It was well known that atomic fluorine and oxygen radicals play a vital role in plasma etching processes. This thesis investigates the density of fluorine and oxygen radical species using different diagnostic techniques in a parallel-plate capacitively coupled reactive ion etching plasma source. The behaviour of absolute atomic fluorine density in SF6/O2/Ar plasma was investigated using appearance potential mass spectrometry as a function of feedstock mixture, gas pressure and applied rf power. Contrary to naive expectations, atomic fluorine density was found to increase with dilution of O2 in SF6 discharge operated at 100 W and reached a peak value at ≈ 20-30% O2 content. This increase in fluorine atom density can be due to decrease in fluorine atom loss rate at walls through surface recombination and production of fluorine atoms through gas- phase reactions involving SFx (where x = 1-5) and atomic oxygen. However, atomic fluorine density was found to decrease with further addition of oxygen to discharge due to decrease in SF6 partial pressure. Absolute fluorine atom density investigated using APMS technique, was found to increase with increase in gas pressure in SF6/O2/Ar discharge (70/26/4 %) operated at 100 W rf power. Increase in fluorine atom density can be mainly due to increase in SF6 partial pressure as electron density was found to be in weak correlation with pressure. A non-invasive, compact and low cost industrial sensor would always be essential to monitor any variations in the plasma. Optical emission spectroscopy was one of such diagnostic tool and using it relative variations in radical density in plasma discharge can be monitored by a popular technique known as actinometry. In this thesis, fluorine actinometry technique was adopted to investigate relative fluorine density variations in capacitively coupled discharge and for validation of fluorine actinometry, relative [F] measurements were compared with APMS measurements. At low pressure conditions, actinometric limitation was violated as relative [F] measured using actinometry had poor correlation with APMS measurements. This could be largely due to additional excitation contribution to fluorine actinometric signal. With negligible changes to discharge characteristics, addition of significant amount of O2 to SF6 discharge could possible suppress additional excitation contribution to fluorine signal as relative [F] measured using actinometry and APMS established good agreement. At higher pressures, fluorine actinometric data was proportional to APMS measurements and thus, fluorine actinometric behaviour was validated. Proportionality constant (K) required for fluorine actinometry, was evaluated and its dependence at different discharge conditions were analysed. Kinetics behaviour of electrons in O2 discharge as a function of gas pressure was investigated using Langmuir probe. Discharge transition in oxygen plasma was investigated from pressure evolution of electron energy probability function (EEPF), electron density and temperature. However, structure was observed in measured EEPF for pressure > 400 mTorr at 200 W rf power and EEPF tail found to enhance which violated inverse proportionality between ionization rate and neutral gas density in accordance to particle balance equation. Such unusual EEPF characteristic was caused mainly due to inadequate rf compensation in the probe and unaccounted variations in plasma potential when probe tip was biased. In addition to this, the behaviour of absolute [O] was investigated as a function of gas pressure using actinometry and Langmuir probe. Good agreement was achieved when relative variations of [O] from actinometry were compared to TALIF measurements under similar experimental conditions. Actinometric technique was successfully validated in monitoring the variations of ground state fluorine and oxygen for discharge pressures ≥ 200 mTorr and can be adopted in industry to monitor radical species in processing plasmas. It should be mentioned that optical emissions lines used in this work to monitor [F] and [O] could have significant dissociative contribution at low pressures.