Abstract
H3+ consists of two electrons and three protons in an equilateral triangle configuration. Because it is the simplest stable polyatomic molecules, theoretical calculations of H3+ serve as benchmarks for calculations on other polyatomic molecules. Moreover, H3+ spectroscopy has been widely used for investigation of interstellar clouds, interstellar chemistry and planetary science. The main purpose of this dissertation is to improve the frequency accuracy of H3+ ν2-band transitions. The results of this study should be useful to not only refine the theoretical calculation but also improve Doppler shift measurement in astrophysics. In this experiment, we constructed a tunable mid-IR difference frequency generation (DFG) source with a wavelength tuning range of 2.66 ~ 4.77 μm and an output power of ~ 1 mW at 3 μm by mixing the radiation from a Nd:YAG laser of power ~ 1W at 1064 nm and a Ti:Sapphire laser of power ~ 1.5 W in 760 ~ 870 nm in a PPLN (periodically-poled lithium niobate) crystal. The Ti:Sapphire laser was stabilized to a Fabry-Perot cavity and its frequency was measured by an optical frequency comb (OFC). The 1 W Nd:YAG laser was offset locked to a 450 mW Nd:YAG laser stabilized to a hyperfine component of iodine transition at 532 nm. The accuracy of our DFG source was better than 30 kHz. Twelve absorption lines in the fundamental ν2-band of H3+ were observed with the DFG source and a positive column discharge using the concentration modulation technique. The accuracy of our measurements on the transition frequency was about 10 ~ 20 MHz which is one order of magnitude better than previous results. The Doppler shift and drift velocity of H3+ ion in a H2 discharge were also determined. Finally, we attempt to search for the infrared saturation spectrum of H3+ to achieve an accuracy better than 1 MHz. To increase the saturation effect, the DFG power was boosted to 6 mW by seeding the Nd:YAG laser output into a fiber amplifier and the positive column discharge was replaced by a hollow cathode discharge to produced sufficient H3+ concentration at few tens mtorr pressure. In addition, the hollow cathode discharge tube was placed inside to White type multipass cell to increase the signal. Up to now, our research has not succeed due to some problems in our discharge tube. In addition, the absolute frequencies of 27 hyperfine transitions of the bands (0-12)and (0-13) of I2 in the wavelength region from 750 to 780 nm were also presented in this work. The results of frequency measurements have an accuracy of 200 kHz. Using our measurements, Dr. H. Knockel and Prof. E. Tiemann of University Hannover propose an improved model description of the iodine spectra in the range from 755 to 815 nm. The new model reduces the differences between measurements and predictions from 38 MHz to 4.4 MHz.