The thermal decomposition of vinylacetylene (C4H4) has been studied in the shock tube with two time-resolved diagnostics, laser-schlieren (2% and 4% C4H4-Kr, 1650-2500 K, 110-427 Torr) and time-of-flight mass spectrometry (2% C4H4-Ne, 1500-2000 K, 150-300 Torr). The time-of-flight mass spectra show dominant products C2H2 and C4H2 with a very consistent 5:1 ratio of C2H2 to C4H2, in essential agreement with earlier shock tube results. The laser-schlieren semilog density gradient profiles are all concave-upward, showing no trace of chain acceleration. Analysis of these profiles also sets the effective heat of reaction between 30 and 50 kcal/mol. Rate constants calculated from the zero-time gradients assuming 7/°298 = 40 kcal/mol are fit with a routine RRKM model which indicates a barrier E0 = 79.5 ± 3 kcal/mol. These observations and the time-of-flight product profiles are consistent with the molecular reactions C4H4 ->-2C2H2 ( /7°298 = 39 kcal/mol) and C4H4 -*• C4H2 + H2 ( #ß298 = 42 kcal/mol). Detailed balance rate constants for 2C2H2 -* C4H4 are in good agreement with the extensive previous data on the second-order acetylene reaction, confirming that vinylacetylene is a direct and dominant product of C2H2 dimerization for moderate temperatures. Above 1500 K, the C4H4 dissociation shows significant falloff even for high pressures, and the now dominant C4H2 in C2H2 pyrolysis may then be formed in part through 2C2H2 -G4H2 + H2. It is proposed that vinylacetylene dissociates as a substituted ethylene, either by 1,1-elimination of molecular hydrogen, leaving vinylidene acetylene which rapidly isomerizes to C4H2, or by 2,2-elimination of C2H2, followed by rapid isomerization of the vinylidene to a second C2H2. This mechanism is consistent with the observed barrier, provides a rationale for the constancy of the C2H2/C4H2 ratio, and offers a reasonable explanation for the magnitude (A factor) of the observed rate constants.
The structure of MnxSi1-x magnetic semiconductor thin films prepared by molecular beam epitaxy(MBE) on Si(100) substrate at 600 ℃ has been studied by X-ray diffraction (XRD) and X-ray absorption near edge structure (XANES) technique. The XRD results show that in the MnxSi1-x thin films with high Mn doping concentrations (x=0.08 and 0.17), only diffraction peaks of crystalline Mn4Si7 are observed. XANES results indicate that all the Mn K-edge XANES spectra of MnxSi1-x thin films with different Mn doping concentrations (x=0.007, 0.03, 0.08 and 0.17) show the similar feature. XANES calculation based on multiple-scattering theory further reveals that the experimental spectra for samples with different Mn doping concentrations are reproduced by the calculated Mn4Si7 spectrum. These results reveal that for the MnxSi1-x magnetic semiconductor thin films, Mn atoms mainly exist in the Si thin film substrate in the form of Mn4Si7 nanocrystalline grains, the substitutional or interstitial Mn atoms scarcely exist.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.