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.     

An experimental study on the formation of polycyclic aromatic hydrocarbons in laminar coflow non-premixed methane/air flames doped with four isomeric butanols

Experimental measurements were conducted for temperatures and mole fractions of C1–C16 combustion intermediates in laminar coflow non-premixed methane/air flames doped with 3.9% (in volume) 1-butanol, 2-butanol, iso-butanol and tert-butanol, respectively. Synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) technique was utilized in the measurements of species mole fractions. The results show that the variant molecular structures of butyl alcohols have led to different efficiencies in the formation of polycyclic aromatic hydrocarbons (PAHs) that may cause the variations in sooting tendency. Detailed species information suggests that the presence of allene and propyne promotes benzene formation through the C₃H₃ + C₃H₄ reactions and consequently PAH formation through the additions of C2 and C3 species to benzyl or phenyl radicals. As a matter of fact, PAHs formed from the 1-butanol doped flame are the lowest among the four investigated flames, because 1-butanol mainly decomposes to ethylene and oxygenates rather than C3 hydrocarbon species. Meanwhile, the tert-butanol doped flame generates the largest quantities of allene and propyne among the four flames and therefore is the sootiest one.     

Application of benzophenones to elucidate the photochemical reaction pathways of hydrogen peroxide with dimethylsulfoxide

Benzophenone ((C₆H₅)₂CO) and decafluorobenzophenone ((C₆F₅)₂CO) were applied to elucidate the photochemical reaction pathway of hydrogen peroxide (H₂O₂) with dimethylsulfoxide (DMSO). When a solution of benzophenone in DMSO was excited with the 355 nm laser light, three transient species were observed in the time-resolved electron paramagnetic resonance spectra: benzophenone ketyl (C₆H₅)₂COH^⋅, methyl^⋅CH₃, and methylsulfinic methyl^⋅CH₂SOCH₃ radicals. However, when decafluoro-benzophenone was used with DMSO, only ketyl and methylsulfinic methyl radicals were observed under the same experimental conditions. When the reaction of benzophenone and DMSO was carried out in the presence of H₂O₂, different time profiles of^⋅CH₃ radicals were observed. In the reaction of decafluorobenzophenone-DMSO-H₂O₂, the time profiles of the radicals were not affected by the presence of H₂O₂. Thus, these results verify that^⋅CH₃ radicals are regenerated in a cyclic pathway, in which^⋅CH₃ radicals attack H₂O₂. The regeneration pathway allows us to observe f-pair polarization throughout the lifetime of^⋅CH₃ radicals, which last several microseconds, an order of magnitude longer than theT ₁ relaxation time of^⋅CH₃ radicals.     

Structure and dynamics of radicals in zeolite matrices: ESR and theoretical studies

Amine radical cations of the type R₃N^·+ and [R₃NCH₂]^·+, R=CH₃, C₃H₇, and nitric oxide, NO, have been used to probe the bonding to the surface and the dynamics of the radicals trapped in the confined space of cages or channels in the zeolite. Regular continuous-wave electron spin resonance (ESR) was employed to study the internal motion of the cation radicals formed by γ-irradiation of amines and related ammonium ions, introduced during the synthesis of the zeolites Al-offretite, SAPO-37, SAPO-42 and AlPO₄-5. The ESR spectra of [(CH₃)₃NCH₂]^·+ radical cation in several studied systems changed reversibly with temperature, indicating dynamical effects. Free rotation about the >N?CH₂ bond of the [(CH₃)₃NCH₂]^·+ species was found to occur in the temperature range of 110 to 300 K, while the rotation about the >N?CH₃ bonds was hindered. The observations confirm the theoretical prediction on the basis of density functional theory calculations, which indicate that the corresponding barriers are of the order of 0.3 and 7 kJ/mol, respectively. The radical cations of the type R₃N^·+ with R=C₂H₅, C₃H₇ were found to undergo a different type of dynamics, involving a two-jump process of the methylene hydrogens next to the nitrogen. A cage or channel size effect on the stability and molecular dynamics was inferred in some cases. Pulsed ESR was employed to study the (NO)₂ triplet-state dimers in Na-A type zeolite, with the purpose to resolve the interaction with surface groups, and to elucidate the role of the zeolite on stabilizing the triplet rather than the usual singlet state. Measurements performed at 5 K gave rise to Fourier transform spectra that were assigned to the dimer species interacting with one or more²³Na nuclei, with approximative parameters A(²³Na)=(4.6, 4.6, 8.2) MHz and Q(²³Na)=(?0.3, ?0.3, 0.6) MHz for the hyperfine and nuclear quadrupole coupling tensors, respectively. The values are of similar magnitude as those determined for the NO?Na⁺ complex. The stability of the triplet-state structure was attributed to unusual geometric structure imposed by the zeolite matrix, with the N?O bonds along a line as in [O?N?Na⁺?N?O], which according to UHF ab initio calculations has a triplet ground     

