Construction Of A Continuous Wave Cavity Ring-down Spectrosmeter And Its Application In Quantitative Measurements Of HO₂ And OH Radicals In Non-Equilibrium Plasmas

In non-equilibrium plasmas, the electrons may be much hotter (with temperatures in the range of tens of thousands up to a hundred thousand Kelvin) than the ions and neutrals, whose translational temperatures are essentially equal and typically range from room temperature to a few times the room temperature. Non-equilibrium plasmas thus represent environments where very energetic chemical processes can occur (via the plasma electrons) at low ambient temperatures. The generation of chemically reactive free radicals by electron impact dissociation in molecular plasmas is an important precursor for plasma chemical reactions. 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 and HO₂ radicals in dielectric barrier discharge plasmas via continuous wave cavity ring-down spectroscopy. The main results presented in the dissertation have been summarized as followings:1. An apparatus of continuous wave cavity ring-down spectroscopy (cw-CRDS) has beenconstructed for the fisrt time in China. The minimum measurable absorption coefficient for our setup is about 3×10^-8 cm^-1 in DBD plasmas, corresponding to an HO₂ radical number density of- 1×10¹¹ molecules cm^-3, which shows that the main technological characteristics of this instrument have reached the international level.2. HO₂ radical number density in non-equilibrium plasmas has been experimentally determined for the first time via cw-CRDS. HO₂ radicals were observed in discharges of HCHO/O₂/H₂O/N₂ mixtures around 6625.7 cm^-1 in the first H-OO stretching overtone, (2, 0, 0) - (0, 0, 0), of its ground electronic state (?)²A". At certain discharge conditions (a.c. frequency of 5 kHz, peak-to-peak voltage of 6.5 kV, 1900 ppm HCHO, 3.5% H₂O in N₂, P_total= 30 Torr), HO₂ concentration rises as oxygen concentration increases initially but declined with its further increase. The effects of water concentration, the discharge voltage and the discharge gas pressure on the concentration of HO₂ radicals have been investigated.The temporary evolution of HO₂ concentration within a half period of the sine wave applied voltage (U_P = 4.5 kV,f_a.c. = 5 kHz, 1900 ppm HCHO, 20% O₂, 3.5% H₂O in N₂, P_total = 30 Torr) has been studied using the method of "time window". The result shows that the concentration of HO₂ radical increases rapidly during the first 5μs following the first discharge current pulse. After its peak value, HO₂ concentration decreases slowly.3. The hydroxyl radical, generated in atmospheric DBD plasmas, has been determined via cw-CRDS for the first time. The P-branches of OH X²Î _i (v' = 2←v" = 0) bands were used for its number density measurements. P₁(7.5)e, P₁(6.5)e, P₁(6.5)f and P₁(5.5)e lines of X²Î _3/2 transition together with P₂(6.5)f and P₂(5.5)f lines of X²Î _1/2 transition were used to measure the OH rotational temperature (T_R). At certain experimental conditions (a.c. frequency of 70 kHz, 6700 ppm H₂O in He, 1 atm.), when the peak-to-peak discharge voltage varied from 6 kV to 10.4 kV, the determined OH radical concentration increased from (2.1±0.1)×10¹³ molecules cm^-3 to (3.7±0.1)×10¹³ molecules cm^-3. The plasma gas temperature, derived from the Boltzmann plots of OH rotational population distributions, ranged from 312±10 K to 363±10 K, when the discharge voltage was raised in the above range. The influences of O₂ and N₂ addition on the production of OH radicals have been also investigated.