Abstract
H3+, consists of three protons and two electrons, is the simplest polyatomic molecule. Due to its simple structure, the theoretical calculation to a very high accuracy can be performed which can also play a role as a benchmark molecule in other calculations for other triatomic species. Currently, the accuracy of theoretical calculation and experimental results can be achieved to be better than 3 GHz and 150 MHz, respectively. On the other hand, for astronomy, matters consist of hydrogen or its compounds in various forms. For H3+, it is not only the most abundant species in our universe but also exists to be stable at the low temperature and low pressure in the interstellar environments. In the early time of cosmos, H3+ plays a crucial role in cooling down the environment and also in forming the first star. Especially, H3+ interacts with carbon and water and forms carbohydrate which is one of the essential elements of life. Through the technique of high resolution spectroscopy, it is possible to provide a platform to develop or to improve the knowledge of quantum chemical calculations, interstellar chemical reaction, planetary science, and astronomical observation. In this dissertation, we built up a single frequency, continuous-wave and widely tunable, mid-infrared optical parametric oscillator for our experiments. The frequency tuning range can cover between 2.6~4.2 μm and the output power can be achieved to 900 mW at 12 W pump power (1055 ~ 1064 nm) near 3.66 μm through the nonlinear process of optical parametric generation by a periodically poled lithium-niobate. The pump wave, emitted by an ECDL, was sent into a signal-wave-resonant ring cavity for enhancing the power of idler wave. The method of producing molecular ions we used is called extended negative glow discharge. The advantages of extended negative glow discharge are its high glow intensity, high concentration of molecular ion and field-free which is beneficial for the application of high resolution spectroscopy. The absolute frequency measurement of laser source relied on a fiber-based mode-locked laser pumped by 1.55 μm laser diodes. In order to measure the frequency of pump wave and signal wave simultaneously, the spectrum of mode-locked laser was expanded to between 1 μm ~ 2.2 μm (an octave) by a microstructure fiber and the stability can be achieved to be better than 10-12 @ 1000 seconds. On the other hand, for observing the saturation signal of H3+, the experiment was implemented by the pump-probe scheme. We measured several transitions of H3+ in the ν2 fundamental band and the accuracy was 250 kHz. The reaction rate between H3+ and H2 was also acquired by investigating the pressure broadening parameter. Besides, we also measured several transitions of HeH+ in the fundamental band. HeH+ is the simplest diatomic molecule. The accuracy of theoretical computations and the experimental observations can be achieved to be better than 100 MHz and 60 MHz, respectively. In our system, we also improved the accuracy of frequency measurement of HeH+. However, due to poor signal-to-noise ratio, the absolute frequencies need to be confirmed further after we reform our system.