High-resolution Laser Spectroscopy Of The Silicon-bearing Molecules

Silicon-bearing molecules are considered as precursor molecules for the formation of interstellar silicon-containing dust.High-resolution spectroscopic studies of siliconbearing molecules are the basis for in-depth understanding of molecular structure and bonding properties,and provide direct data support for the study of interstellar silicon chemistry model.This thesis is mainly organized in two major categories:Construction of slit jet plasma cavity ring-down(SJP-CRD)spectroscopic instrument.The high-resolution spectroscopy of some small silicon-bearing molecules are studied by combining the SJP-CRD and laser induced fluorescence(LIF)spectroscopy.We systematically studied the electronic transition spectra of Si2,r-Si4,SiC2,and l-Si2C2 molecules.The main contents and related achievements are as follows:1.Construction of SJP-CRD spectroscopic instrumentA high sensitive slit jet plasma cavity ring-down(SJP-CRD)spectroscopy instrument is constructed to perform high-resolution laser spectroscopy of transient molecules.There are two parts of this instrument:the supersonic jet plasma for the efficient preparation of ro-vibration cooling free radicals and the cavity ring-down for high sensitive detection.The supersonic jet plasma is generated by a 30 mm×0.2 mm slit discharged nozzle.In our test experiment,negative high voltage pulses are used to discharge a high pressure mixture gas of 1%C2H2/Ar to achieve the high production of the C6H transient molecules with a concentration as high as 1012 molecuIes/cm3 and<40 K cooling temperature.Using the SJP-CRD instrument,the recorded high-resolution spectra of Si4 molecules shows that Doppler linewidth can be compressed to?v/v-6×10-7.2.Spectroscopic survey of Si2 moleculesRotationally-resolved spectra of the H3?u--X3?g-electronic transition bands of Si2 have been experimentally studied using LIF in the 380-520 nm range.We experimentally record 44 bands and 11 bands belonging the 28Si2 and two isotopologues(29Si28Si and 30Si28Si)to the H3?u--X3?g-electronic transition system,respectively.The good agreement between the experimentally determined spectroscopic constants of the isotopologue bands and those calculated from the main isotopologug 28Si2 shows that the assignment of all bands are uncontroversial.Detailed analyses on the high-resolution spectra enable to produce the accurate spectroscopic constants for both the H3?u-and the ground state,including the spin-splitting constants determined for the first time.Franck-Condon factors(qv'v"),Einstein coefficients(Av'v"),and oscillator strengths(fv'v")are experimentally determined using the molecular constants and the dispersed fluorescence spectra.3.Spectroscopic survey of r-Si4 moleculeThe absorption spectra of r-Si4 has been investigated using the slit jet plasma cavity ring-down(SJP-CRD)spectroscopy.The spectroscopic survey shows that two electronic transitions of the 1B1u-X1Ag and 1B3u-Ag are observed.In total,56 bands are experimentally recorded,among which 18 bands are hot bands transitions of vibrationally-excited state.Two frequencies of v1" and v2" for the ground state,Two frequencies of v1' and v2' for 1B1u state,and v2" for 1B3u state are obtained by analyses of vibrational assignment.The rotational constants of the 1B3u and the ground state for the r-Si4 are firstly experimentally determined by fully resolved rotational analysis.The Pseudo Jahn-Teller effect of molecule is Si4 determined from the rotational constants which obtained from current experiment.4.Spectroscopic study of vibrationally-excited states of SiC2Rotationally-resolved spectra of 1v1",1v2" and 6v3" of the SiC2 ground state are experimentally recorded using high sensitive detection of stimulated emission pumping(SEP)spectroscopy.By means of pumping the 20 band and probing the 110201 band,the rotational levels transition of the 1v1" vibrationally-excited state are measured and the accurate spectroscopic constants are obtained by rotational analysis and the assist of two microwave spectral line and the theoretical data.By means of pumping the 000 band and probing the 210+360 bands,rotational spectra of two fermi resonant states 1v2" and 6v3" are experimentally recorded which enable to produce accurate spectroscopic constants and rotational levels transition frequency.The rotational analysis of combination fits enable to produce the accurate spectroscopic constants for 1v2" and 6v3" vibrationally-excited states.The Fermi resonance strength are also determined from high-resolution dispersed fluorescence and rotational analysis.5.The C3?u--X3?g-transition spectra of l-Si2C2High-resolution spectra of l-Si2C2 molecule are experimentally recorded with 5 bands belonging to C3?u-X3?g-transition by using slit jet plasma cavity ring-down(SJP-CRD)spectroscopy,among which 2 bands are vibrationally-excited state transition.Detailed analyses on the highresolution spectra enable to yield an accurate determination of spectroscopic constants for both the C3?u-and the ground state,including the spin-splitting constants,spin-rotation interaction constants and centrifugal distortion constants.Using the experimentally determined spectroscopic constants,optical spectra with different rotational excitation temperatures are reproduced and compared with stellar spectra of evolved carbon stars where the triatomic SiC2 are known to be abundant.