Diagnosis Of The OH Radical In Microwave Discharge Plasmas Using Cavity Ring Down Spectroscopy

The generation of chemically reactive free radicals in molecular plasmas is an important precursorfor plasma chemical reactions.Non-equilibrium plasmas thus represent environments where very energetic chemical processes can occur(via the plasma electrons) at low ambient temperatures.Therefore,measurements of the radical number density are needed to characterize experiments and study physico-chemical processes.However,determining the radical concentration is a challenging problem in the atmosphere or in laboratory experiments because of its short lifetime and low concentration.In order to improve the detection sensitivity,various ultra-sensiive techniques have been developed.The technique of cavity ring-down spectroscopy(CRDS) has been developed in 1980s.The advantage of CRDS over normal absorption spectroscopy results from,firstly,the intrinsic insensitivity to light source intensity fluctuations and,secondly,the extremely long effective path lengths.In the past decades,the CRD technique has been especially powerful in gas-phase spectroscopy for absolute concentration measurements of trace gas.The present work reports the determination of OH radicals in microwave plasmas via continuous wave cavity ring-down spectroscopy.In this work,the P-branches of OH X²âˆ_i(v′=2←v″=0) bands were used for its number density measurements,and the OH rotational temperature(T_R) was measured by Boltzmann plots.The influence of pressure and microwave power on number density of OH radicals has been investigated.The results show that at certain experimental conditions(in H₂O/N₂ system, 500W of microwave power),in the range of 5-30 Torr,the OH number density increases before 15 Torr with increasing pressure and decreases at higher pressure.The OH number density increases with increasing microwave power at 15 Torr.The influences of O₂ and He addition on the production of OH radicals have been also investigated.In this work,initially cavity enhanced absorption spectroscopy(CEAS) has also been discussed.It is showed that CEAS is a highly sensitive spectrum technology which has the advantages of simple experimental setup and easy operation.