Microwave and Diode Laser Spectroscopy of Discharge and Flame Plasma

Abstract

The gas phase pure rotational spectrum of the monoiodomethyl radical (CH2I· radical), has been observed for the first time in the millimeter-wave region using a frequency modulated microwave spectrometer equipped with a 2 m long free space absorption cell. CH2I· radical was generated by the reaction of either diiodomethane (CH2I2) or iodomethane (CH3I) with atomic chlorine (Cl·) by microwave discharge 2450 MHz products of chlorine molecules (Cl2) diluted in helium. The 331 millimetre-wave a-type R-branch transitions have been observed with fully resolved fine and partly resolved hyperfine components for N’ ≤ 35, Ka ≤ 6 in the ground vibrational state. The small positive inertial defect, 0 = 0.03665(3) amu.A2, calculated from the rotational constants obtained for CH2I· radical indicates that the radical is planar in the ground vibronic state. The observed fine and hyperfine interaction constants are consistent with 2B1 symmetry, i.e. with the unpaired electron occupying a pπ, orbital extending perpendicular to the molecular plane. An experimental set-up for diode-laser infrared spectroscopy was optimized. The molecular ion ArD+, CN· radical and molecules C3H4 and OCS were detected and identified. Based on the mathematical evaluation of the calculated signal to noise ratios of absorption spectra recorded with the use of lock-in detection optimal values of frequency and amplitude modulation were found. The sensitivity of the detection method is also discussed in the frame of this part as well as an optimization of experimental set up based on the optical path length. The concept of the Allan variance has been utilized to test the detection possibilities of two powerful experimental methods for trace gas monitoring - the diode-laser and the CO2 laser photoacoustic spectroscopy. Detection of stable molecules (C3H4, OCS, CH3OH), and reactive unstable species (O3·, CN·, ArD+) have been compared by Allan variance calculations. An approach for studies of influence of the reactivity, respectively lifetime on the optimal averaging time for the minimum detectable concentration is demonstrated.