Using the method of reabsorption the absolute densities of argon, atomic and molecular carbon are determined during the deposition of amorphous hydrogenated (diamond-like) carbon coatings by an expanding thermal are plasma. Depending on the gas mixture argon/methane or argon/acetylene and the manner of hydrocarbon injection, the stationary negative or positive absorption between the quantum states Ar(3p54p to 3p54s). C(2p3s to 2p2) and C2(d3 Pi g, v'=0 to a3 Pi u, v''=0) were obtained. From the line absorption the absolute population densities of the radicals in the plasma have been determined. The density of the argon first excited state Ar(3p54s, 3P2) is approximately=1016-1017 m-3 the atomic and diatomic molecular carbon densities are of the order of 1018-1019 m-3.