High-resolution Laser Spectroscopy On Several Free Radicals

Free radicals have long been known to play an important role in many chemical processes,such as the interstellar medium,atmospheres of exoplanets and cool stars,plasmas,combustion flames,etc.High resolution spectroscopic study of free radicals can provide important information in understanding their structures,bonding properties,chemical reactivity,as well as the nature of intramolecular interactions.New spectroscopic data are also necessary inputs required in many applications.The main contributions presented in this dissertation can be divided into two aspects:instrumental modifications of the laser-induced fluorescence?LIF?and cavity ringdown spectroscopy?CRDS?combined setup,and spectroscopic studies on the free radicals of YO,CuH,SnO and carbon chains radicals.Improvements in the experimental setupTo improve the performance of our experimental setup and the resolution of the spectra,we have made a series of instrumental modifications,including:Combing CRDS and LIF techniques on the same experimental setup,improving the spectral resolution of the dye laser,applying the home-made single-longitude-mode optical parametric oscillator?OPO?to high resolution spectroscopic study,using a high-precision wavelength meter to calibrate the wavelength during the expremental running.Yttrium monoxide?YO?YO radicals are produced by corona discharge of an oxygen gas mixture between the tips of two yttrium needles in a supersonic jet expansion.The LIF spectra of YO radicals are recorded in the wavelength range of 370-550 nm.In total,111 vibrionic bands with fine rotational structures are observed in the present experiment,and assigned to the B²?⁺-X²?⁺,C²?-X²?⁺,D²?⁺-X²?⁺ transition.The C²?-X²?⁺ and D²?⁺-X²?⁺ electronic transitions are experimentally observed for the first time.By using the single-longitude-mode OPO and/or the optimized dye laser,fully spin-rotationally resolved spectra of B²?⁺-X²?⁺ and D²?⁺-X²?⁺ transition bands have been recorded.Rotational and vibrational analyses of our experimental spectra result in an accurate determination of a series of molecular parameters,including band origins,rotational constants,centrifugal distortion and spin-rotational constants for X²?⁺,B²?⁺and D²?⁺ states.The severe perturbations observed in B²?⁺ v'= 2,3,6 and D²?⁺ v' =0-2/4,5 states are also discussed.A Hamilton model with inclusion of the perturbation term is also built to describe the interaction between B²?⁺ and C^<sup>2?_1/2 states.We have also measured the radiative lifetimes of B²?⁺ and D²?⁺ states,based on which the bands oscillator strengths of B²?⁺-X²?⁺ and D²?⁺-X²?⁺ transitions have been determined.Copper monohydride?CuH?A combined LIF and CRDS spectroscopic study on the A¹?⁺-X1?⁺ transition of CuH and CuD is presented.The CuH?or CuD?radicals are produced by corona discharge of the H₂?or D²?gas mixture between the tips of two copper needles in a supersonic jet expansion.Different profiles of relative line intensities are observed between the measured LIF and CRDS spectra,providing an experimental evidence for the predissociation behavior in the A¹?⁺ state of CuH.The lifetimes of individual upper rotational levels are measured by LIF,from which the J'-dependent predissociation rates are obtained.Based on the previous theoretical calculations,the predissociation mechanism is concluded to due to the strong spin-orbit coupling between the A¹?⁺ state and the lowest-lying triplet 3?⁺ state,and a tunneling effect may also be involved in the predissociation.Similar experiments have also been performed for CuD.It is found that the A¹?⁺ state of CuD does not undergo a predissociation process.Tin monoxide?SnO?SnO radicals are produced by corona discharge of an oxygen gas mixture between the tips of two tin needles in a supersonic jet expansion.The LIF spectra of SnO radicals are recorded in the wavelength range of 265-340 nm.The vast majority of the observed bands are assigned to the A¹?-X1?⁺ and E¹?⁺+-X1?⁺ transitions.We have measured the radiative lifetimes of the A¹? and E¹?⁺ states,from which the A¹?v'=3,4,8 levels are found to have longer lifetimes.In addition,we have also observed the perturbation sources of v' = 3,4 states and measured their lifetimes.Based on these experimental results,we conclude that the 'dark' perturber state borrows transition intensity from the A¹?v'=3,4 states and becomes 'bright'.Benefitted from the low rotational temperature in the supersonic jet,we observed and analyzed the E¹?⁺-X1?⁺?0,0?band for the first time.By measuring the spectral intensities of the?0,0?band at the different rotation temperatures,we have been able to explain of the absence of?0,0?band in previous studies to be due to the small Franck-Condon factor of this band,and due to the limited population of X1?⁺v" = 0 state in high-temperature experiments.Spectroscopy of carbon chains radicalsBy the pulsed discharge of the C²H₂/Ar/He?or C²H₂/CS₂/Ar/He?gas mixture using the newly designed discharge nozzle with two flat-tip electrodes,Cs,C₆H,C₉H₃,C₁₁H₃,HC₄S and C₄S radicals are succesfully produced in a supersonic jet expansion.We have detected the electronic spectra of these carbon chain radicals by CRDS.These experimental spectra have allowed us to optimize experimental conditions for efficient production of carbon chains radicals and for direct spectroscopic studies in the near future.