Mechanism of sonochemical reduction of permanganate to manganese dioxide in aqueous alcohol solutions: Reactivities of reducing species formed by alcohol sonolysis

The sonochemical reduction of MnO₄⁻ to MnO₂ in aqueous solutions was investigated as a function of alcohol concentration under Ar. The rate of MnO₄⁻ reduction initially decreased with increasing alcohol concentration, and then increased when the alcohol concentration was increased further. The concentrations at which the reduction rates were minimum depended on the hydrophobic properties of the added alcohols under ultrasonic irradiation. At low concentrations, the alcohols acted as OH radical scavengers; at high concentrations, they acted as reductant precursors: R_ab, formed by abstraction reactions of the alcohols with sonochemically formed OH radicals or H atoms, and R_py, formed by alcohol pyrolysis under ultrasonic irradiation. The results suggest that the reactivity order of the sonochemically formed reducing species with MnO₄⁻ at pH 7–9 is the sum of H₂O₂ and H > R_py > R_ab. The peak wavelengths of MnO₂ colloidal solutions formed at high 1-butanol concentrations shifted to shorter wavelengths, suggesting the formation of small particles at high 1-butanol concentrations. The rates of sonochemical reduction of MnO₂ to Mn²⁺ in the presence of 1-butanol were slower than that in the absence of 1-butanol, because the sonochemical formation of H₂O₂ and H, which act as reductants, was suppressed by 1-butanol in aqueous solutions.     

Reactions and anion desorption induced by low-energy electron exposure of condensed acetonitrile

Thermal desorption spectrometry (TDS) and electron stimulated desorption (ESD) are employed to investigate mechanisms responsible for the formation of C₂H₆ in electron irradiated multilayer films of acetonitrile (CH₃CN) at 30 K. Using a high sensitivity time-of-flight mass spectrometer, we observe the ESD of anionic fragments H⁻, CH₂ ⁻, CH₃ ⁻ and CN⁻. Desorption occurs following dissociative electron attachment (DEA) via several negative ion resonances in the 6 to 14 eV energy range and correlates well with a “resonant” structure seen in the TDS yield of C₂H₆ (i.e., at mass 30 amu). It is proposed that C₂H₆ is formed by the reactions of CH₃ radicals generated following DEA to CH₃CN which also yields CN⁻. Between 2 and 5 eV, a second resonant feature is seen in the C₂H₆ signal. While DEA is observed in the gas phase at these energies, no anion desorption occurs since anionic fragments likely have insufficient kinetic energy to desorb. Since the CH₂ ⁻ ion has not been observed in gas-phase measurements, we propose that it is formed, along with HCN (that is detected in TDS) when dissociation into CH₃ ⁻ and CN is hindered by adjacent molecules.     

