The reaction chemistry in the afterglow of a non-equilibrium, capacitive discharge, operated at 600 Torr total pressure with (0.5 to 5.0) x 10(17) cm(-3) of oxygen in helium, has been examined by ultraviolet absorption spectroscopy, optical emission spectroscopy, and numerical modeling. The densities of the active species, O(P-3), O-2((1)Delta(g)), O-2((1)Sigma(g)(+)), and O-3, have been determined as a function of the operating conditions. At RF power densities between 6.1 and 30.5 W/cm(3) and a neutral temperature of 100 +/- 40 degrees C, the plasma generated (0.2 to 1.0) x 10(16) cm(-3) of O(P-3) and O-2((1)Delta(g)), (0.2 to 2.0) x 10(15) cm(-3) of O-2((1)Sigma(g)(+)), and (0.1 to 4.0) x 10(15) cm(-3) of O-3. After the power was turned off, the singlet-sigma and singlet-delta states decayed within 0.1 and 30.0 Ins, respectively. The concentration of oxygen atoms remained constant for about 0.5 ms, then fell rapidly due to recombination with O-2 to form O-3. It was found that the etching rate of polyimide correlated with the concentration of oxygen atoms in the afterglow, indicating that the O atoms were the active species involved in this process.