H3+中紅外速度調制光譜

Mid-Infrared velocity modulation of H3+

黃泓昇(Hung-Hsing Huang)

指導教授 ： 施宙聰

國立清華大學/理學院/物理學系/碩士(2015年)

https://doi.org/10.6843/NTHU.2015.00243

摘要

在早期宇宙中，H3+在冷卻星際環境及行星的形成扮演重要的角色，尤其H3+會和碳與水分子發生反應，進而生成碳水化合物，是生命起源不可或缺的重要成分。並且由於H3+是由三個質子與兩個電子組成，因其簡單對稱的結構，對於三原子分子的理論提供一個計算的基礎。目前H3+在理論計算的準確度約為300 MHz，而在實驗上，最準確可以達到~1 MHz。我們希望透過量測大量的H3+譜線，提供更多相對精準的數據，在分子理論計算以及天文觀測提供一個發展的舞台。
到目前為止，大部分H3+的躍遷是利用速度調制光譜方法觀測，準確度約150 ~ 300 MHz。最近，我們實驗室及McCall研究團隊分別觀測到H3+的飽和光譜，並使用光頻梳系統將躍遷頻率的準確度推進至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H3+分子離子的振動轉動吸收譜線，將雷射鎖在譜線中心，並藉由光頻梳系統來測量躍遷頻率，我們預期不準度可小於20 MHz。
我們實驗室的鈦藍寶石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，長時間下其準確度高達10^-12，鈦藍寶石光頻梳可用來量測波長為500 ~ 1200 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7 mHz和10 mHz。我們系統經過更新並增加其穩定度，使光頻梳能同時鎖頻的時間達4小時。
　　我們使用的差頻光源為Ti:sapphire雷射及Nd:YAG雷射重合打入PPLN晶體產生，可提供3 mW 2.66 ~ 4.77 μm的中紅外光，利用光頻梳與碘穩頻分別得知Ti:sapphire雷射及Nd:YAG雷射的絕對頻率，並使用速度調制法，將Ti:sapphire雷射的頻率鎖在H_3^+譜線解調訊號的零交點，量測譜線R(1,0)、R〖(3,2)〗^l和R〖(3,3)〗^l，其量測值與McCall研究團隊的結果相比，差異小於10 MHz。
　　未來的短期目標我們將繼續量測H3+ R branch中其他吸收較弱的譜線，由於我們的中紅外差頻光源可調的範圍涵蓋2.66 ~ 4.77 μm，未來也將量測Q branch躍遷。

關鍵字

速度調制光譜

並列摘要

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+.

並列關鍵字

velocity modulation

參考文獻

37.李明漢，“飛秒光頻梳的改進”，國立清華大學光電工程研究所碩士論文(2008)。

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