared with the H2 column densities inferred from dust emission. We find that while non-LTE intensity ratios of different components of the N=1-0 and N=2-1 lines are present, they are of minor importance and do not impede CCH column density determinations based upon LTE analysis. Moreover, the comparison of our Monte-Carlo calculations with observations suggest that the non-LTE deviations can be qualitatively understood. For L1498, our observations in conjunction with the Monte Carlo code imply a CCH depletion hole of radius 9 x 10^{16} cm similar to that found for other C-containing species. We briefly discuss the significance of the observed CCH abundance distribution. Finally, we used our observations to provide improved estimates for the rest frequencies of all six components of the CCH(1-0) line and seven components of CCH(2-1). Based on these results, we compute improved spectroscopic constants for CCH. We also give a brief discussion of the prospects for measuring magnetic field strengths using CCH.