Experimental and kinetic modeling study of C2H4 oxidation at high pressure

A detailed chemical kinetic model for oxidation of C₂H₄ in the intermediate temperature range and high pressure has been developed and validated experimentally. New ab initio calculations and RRKM analysis of the important C₂H₃ + O₂ reaction was used to obtain rate coefficients over a wide range of conditions (0.003-100 bar, 200-3000 K). The results indicate that at 60 bar and medium temperatures vinyl peroxide, rather than CH₂O and HCO, is the dominant product. The experiments, involving C₂H₄/O₂ mixtures diluted in N₂, were carried out in a high pressure flow reactor at 600-900 K and 60 bar, varying the reaction stoichiometry from very lean to fuel-rich conditions. Model predictions are generally satisfactory. The governing reaction mechanisms are outlined based on calculations with the kinetic model. Under the investigated conditions the oxidation pathways for C₂H₄ are more complex than those prevailing at higher temperatures and lower pressures. The major differences are the importance of the hydroxyethyl (CH₂CH₂OH) and 2-hydroperoxyethyl (CH₂CH₂OOH) radicals, formed from addition of OH and HO₂ to C₂H₄, and vinyl peroxide, formed from C₂H₃ + O₂. Hydroxyethyl is oxidized through the peroxide HOCH₂CH₂OO (lean conditions) or through ethenol (low O₂ concentration), while 2-hydroperoxyethyl is converted through oxirane.     

EFFECT OF ATOMIC HYDROGEN ON THE ACETYLENE-ADSORBED Si(100)(2×1) SURFACE AT ROOM TEMPERATURE

High resolution electron energy loss spectroscopy, low energy electron diffraction and quadrupole maas spectrometer (QMS) have been employed to study the effect of atomic hydrogen on the acetylene-saturated pre-adsorbed Si(100)(2×1) surface and the surface phase transition at room temperature. It is evident that the atomic hydrogen has a strong effect on the adsorbed C₂H₂ and the underlying surface structure of Si. The experimental results show that CH and CH₂ radicals co-exist on the Si surface after the dosing of atomic hydrogen; meanwhile, the surface structure changes from Si(100)(2×1) to a dominant of (1×1). These results indicate that the atomic hydrogen can open C=C double bonds and change them into C-C single bonds, transfer the adsorbed C₂H₂ to C₂H_x(x = 3,4) and break the underlying Si-Si dimer, but it cannot break the C-C bond intensively. The QMS results show that some C₄ species axe formed during the dosing of atomic hydrogen. It may be the result of atomic hydrogen abstraction from C₂H_x which leads to carbon catenation between two adjacent C-C directs. The C₄ species formed are stable on Si(100) surfaces up to 1100 K, and can be regarded as the potential host of diamond nucleation.     

A pyrolysis study on C4–C8 symmetric ethers

Pyrolysis of diethyl (C₄), di-n-propyl (C₆), di-isopropyl (C₆) and di-n-butyl (C₈) ethers were studied in a jet-stirred reactor between 720 and 1140 K, at 10 atm with an initial ether mole fraction of 0.1%. Major common pyrolysis products were observed to be CO, CH₄, H₂, and C₂H₄. All ethers produced the n/2 alcohol and olefin as products of molecular reaction to a small extent. Under pyrolysis conditions at 10 atm, hydrogen abstraction reactions by H atoms and CH₃ radicals were found to be important. Acetylene and benzene were formed for all ethers when T > 1000 K. A kinetic mechanism is used to represent these results. This study shows that there is need of systematic studies in determining site specific rate constants of important fuel related reactions of ethers.     

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.     

Structures and thermochemistry of methyl ethyl sulfide and its hydroperoxides: HOOCH2SCH2CH3, CH3SCH(OOH)CH3, CH3SCH2CH2OOH,and radicals

