Quantitative measurements ofC2(a3&Pgr;u),C3(X,1&Pgr;),andCH(X,2&Pgr;)have been made by calibrated linear laser-induced fluorescence in the plume of a dc-arcjet (Ar/H2/CH41.1:1:0.005) reactor during the chemical vapor deposition of diamond. The peak number density in the arcjet plume for CH is(3.7±0.8)×1012 molecules/cm3,forC2(3a)(3.1±1.3)×1010 cm−3,and forC3∼3×1012 cm−3with 25 Torr reactor pressure. The radial spatial distributions forC2and CH have a maximum in the center of the plume; however,C3is distributed as a hollow cylinder with a pronounced minimum at the center. The variation in number density and in spatial distribution is investigated for changes in chamber pressure, methane flow, and distance from the nozzle. ©1997 American Institute of Physics.