Chemical reactions of carbon atoms and molecules from laser-induced vaporization of graphite, TaC and WC

Abstract

The vapor composition above graphite, tantalum carbide and tungsten carbide together with the chemical reactions of C1, C2 and C3 have been studied by the techniques of cw laser vaporization, matrix isolation, and FTIR spectroscopy. The carbon species were scavengered by cocondensing reactive gases (N2, H2 and CO) with the vapors of graphitic samples in Ar or N2 matrices and then identified in the infrared. From these experiments, N2 is shown to be an ideal scavenger molecule for detecting C1, by the formation of CNN, while H2 is shown to scavenge only the C2 molecule, by forming acetylene. However, CO is shown to be potentially the most useful scavenger molecule since it reacts with both C1 and C2 to form C2O and C3O, respectively. The formation of CNN and C2H2 in N2 matrices is explained by excited state carbon reactions, whereas the formation of all products in reactant gas/Ar matrices is explained by ground state carbon reactions. By using neat N2 matrices, we have measured the CNN:C3 ratio for graphite, TaC and WC samples. From these results, the activity of carbon in WC was determined to be 0.4 while the activity of carbon in TaC was 0.2. The results of this study provide a foundation for understanding the elementary reactions of carbon atoms and molecules and for determining the vapor composition over graphites and carbides at very high temperatures.