Hydroperoxides and the corresponding peroxy radicals are important intermediates during the partial oxidation of methyl ethyl sulfide (CH₃SCH₂CH₃) in both atmospheric chemistry and in combustion. Structural parameters, internal rotor potentials, bond dissociation energies, and thermochemical properties (ΔH_f^o, S^o and C_p(T)) of 3 corresponding hydroperoxides CH₂(OOH)SCH₂CH₃, CH₃SCH(OOH)CH₃, CH₃SCH₂CH₂OOH of methyl ethyl sulfides, and the radicals formed via loss of a hydrogen atom are important to understanding the oxidation reactions of MES. The lowest energy molecular structures were identified using the density functional B3LYP/6‐311G(2d,d,p) level of theory. Standard enthalpies of formation (ΔH_f^o₂₉₈) for the radicals and their parent molecules were calculated using the density functional B3LYP/6‐31G(d,p), B3LYP/6‐31 + G(2d,p), and the composite CBS‐QB3 ab initio methods. Isodesmic reactions were used to determine ?H_f^o values. Internal rotation potential energy diagrams and rotation barriers were investigated using the B3LYP/6‐31G(d,p) level theory. Contributions for S^o₂₉₈ and C_p(T) were calculated using the rigid rotor harmonic oscillator approximation based on the structures and vibrational frequencies obtained by the density functional calculations, with contributions from torsion frequencies replaced by internal rotor contributions. The recommended values for enthalpies of formation of the most stable conformers of CH₃SCH₂CH_2, CH₂(OOH)SCH₂CH₃, CH₃SCH(OOH)CH₃, and CH₃SCH₂CH₂OOH are ?14.0, ?33.0, ?37.2, and ?32.7 kcal/mol, respectively. Group additivity values were developed for estimating properties of structurally similar and larger sulfur‐containing peroxides. Groups for use in group additivity estimation of sulfur peroxide thermochemical properties were developed.     

Veredelung industrieller Erzeugnisse durch den Ersatz von Wasserstoff durch Deuterium

Es wurden folgende isotopisomere Formen des Bernsteinsäuredibenzylesters synthetisiert:

C₆H₅CH₂–OOCCH₂CH₂COO–CH₂C₆H₅, (I)

C₆H₅CD₂-OOCCH₂CH₂COO–CD₂C₆H₅, (II)

C₆H₅CH₂–OOCCD₂CD₂COO7ndash;CH₂C₆H₅, (III)

C₆H₅CD₂-OOCCD₂CD₂COO–CD₂C₆H₅. (IV)

Die Gesehwindigkeit der Autoxydation dieser Verbindungen bei 191 °C zeigt Isotopieeffekte der Größe k₁/k₁₁ = 4,3 ± 0,5; k₁/k₁₁₁ = 1,0 und k₁/k_iv = 4,6 ± 0,5, bezogen auf die Maximalgeschwindigkeit. Durch die Anwesenheit des Inhibitors Phenyl-a-naphthylamin (0,1%) werden die maximalen Geschwindigkeiten der Autoxydation nicht beeinflußt. Die Induktionsperioden werden hingegen bei den Verbindungen (I) und (II) in sehr unterschiedlicher Weise verändert, so daß die Induktionsperiode von (II) 40mal länger als die von (I) wild.     

A study of the relative brönsted acidities of surface complexes on Ag(110)

The relative acidities of a number of Brönsted acids have been established on the Ag(110) surface under UHV conditions. For acids which react completely with adsorbed oxygen atoms on this surface to form H₂O, relative acidities were determined by means of acid-base titration reactions. Adsorbed species such as carboxylates, alkoxides, etc., were formed by reaction of the parent acids with O(a) and then displaced from the surface by titration with stronger acids. Relative acidities of the acids which did not react to completion with O(a) were established on the basis of their relative extents of reaction. The relative acidity scale on Ag(110), according to the reaction BH(g) + B'(a) B'H(g) + B(a) was found to be HCOOH ≈ CH₃COOH>C₂H₅OH> C₂H₂>CH₃OH>C₃H₆, H₂O>C₂H₄, C₂H₆, H₂. This order is in excellent agreement with the acidity scale for these species in the gas phase according to BH(g)B^?(g) + H⁺ (g); it cannot be explained by aqueous dissociation constants or homolytic bond dissociation energies. This result is in accord with the appreciable anionic character of the adsorbed species, since the electron affinity of the base, B, is a strong thermodynamic factor in the acidity in the gas phase. Both XPS and UPS results for adsorbed species on the Ag(110) surface are consistent with this interpretation.     

