Laser Spectroscopy Of Resonance Stabilized Radicals

Resonance stabilized radicals(RSRs)play an important role in processes of combustion chemistry,interstellar chemistry and atmospheric chemistry.Spectroscopic studies of RSRs can help for a comprehensive understanding of bonding and electronic properties,and provide necessary information for the study of interstellar chemistry and combustion kinetics.This thesis focuses on the high-resolution gas-phase spectroscopy of the1,4,6-heptatrienyl radical(C7H9),indenyl radical(C9H7),1-indanyl radical(C9H9)and indene cation(C9H8+)studied by using laser-induced fluorescence(LIF)and cavityring down spectroscopy(CRDS).Meanwhile,a new experimental setup combining a jet-stirred reactor and CRDS has also been constructed and applied for online detection of the intermediate products of indene pyrolysis.1.Electronic spectra of the 1,4,6-heptatrienyl radicalThe rotationally resolved gas-phase absorption spectra of the A2B1-X2A2 transition of the 1,4,6-heptatrienyl radical are reported for the first time.Vibronic assignments of the spectra are made based on an accompanying Franck-Condon calculation of the excitation spectrum.The rotational constants for both A2B1 and X2A2 states are determined from analysis of the experimental spectra.By combination with available data for the ally radical and 1,4-pentadienyl radical,we apply a new exponential function to fit the curve of excitation energy gap change versus the chain length,which yields the excitation gap converging to 0.8 eV for the C2n-1H2n+1 chain as n goes to infinity.2.Gas-phase optical absorption spectra of indenyl and indene cationIndene and related radical/cationic species play an important role in the interstellar chemistry.We report for the first time the gas-phase optical absorption spectra of indenyl in the 4300-4700 ? region and indene cation(In+)in the 5500-5800 ? region,respectively.The spectra are experimentally recorded using sensitive cavity ring-down spectroscopy in a supersonically expanding planar plasma.A set of spectroscopic parameters for both species have been obtained from analyses of the experimental data in combination with quantum chemical calculations.Upper limits for indenyl and In+column densities in diffuse interstellar medium have also been estimated for six sightline spectra.3.Rotationally resolved spectrum of the A2A"-X2A" 000 band of 1-indanyl and its resonance stabilityThe 1-indanyl radical is a benzylic chromophore.It has attracted great interest to study the electronic spectra of 1-indanyl due to its important role in understanding the bonding properties of benzylic moieties.The rotationally resolved spectrum of the A2A"-X2A" 000 band of jet-cooled 1-indanyl near 473 nm has been recorded by laser induced fluorescence with a spectral resolution of～0.014 cm-1.The accurate spectroscopic constants for both A2A" and X2A" states of 1-indanyl are determined by a rotational analysis of the experimental spectrum.These indicative spectroscopic parameters are applied to test the calculated structure of 1-indany1.The calculated geometries,molecular orbital configurations and spin densities provide theoretical evidence that the bonding properties and the pπ electron delocalization in the 1-indanyl is very similar with the benzyl radical.The high stability of 1-indanyl is due to the increased pπ electron delocalization through a conjugated π-system.4.Development of a JSR-CRDS instrumentThe quantitative and online measurement of fuel pyrolysis intermediates is the key to building combustion chemistry models.We develop a new instrument that couples a jet-stirred reactor with cavity-ring spectroscopy(JSR-CRDS),aiming for an online detection of the intermediate products in combustion and pyrolysis.The instrument can be used to study the kinetics of oxidation of hydrocarbons at temperature up to 1200 K.To test the performance of the instrument,we have studied the oxidation of butane at 650 K.The CH2O,H2O,and CH4 molecules have been identified by analyzing the absorption spectra.The concentration of CH2O is determined to be 8 × 1015 molecule/cm3.5.The optical diagnosis of indenyl in indene pyrolysisIndene(C9H8)is a typical bicyclic aromatic species,plays an important role in the formation of PAHs.A clear map of indene kinetics can improve our knowledge of large PAHs.The indenyl radical is the dominant decomposition intermediate and also the main contributor to the further growth of aromatic molecules in the pyrolysis of indene.Using the new JSR-CRDS setup,we have successfully recorded the spectra of the intermediate indenyl radical by pyrolysis and oxidation of indene.By using the calculated oscillator strength(f=0.016)for the electronic transition of indenyl,the concentration of the indenyl radical in JSR is determined to be 3 × 1010 molecule/cm3.