| Summary: | 碩士 === 國立清華大學 === 物理系 === 103 === In the early time of cosmos, H3+ plays a crucial role in cooling down the environment. Especially, H3+ interacts with carbon and water and form carbohydrate which is one of the essential elements of life. H3+, consists of three protons and two electrons, is the simplest polyatomic molecule. Due to its simple structure, the theoretical calculation can be performed to very high accuracy. Currently, the accuracy of theoretical calculation and experimental results can be achieved to be better than 300 MHz and 1 MHz, respectively. We hope to provide more accurate data through massive measurements of H3+. Our work will provide a platform to improve the quantum calculations and astronomical observation.
So far, most of H3+ transitions are observed using the velocity modulation spectroscopy. The transition frequency accuracy is about 150~300 MHz. Recently, our lab and McCall’s group have measured the saturated absorption spectrum of H3+. Those works achieve frequency accuracy less 1 MHz with the help of optic frequency comb (OFC). Although above methods provide high accuracy, but the systems are more complex and the signal are smaller. In this dissertation, we use velocity modulation spectroscopy to detect the signal of vibration-rotation absorption transition of H3+ molecular ion, and measure the frequency by an OFC system. We expect that the frequency accuracy can be improved to < 20 MHz.
The repetition rate and offset frequency of our Ti:sapphire-based OFC are phase-locked to a global positioning system (GPS) disciplined Rb clock. The accuracy of our OFC is better than 10^-12 at 1000 sec. It can be used to measure the absolute frequency of wavelength from 500 to 1200 nm. After phase locking, the standard deviation of repetition rate and offset frequency are 7 mHz and 10 mHz respectively at 1 s gate time.
Our light source is a PPLN difference frequency generation (DFG) laser generated using a Ti:sapphire laser and a Nd:YAG laser. And we use the velocity modulation spectroscopy to measure the transition frequency of R(1,0), R〖(3,3)〗^l, R〖(3,2)〗^l.The difference of our results and McCall’s measurements are less than 10 MHz. In the future, we will measure other weak absorption line of H3+. Because our DFG source has wide tuning range (2.66 ~ 4.77 μm), we will also measure the Q branch transitions of H3+.
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