The carbon molecules C4, C5, C6, and perhaps C9, have been formed in argon matrices by the reaction of C, Cz, and C3. Highly enriched I3C graphite was vaporized to form the isotopically substituted molecules. Analysis of the ir spectra of" pure" and mixed-isotope molecules made possible definite vibrational assignments for the &+ frequencies. For IC 4, Ws, and 12C6 these are 2164; 1952, 1544; and 1997, 1197 cm-l, respectively. The data on each molecule, including mixed-isotope species, were? sed to calculate stretching force constants. C4 was found to have a strong CsC outer bond withf= 16.5 mdyn/A, in general agreement* with theory. The force constants for the two outer bonds of C6 were reliably determined to be 11.9 and 8.1 mdyn/A. Larger carbon molecules were formed but not identified. t has been established that the vapor over graphite at 2500 K contains C, CZ, and C3 m~ lecules~-~ which have now been rather thoroughly studied. 51~ Mass spectrometry'has also detected very small quantities (3) zyxwvutsrqponmlkjihg