Theoretical studies on the nature and strength of an intermolecular non-covalent Te•••π interaction

ABSTRACT

DFT and MP2 calculations were used to determine the nature of non-covalent tellurium–π interactions in R₂Te???C₆H₆ (R = H, F, Cl, CH₃) and C₄H₄Te???C₆H₆ systems. The results showed that the strength of Te···π interaction follows the order F₂Te > Cl₂Te > tellurophene > H₂Te > Me₂Te. Also, the F₂X···π system complexes (X = Te or Se, π system = C₆F₆, C₆(CH₃)₆, Cr(C₆H₆)₂ and coronene) were studied for investigating the direction of charge flow in Te···π interaction. The obtained data expressed that the existence of electron withdrawing group on Te atom increases the strength of Te???π interaction while the fluorine atoms on benzene ring decrease it. The breakdown of ΔE_int in the R₂Te···C₆H₆ (R = H, F, Cl, CH₃) and C₄H₄Te···C₆H₆ systems using two dispersion corrected DFT methods showed that when the R group on divalent tellurium atom is an electron withdrawing substituent, the contribution of ΔE_elstat and ΔE_orb in total interaction energy increases and the value of ΔE_int is relatively large. The present data also showed that the intermolecular Te···π interactions are slightly stronger than corresponding Se···π interactions.     

Hydrogen-assisted fabrication of spherical gold nanoparticles through sonochemical reduction of tetrachloride gold(III) ions in water

Spherical gold nanoparticles (AuNPs) were selectively synthesized through sonochemical reduction of tetrachloride gold(III) ions ([AuCl₄]⁻) in an aqueous solution of hydrogen tetrachloroaurate(III) tetrahydrate (HAuCl₄·4H₂O) with the aid of hydrogen (H₂) gas in the absence of any additional capping agents. On the other hand, various shaped-AuNPs such as spherical nanoparticles, triangular and hexagonal plates were formed from sonochemical reduction of [AuCl₄]⁻ in argon (Ar)-, nitrogen (N₂)- or oxygen (O₂)-purged aqueous [AuCl₄]⁻ solutions. The selective fabrication of spherical AuNPs assisted by H₂ gas is most likely attributed to the generation of hydrogen radicals (H) promoted by the reaction of H₂ introduced and hydrogen oxide radicals (OH) produced by sonolysis of water.     

Oxidation of di-n-propyl ether: Characterization of low-temperature products

The oxidation of di-n-propyl-ether (DPE) was performed in a jet-stirred reactor at 1 and 10 atm, at residence times of 1 and 0.7 s, respectively, and initial fuel concentrations of 5000 and 1000 ppm at 1 and 10 atm, respectively. Atmospheric pressure experiments were used for characterization of cool flame products. The 10 atm experiment provided KHPs profile vs. temperature and mole fraction profiles of stable species which were obtained through sonic probe sampling, gas chromatography, Fourier transform infrared spectrometry analyses. High resolution mass spectrometry analyses (HRMS) with syringe direct injection or ultra-high-pressure liquid chromatography coupling was used to characterize hydroperoxides (C₃H₈O₂, C₆H₁₄O₃), diols (C₆H₁₄O₃), ketohydroperoxides (C₆H₁₂O₄), carboxylic acids, and highly oxygenated molecules (C₆H₁₂O₆, C₆H₁₂O₈) resulting from up to four O₂ additions on fuel's radicals. Heated electrospray and atmospheric pressure chemical ionizations (HESI and APCI) were used in positive and negative mode. Whereas the CH₂ groups neighboring the ether function are the most favorable sites for H-atom abstraction reactions, speciation indicated that other sites can react by metathesis forming a large pool of intermediates. Our kinetic reaction mechanism represents the experimental data for most of the stable species but need to be expended for simulating the formation of newly detected species.     

Exceptionally high decarboxylation rate of a primary aliphatic acyloxy radical determined by radical product yield analysis and quantitative 1H‐CIDNP spectroscopy

Symmetrical (RCO₂CO₂R; R = XCH₂CH₂) and asymmetrical (RCO₂CO₂R′; R = C₉H₁₉CH₂CH₂, R′ = CH₃ or m‐ClC₆H₄) primary diacyl peroxides were thermally decomposed under different conditions to analyze the decarboxylation rates of the thermally generated acyloxy radicals. Quantitative models of the geminate product yields, and qualitative and quantitative ¹H‐CIDNP spectroscopy were used to obtain the decarboxylation rate estimates. Results reported here suggest that, unlike short chain acyloxy radicals such as propanoyloxyl, long chain acyloxy radicals possess the highest decarboxylation rates of all known acyloxy radicals, estimated at (0.5–1.5) × 10¹² s^?1 between 80 and 140 °C. Given the nature of the dissociative state of acyloxy radicals, such rates appear to be the result of destabilization of the former by the steric bulk of the long chain substituents. Additionally, the rate of this order of magnitude suggests a nearly concerted decarboxylation of primary diacyl peroxides. Copyright © 2011 John Wiley & Sons, Ltd.     

