Molecular Ion Velocity Modulation Spectrometer

• Main Authors: Hu, Ko-Bin, 胡克斌
• Other Authors: Shy, Jow-Tsong
• Format: Others
• Language: zh-TW
• Published: 2014
• Online Access: http://ndltd.ncl.edu.tw/handle/50907345535770605715

碩士 === 國立清華大學 === 物理系 === 102 === H_3^+, consists of three protons and two electrons, is the simplest polyatomic molecule. Due to its simple structure, it is the benchmark of highly accurate calculation. In the early time of cosmos, H_3^+ plays a crucial role in cooling down the environment and also in forming the first star. H_3^+ interacts with carbon and water and forms carbohydrates which are the essential elements of life. Through the technique of high resolution spectroscopy, it provide precise values for theoretical calculation. It also has important impacts on quantum chemical calculations, planetary science, and astronomical observation. 　　So far, most of H_3^+ transitions are observed by velocity modulation spectroscopy which eliminates the strong absorption line of neutral gas. The transition frequency accuracy is about 150~300 MHz. Recently, our lab and McCall’s group have measured the saturated absorption spectrum of H_3^+. Schlemmer’s group in Germany has observed H_3^+ transition using cryogenic ion trap. 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 the method of velocity modulation spectroscopy to detect the signal of vibration-rotation absorption transition of H_3^+ molecular ion, and measure the frequency by an OFC system. We expect that the frequency accuracy of the weak absorption transitions can be improved to < 10 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 1100 nm. After phase locking, the standard deviation of repetition rate and offset frequency are 2 mHz and 10 mHz respectively at 1 s gate time of the frequency counter. We measure R(56)32-0 a_10 the frequency of hyperfine line the molecular iodine to check our OFC. Our result show the difference is less 17 kHz from CIPM. 　　Our light source is a PPLN difference frequency generation (DFG) laser generated using a Ti:sapphire laser and a Nd:YAG laser. In order to improve the signal-to-noise ratio of absorption spectrum, we split the DFG light into two beams which go through the AC discharge tube with opposite direction. Then we collected the signals by two different detectors. The signals from the two detectors are substracted to increase the signal and decrease the noise. The zero point of absorption signal is also nearer to the line center than the one beam configuration. For the R(1,0) line of H_3^+, the frequency measured is 1.5 MHz less than McCall’s result. 　　In the feature, we will improve the stability of the frequency locking of our Ti:sapphire frequency comb and PPLN DFG and test the measurement accuracy of a weak absorption line. Finally, we will measure extensively the weak absorption line of H_3^+.