Line Shape Study For Cavity Ring-down Spectroscopy Of Atmospheric Gases

Molecular spectroscopy has been used to reveal the structure of molecules and the interactions between them, making it of great importance to not only basic sci-ence (chemistry, physics and biology) but also applied science (e.g. medical tests and astrophysics). Over the years, absorption spectroscopy has become one of the most widespread analytical technique, especially in monitoring greenhouse gas emission (e.g. carbon dioxide), tracing the atmospheric pollutants and the astrophysics of interstel-lar space. Meanwhile, developments in high precision spectroscopy now provide new methods to measure the fine structure and Boltzmann constants from atomic and molec-ular spectra. All these applications rely on highly precise spectral parameters retrieved from dif-ferent line profile models. The HITRAN database, for instance, employs the commo-nly-used Voigt profile to obtain parameters such as the position, intensity, lineshift and broadening coefficients. However, progress in spectroscopic techniques and the need for greater precision in the results from atmospheric spectroscopy has led to the widespread recognition that the Voigt profile cannot give a fully accurate representation of the spectral line shape. In response, increasingly complex and precise line profiles (Galatry, Rautian, SDVP etc) have been developed.This dissertation is mainly focused on the cavity ring-down spectroscopy of at-mospheric gases. Spectral line profiles appropriate to different signal-to-noise ratios, sample pressure and spectral frequency precisions have been used to analyze spectra of different molecules to obtain spectral parameters. A comprehensive study of lineshape profiles has been carried out and is expected to be of use in forthcoming measurements of the Boltzmann constant by spectroscopic methods. The contents of this dissertation are organized as follows:Chapter 1 introduces the various line profiles currently used in the spectroscopic literature. We discusses the main causes of line broadening and how a consideration of these causes has led to development of different line profiles, including the Voigt, Galatry, Rautian, and HTP etc. The meaning of various spectral parameters and their dependence on pressure and temperature is also explained. Finally, it gives a brief summary of molecular energy levels and the selection rules of molecular ro-vibrational transitions in the infrared.Chapter 2 briefly introduces several current high-resolution spectroscopic tech-niques, describing both the theory behind cavity ring-down spectroscopy (CRDS) and the development/evolution of high-precision CRDS system we built in our laboratory. This ultra-high vacuum cavity ring-down spectrometer provides both high sensitivity (equivalent to 10-11/cm) and high frequency accuracy (sub-MHz) in the detection range of 780-850 nm.Chapter 3 describes the cavity ring-down spectroscopy of atmospheric gases, such as water and its isotopes, carbon dioxide and carbon monoxide in the relevant detec-tion range of 780-850nm. The Gaussian and Voigt profile were used to retrieve the spectral parameters which include the line position, intensity, shift and broadening coefficients etc.Chapter 4 describes the high-precision measurements of the electric quadrupole transitions of molecular hydrogen near 784-852 nm. Complex line profiles, such as the Calatry, Rautian, SDVP etc, were used to fit the recorded spectrum of the second overtone transitions of hydrogen and obtain the line position, intensity, broadening and narrowing parameters. The effects of collisions between hydrogen and varous foreign gases on the quadruple transition of H2S3(1) were also investigated. It was discov-ered that these intermolecular collisions produce not only include Dicke narrowing but also non-negligible speed-dependent effects. The SDHC line profile based on a speed-dependent hard sphere collisional model was developed to fit the spectrum produced by hydrogen in the presence of these foreign gases and obtain the pressure shift, broadening and collisional narrowing coefficients.