Transient Molecular Speed Modulation And Zeeman - Magnetic Rotation Modulation Laser Spectroscopy

Transient Molecules are short-lived species, which often exist as intermediates of chemical, physical and biological processes. They are the keys for deep understanding of chemical dynamics and molecular structure, therefore they have been attracting interests for a long time. Transient molecules consist of free radicals, molecular ions, molecules in excited states, and other short-lived neutrals. The objective of this Ph.D. project is to develop high-resolution, high-sensitive spectroscopic technique for selective detection of transients of different kinds. Two novel differential detection methods for either molecular ion or neutrals were proposed and tested with thespectrum of N₂⁺ using this technique. Spectra of A²Î â†X²Î£(1,0) band ofCS⁺were obtained for the first time. In addition, a Zeeman-magnctic rotation modulation spectrometer was constructed and its properties were investigated. This thesis covers the following content.In the introductory chapter, the history of transient molecules is reviewed, so is the related spectroscopic techniques, such as velocity modulation, concentration modulation, laser magnetic resonance, and the relevant generation methods of transients.Chapter 2 and 3 detailly discuss velocity modulation(VM) and concentration modulation(CM) spectroscopy in both theory and experiment, especially the noveldifferential VM and CM approaches. N₂⁺ and Ar^* spectra were used to examine theproperties of these two techniques, including the dependence of the spectral signal on experimental conditions. The results confirm that the S:N ratios for the new CM and VM are as 30 to 60 times or more respectively as the traditional CM and VM. Thisbreakthrough would make used-elusive transients accessible like CS⁺, which will be discussed in the next chapter.Chapter 4 is devoted to CS⁺. The novel VM technique allowed the first observation of its high resolution absorption spectrum since its emission spectrum in 1978. Over 300 lines of A²Î â†X²Î£(1,0) band of were recorded and assigned. A depertubation analysis of these lines yielded precise molecular constants, which will beof great help for astronomers who are interested in searching for CS⁺ spectrum in space.Chapter 5 and 6 briefly introduce Zeeman-magnetic rotation modulation spectroscopy, discuss its features and lineshapes. In this chapter, a tentative theory will be proposed to explain the experimental lineshapes by considering both Faraday dichroism and Voigt effect. This model agrees with the experiment roughly.Future work is discussed in chapter 7. After the improvement of the techniques, future study should center on the spectral observation of new transients such asPN⁺,CN⁺ and PN etc.