Transformations of hydrocarbon radicals moving along a stainless steel tube

Transport of exhausted thermonuclear fuel in the ITER divertor and pumping duct was modeled on a specially designed dc glow discharge setup using mass spectrometry, optical and electron microscopy, and electron probe microanalysis. Transport and deposition of hydrocarbon radicals transferred in an H₂/C _x H _yx mixture through a hollow stainless steel anode at a total mixture pressure of 8–212 Pa and a methane content to 15 mol % were considered. It was shown that deposition of radicals and ions (CH₃, C₂H₃, C₂H₅) with kinetic energies of 0.03–3 eV on the anode inner surface at 600 K was suppressed to a large extent. In the temperature range of 600–800 K, deposition of ions and radicals with kinetic energy of ~3 eV was partially restored with the formation of soft a-C:H films, while thermalized radicals were not condensed.     

Atomic level analysis of biomolecules by the scanning atom probe

Utilizing the unique features of the scanning atom probe (SAP) the binding states of the biomolecules, leucine and methionine, are investigated at atomic level. The molecules are mass analyzed by detecting a single atom and/or clustering atoms field evaporated from a specimen surface. Since the field evaporation is a static process, the evaporated clustering atoms are closely related with the binding between atoms forming the molecules. For example, many thiophene radicals are detected when polythiophene is mass analyzed by the SAP. In the present study the specimens are prepared by immersing a micro cotton ball of single walled carbon nanotubes (SWCNT) in the leucine or methionine solution. The mass spectra obtained by analyzing the cotton balls exhibit singly and doubly ionized carbon ions of SWCNT and the characteristic fragments of the molecules, CH₃, CHCH₃, C₄H₇, CHNH₂ and COOH for leucine and CH₃, SCH₃, C₂H₄, C₄H₇, CHNH₂ and COOH for methionine.     

Adsorption and thermal reaction of short-chain alkanethiols on GaAs(1 0 0)

By means of temperature-programmed desorption (TPD) and X-ray photoemission spectroscopy (XPS) with synchrotron radiation, we investigated the adsorption and thermal decomposition of alkanethiols (RSH, R = CH₃, C₂H₅, and C₄H₉) on a GaAs(1 0 0) surface. All chemisorbed alkanethiols can deprotonate to form thiolates below 300 K via dissociation of the sulfhydryl hydrogen (-SH). Two types of thiolates species are observed on GaAs(1 0 0), according to adsorption on surface Ga and As sites. The thiolates adsorbed on a Ga site preferentially recombine with surface hydrogen to desorb as a molecular thiol at 350-385 K. The thiolate on the As site exhibits greater thermal stability and undergoes mainly dissociation of the C-S bond at ∼520 K, independent of the alkyl chain length. The decomposition of CH₃S either directly desorbs CH₃ or transfers the CH₃ moiety onto the surface. The surface CH₃ further evolves directly from the surface at 665 K. The dissociations of C₂H₅S and C₄H₉S yield surface C₂H₅ and C₄H₉, which further decompose to desorb C₂H₄ and C₄H₈, respectively, via β-hydride elimination. The complete decomposition of alkanethiol leads to the formation of surface S without deposition of carbon. Adsorption of CH₃SSCH₃ results in the formation of surface CH₃S at initial exposures via scission of the S−S bond. Compared with the adsorption of CH₃SH, the CH₃S on the Ga site exhibits greater thermal stability because surface hydrogen is absent. At a high exposure, CH₃SSCH₃ can absorb molecularly on the surface and decompose to desorb CH₃SCH₃ via formation of a CH₃SS intermediate.