STUDIES OF THE COMPOSITION AND REACTIVITY OF CARBON SPECIES GENERATED THROUGH LASER VAPORIZATION OF GRAPHITE, TANTALUM-CARBIDE AND TUNGSTEN-CARBIDE

[摘要] The vapor compositions above graphite, tantalum carbide and tungsten carbide together with the chemical reactions of C$sb1$, C$sb2$, and C$sb3$ have been studied by the techniques of CW laser vaporization, matrix isolation, and FTIR spectroscopy. The carbon species were scavenged by cocondensing reactive gases (N$sb2$, H$sb2$, O$sb2$, CO and H$sb2$O) with the vapors of graphitic samples in argon or nitrogen matrices. Products were identified from their infrared spectra. Photolysis and annealing studies were also conducted to detect reaction intermediates, formation of carbon clusters, and metastable C$sb1$, C$sb2$ and C$sb3$ adducts with the added reactants. From these experiments CO was shown to be potentially the most useful scavenger molecule, since it reacted with both C$sb1$ and C$sb2$ to form C$sb2$O and C$sb3$O, respectively. CO also reacted with C$sb3$ to form an adduct, which rearranges upon photolysis to form C$sb4$O, a previously unknown molecule. Its identity was confirmed by isotopic studies and comparison of its vibrations to those of related molecules, C$sb3$O, C$sb2$O and CO. In H$sb2$O/carbon studies, only C$sb3$ reacted with H$sb2$O upon cocondensation to form an adduct. In O$sb2$/carbon studies, the formation of CO$sb2$ and CO has been explained in terms of reactions of C$sb1$ and C$sb2$, respectively. At high O$sb2$ concentrations the observance of O$sb3$ was explained as a reaction of C$sb1$ with dimer of O$sb2$ to form CO and O$sb3$. These results were supported by the decrease of CO$sb2$ as the O$sb2$ concentration was increased. In H$sb2$/carbon studies only C$sb2$H$sb2$ was detected upon either cocondensation or photolysis. Therefore, it is suggested that only C$sb2$ reacts with H$sb2$ on a matrix surface at 12$spcirc$K. When graphite, TaC and WC samples were laser vaporized into a nitrogen matrix, the C$sb1$ concentration was detected by the formation of CNN. By measuring the CNN to C$sb3$ peak area ratio and comparing it to that for graphite, the activity of carbon in WC was determined to be 0.4, while the activity of carbon TaC was determined to be 0.2. The IR frequencies of the C$sb4$, and C$sb6$ and C$sb9$ carbon clusters, previously assigned by Weltner, were confirmed in this study. (Abstract shortened with permission of author.)

[发布机构] Rice University