Laser-locked Cavity Ring-down Spectroscopy And Its Application

Absorption spectroscopy of molecules can be used to retrieve the information ofinternal structure and interaction between molecules, such as molecular structure,energy levels, and transition momentum. Taking the advantage of the remarkableprogress of laser techniques, the spectroscopy turns out to be more sensitive and moreaccurate. Extensive applications have been realized in fundamental and appliedstudies. This thesis is organized in 4 chapters. First a brief introduction is given forthe theorem of absorption spectroscopy and spectroscopic methodologies. In thesecond chapter, principles and a review of the cavity ring down spectroscopy (CRDS)are presented. A laser-locked cavity ring down spectrometer was built based on laserfrequency lock and cavity stabilizing. In the third section, we present CRDS study ofhigh overtones of some O¹⁷and O¹⁸substituted carbon dioxide isotopologues. Finally,the CRDS study of the electric quadrupole transitions of molecular hydrogen near0.8μm is presented. The experimental precision has reached and even exceeded that ofthe theoretical calculations with high-level QED corrections included.The cavity ring-down spectroscopy (CRDS) is inherited with large equivalentabsorption length and immune to the fluctuation of incidence light power, which leadsto a very high sensitivity. However, in many applications, high spectral resolution(frequency accuracy) is also essential. A laser-locked cavity ring down spectrometeris built using an ultra-stable etalon, combining the high sensitivity with sub-MHzabsolute frequency precision. Miscellaneous techniques are used, including opticalmode matching, electro-optic modulation, demodulation and feedback servo. Theinvestigation of the water vapor absorption demonstrates the capability of thespectrometer.Carbon dioxide is the most important greenhouse molecule. It is also a major composition of the atmosphere of some planets (such as Mars and Venus).Near-infrared spectra of CO₂is also important in the determination of the opticalthickness and isotope abundance of the planet atmosphere. However there is still largeblank in the quantitative spectroscopy of CO₂in the near-infrared. Ro-vibrationalspectra of(17)^O and(18)^O substituted CO₂isotopologues are investigated here. More than400 transitions are studied, and the spectroscopy constants of the 10⁰52 and 10⁰51bands are obtained.The hydrogen molecule is the simplest neutral molecule, and also one of the besttest grounds for quantum chemistry theory and calculations. Using CRDS integratedwith ultra-stable Fabry-Perot Interferometer, the electric quadrupole transitions in thesecond overtone of H₂is studied with a frequency precision of sub-MHz. Throughspectral profile fitting, the positions, intensities, pressure induced shift and broadeningcoefficients, and Dick-narrowing coefficients are obtained. About 1% accuracy in theline intensities has been obtained and the deviation from theoretical calculationdecreased to 2% from the 10-30% level in literatures. The main source of presentuncertainty is the line profile model used to describe the inter-molecular collisions.Calibrated with a Rb atomic line at 795nm, the absolute frequency of the S(3)transition has been determined with an uncertainty of 1.6 MHz (4ppb), which is themost accurate H₂transition reported so far.