A systematic study of the chemistry of stable molecules and radicals in a low pressure CCl2F2 radio frequency discharge for dry Si etching has been performed. Various particle densities have been measured and modeled. The electron density, needed as an input parameter to model the CCl2F2 dissociation, is measured by a microwave cavity method. The densities of stable molecules, like CClF3, CF4, 1,2-C2Cl2F4 and the etch product SiF4, are measured by Fourier transform absorption spectroscopy. The density of the CF2 radical is measured by means of absorption spectroscopy with a tunable diode laser. Its density is in the order of 10(19) m(-3). All density measurements are presented as a function of various plasma parameters. Moreover, optical emission intensities of Cl and F have been recorded as a function of plasma parameters. It appears that the feed gas (CCl2F2) is substantially dissociated (about 70%-90%) in the discharge. Based on the obtained data the dissociation rates of several molecules have been estimated. The measured total dissociation rate of is 8x10(-15) m(3) s(-1). For this molecule the dissociation rate is substantially higher than the dissociative attachment rate (10(-15) m(3) s(-1)). The dissociation rate for CClF3 is 2X10(-15) m(3) s(-1) about 3x10(-16). The total dissociation rate of C2Cl2F4 is higher than 5X10(-15) m(3) s(-1), Of which 2.5+/-0.5X10(-15) m(3) s(-1) is due to dissociative attachment. Furthermore it has been found that the presence of a silicon wafer strongly affects the plasma chemistry. Optical emission measurements show that the densities of halogen radicals are significantly depleted in presence of Si. Moreover, the densities of several halocarbon molecules display a negative correlation with the density of the etch product SiF4.