Spectroscopy Study Of Several Free Radicals By Cavity Ringdown

The free radicals play an important role in many fields, such as combustion process, atmosphere chemistry, and analytical chemistry, et al.. The spectroscopy of free radicals can provide important information in understanding their structures, detecting them, and identifying the mechanisms and branch ratios of related chemical reactions. The main contributions presented in this dissertation are a newly built cavity ringdown spectroscopy (CRDS) apparatus, and the spectroscopy study on the PH2, AsH2 and transition metal containing radicals, by using CRDS combined with the technique of gas discharge both in flow system and supersonic molecule beam.Building the experimental apparatusA pulsed CRDS apparatus used to study radicals'spectra in the flow system was designed and assembled. The hollow cathode discharge with double anodes was employed to produce target radicals. By the experiments of measuring the reflectivity of cavity mirrors and the absorption spectra of H2O, the reliability of the CRDS apparatus was verified to be perfect: a absorption detection limit of 4×10-3(1-R), and Doppler-limited spectral resolution in the obtained spectra.Another apparatus used in this dissertation, was built by transplanting the CRDS apparatus to an old vacuum chamber which were used to produce the supersonic molecule beam in our lab. The pulsed DC discharge under the nozzle was used to produce target radicals in this apparatus.Spectroscopy study of the PH2 radicalThe absorption spectra of jet-cooled PH2 radicals were recorded in the wavelength range of 410-555 nm by CRDS. The PH2 radicals were produced in a supersonic jet by pulsed DC discharge of a mixture of PH3 and SF6 in argon. Ten vibronic bands with fine rotational structures have been observed and assigned as the 00 0, 2 0n(n=1–6), and 21 n(n=1–3) bands of the A 2 A1 ? X 2B1 electronic transition. From the rotational and vibrational analysis, the molecular parameters, including rotational constants, centrifugal distortion constants, spin-rotation interaction constants and vibrational constants for X 2 B1 21 and A 2 A1 2 n ( n = 0 ? 4)states, were determined with reasonably high precision. In addition, large perturbations observed in each Ka level of the excited vibronic states were briefly discussed. The lifetimes of the rot-vibronic levels in A 2 A1 state of PH2 were obtained by measuring the fluorescence decay rate using laser induced fluorescence (LIF) technology. Based on the comparison of the spectra obtained by LIF and CRDS, respectively, and the obtained lifetimes of the rot-vibronic levels, the perturbations in the A 2 A1 state and the possible predissociation of PH2 were discussed.Spectroscopy study of the AsH2 radicalThe absorption spectra of jet-cooled AsH2 radical were obtained in the wavelength range of 380-510 nm by CRDS. The AsH2 radicals were produced in a molecular beam of a mixture of AsH3, SF6, and argon. Eleven vibronic bands with fine rotational structures have been identified and assigned as the 0 00, 2 0n(n=1–7), and 21 n(n=1–3) bands of the A 2 A1 ? X 2B1 electronic transition. Based on the previous studies of AsH2 radical, rotational assignments and rotational term values for each band were obtained, and the molecular parameters including vibrational constants, rotational constants, centrifugal distortion constants, and spin-rotation interaction constants were also determined with reasonably high precision. By investigating the spectral linewidths of the rotational lines in each band, the lifetimes of the rot-vibronic levels in A 2 A1 state was obtained, and the predissociation pathways in the A 2 A1 state of AsH2 radical were discussed in detail.We also studied the A 2 A1 ? X 2B10 00 band of AsH2 in the room temperature by CRDS. The AsH2 radicals were produced by the hollow cathode discharge of AsH3/Ar. Several high order spectra parameters in A 2 A1 00 state were obtained, and the perturbations in the higher rotational levels in A 2 A1 00 state were briefly discussed. Pulsed CRDS on the transition metal containing radicals in hollow cathode discharge plasmaThe cavity ringdown spectroscopy combined with hollow cathode discharge plasma is presented for the absorption spectroscopy study on the transition metal containing radicals. The performance is demonstrated with the spectra of the CuF B1Σ+ ? X1Σ+ transition and the CuBr b 3Π+ ? X1Σ+ transition. Rotational resolved spectrums with the spectral linewidth of the order of 0.055 cm-1 have been obtained in the room temperature. The absorption detection limit can be up to the order of 4×10-7 absorption.