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High Resolution Molecular Line Shape Studies With Mid-infrared Lasers

Posted on:2009-02-09Degree:DoctorType:Dissertation
Country:ChinaCandidate:W P DengFull Text:PDF
GTID:1118360272462493Subject:Chemical Physics
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Line shape studies have extensive applications,for instance in geophysics and astrophysics.The present work is devoted to the line shape studies of atmospheric molecules.In LPMAA,a cold multi-pass Herriott cell has been newly built for experiments with variable temperature between room temperature and 15 K,operating either in a static or collisional cooling mode.It is coupled with a tunable diode laser(TDL) spectrometer,actively frequency-controlled with a Michelson interferometer allowing measuring the temperature dependence of spectroscopic parameters.Due to the good performance of the cold Herriott cell and the TDL spectrometer,this experimental setup has been applied to obtain the temperature dependence of the line-by-line spectroscopic coefficients of the P9 manifold in the v3 band of 12CH4. The results have been used to investigate the influence of the temperature-dependent parameters on retrieving the atmospheric CH4 volume mixing ratios(VMRs),comparing with the CH4 VMR retrieval results obtained using the parameters from the Hitran04 database.Significant difference of retrieved VMR has been found with the two different sets of spectroscopic parameters.The cold Herriott cell is also used to obtain the helium broadening parameters of the P(2) and R(10) lines in the v3 band of the 13CO2 molecule from room temperature to 70 K.When operating in the collisional cooling mode,different heating powers applied on the injector have been tested.It is found that the thermal conductivity of the helium buffer gas limits the helium pressure,resulting in larger errors on the fitted values,while the extremely low temperatures(down to 20 K) achieved with the collisional cooling technique is also a first step towards the generation of the He:CO2 complexes.On the other hand,a mid-infrared difference frequency generation(DFG) spectrometer has been built in USTC,based on a continuous wave(CW) tunable Ti:Sapphire laser and a CW Nd:YAG laser.For the purpose of line shape studies, the 1064 nm Nd:YAG laser is frequency doubled and then locked to a Doppler-broadened iodine absorption lines to reduce its long-term drift.As a result, the DFG emission is frequency-stabilized to a level of about 1×10-4 cm-1.As a testing,the apparatus function of this DFG emission is determined by recording the CH4 lines at low pressure.The best fit residuals are obtained with a Lorentzian instrumental line shape function with a FWHM width of 1.7×10-4 cm-1.To extend the high-resolution spectroscopy studies to the molecules isolated in condensed phase environments,a setup for para-hydrogen matrix isolation spectroscopy has been built in USTC.The solid hydrogen(without or with embedded guest molecules) can be prepared in a closed cell or on a cold plate,and then investigated with a high-resolution Fourier transform spectrometer or with the DFG laser spectrometer.The spectra of the solid normal hydrogen and the solid hydrogen with high para-hydrogen purity are presented.Following our investigation of the CO2 molecules isolated in solid hydrogen with different ortho-hydrogen concentrations and at different temperatures,a preliminary conclusion was drawn:the absorption of the CO2 molecules isolated in solid hydrogen is rather sensitive to their surrounding environment.The complicated spectra reveal a rich physical information to be explored in such quantum solid system.
Keywords/Search Tags:Line Shape, Frequency Control/Stabilization, Tunable Diode Laser, Cold Herriott Cell, Difference Frequency Generation, Frequency Modulation, Para-hydrogen, Matrix Isolation Spectroscopy, Fourier Transformation Spectrometer, CH4, CO2
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