Sensitive Laser Spectroscopy Based On Optical Heterodyne Technique And Its Application

Molecular spectroscopy is a primary method to study the interior structure and dynamics and physical and chemical properties of molecules. It is widely applied to the scientific investigations and material analysis. The spectroscopic technique is the experimental basic of molecular spectroscopy and can even promote the development of molecular spectroscopy, especially those of high sensitivity. Therefore, the study of high sensitive laser spectroscopy is an important components of molecular spectroscopy.A laser spectroscopy with both high sensitivity and high resolution, optical heterodyne detected velocity modulation molecular ionic spectroscopy (OH-VMS), is developed and employed to observe the rovibrantional spectra of O₂⁺ and O₂^- in the infrared region in this thesis. That the lineshape of OH-VMS is of the second derivative of Gaussian profile and its sensitivity is 3.4×10^-8 (1s integrated) are theoretically analyzed, and they are both in good agreement with our experimental results.Furthermore, CO⁺ is taken as an example to study the influence on the OH-VMS spectral intensity of the Penning discharging current and the partial gas pressure of the mixture which comprises minimal parent gas CO/CO₂ and vast buffer gas helium. After optimized, some plasma parameters, e.g. the temperature and the electric field intensity in the positive column were determined. Thus, the OH-VMS, as a good non-intrusive spectral diagnosing method, will play an important role in the plasma diagnosis.In recent years, cavity ring-down spectroscopy (CRDS) has been developed as a primary method for the detection of trace absorption and employed to many different areas, such as molecular spectroscopy, the detection of trace absorption and quantitative analysis. However, conventional CRDS consists of a DC detection caused by the loss of the cavity, which limits its sensitivity. Optical heterodyne detected cavity ring-down spectroscopy (OH-CRDS) was presented in the thesis. The ultra-high sensitivity approaching the shot noise limit is expected due to its pure ac detection. For some objective reasons, only some elementary works, both theoretical and experimental, are presented.