H_3^+分子離子是由三個質子及兩個電子所組成的三原子分子，因其簡單結構，對於三原子分子系統它提供一個理論計算的檢驗基礎。在早期的宇宙中，H_3^+會冷卻星際環境，在第一個星球的形成中扮演了重要角色。H_3^+在宇宙中會和碳與水分子發生反應，進而生成碳水化合物，在生命的起源也是不可或缺的元素之一。透過高解析度的光譜測量可以提供給理論計算更精確的數值，在未來中能為分子理論計算、行星科學、及天文觀測上帶來應用與發展。
　　到目前為止，大部分H_3^+躍遷的是利用速度調制光譜方法觀測，以中性氣體之吸收譜線校正頻率，準確度約150~300 MHz。最近，我們實驗室及McCall研究群分別觀測H_3^+的飽和光譜，德國Schlemmer研究群利用離子井觀測的低溫(約10 K) H_3^+的光譜，使用光頻梳可以將躍遷頻率的準確度推進至小於1 MHz。這些方法具備非常高的準確度，但系統較複雜、訊號較小，不太適用於弱的吸收譜線。本論文中，我們利用速度調制法來偵測H_3^+分子離子的振動轉動吸收譜線訊號，並藉由光頻梳系統來測量躍遷頻率，期望能提昇H_3^+分子離子弱吸收譜線躍遷頻率的準確度。
　　我們的鈦藍寶石(Ti:sapphire)光頻梳系統的脈衝重複率及偏差頻率皆鎖相至全球定位(GPS)調控的銣原子鐘，長時間下其穩定度高達〖10〗^(-12)，主要用來量測波長為500~1100 nm可見光到近紅外光的絕對頻率，在計數器的gate time為1秒時，脈衝重複率及偏差頻率的標準差分別為2 mHz和10 mHz。我們以碘分子R(56) 32-0 a_10譜線來校準光頻梳，量測的結果與CIPM給定標準值差異小於17 kHz。
　　我們使用的光源為Ti:sapphire雷射及Nd:YAG雷射重合打入PPLN晶體產生的差頻(DFG)光源。為了提高速度調制吸收光譜訊號之訊噪比，我們將DFG光分成兩束，反向射入交流放電管中，再分別利用不同偵測器收集光訊號，將兩偵測器的訊號相減可使訊號增加並減少雜訊，且如此得到吸收譜線訊號之零點相較於單方向的方式，更接近譜線中心。對於H_3^+之R(1,0)譜線，我們測量的值與McCall研究群所測量的值相比，差異約∆ 1.5 MHz。
　　未來我們將改進Ti:sapphire OFC和PPLN DFG光譜的鎖頻的準確度，並檢驗弱吸收譜線的測量準確度，最後我們將廣泛地量測H_3^+的弱吸收譜線。

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^+.

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