H3+和HeH+中紅外飽和吸收光譜

Mid-Infrared Saturated Absorption Spectroscopy of H3+ and HeH+

官鈺禪(Yu-Chan Guan)

指導教授 ： 施宙聰

國立清華大學/電機資訊學院/光電工程研究所/博士(2018年)

摘要

本論文詳細的描述我們建立的中紅外分子離子飽和吸收光譜儀。首先產生離子的方式是利用一個延伸負光輝區放電管(extended negative glow discharge tube)，其優點在於可產生的離子濃度較高且負光輝區域中電場較小。然後我們利用自製的高功率光學參量放大器OPO (optical parametric oscillator)作為雷射光源，量測離子的飽和吸收光譜。為了決定雷射光源的絕對頻率，我們把OPO signal光的頻率鎖在一套光頻梳(optical frequency comb)系統上，並且利用一個可調的偏差鎖頻系統(tunable offset locking system)把OPO pump光的頻率鎖在碘分子穩頻(iodine-stabilized)的Nd:YAG雷射上，利用可調的偏差鎖頻系統，OPO idler光的頻率可以精準的scan並得到飽和吸收光譜。為了校正光譜儀的精確度，我們量測CIPM視為頻率標準的甲烷ν3 band的P(7)躍遷頻率，其量測結果證明光譜儀頻率量測的精確度可達7 kHz以下。此外，此套光譜儀還加入光強度調制跟離子濃度調制來有效的增加量測到離子飽和吸收光譜的訊噪比。
利用這套光譜儀我們量測了16條H3+ ν2基帶(fundamental band)譜線的躍遷頻率和9條HeH+基帶譜線的躍遷頻率。對於訊噪比足夠的光譜，其中心躍遷頻率精確度的決定可達1 MHz以下，這樣的精確度比當前最好理論計算的精確度(~300 MHz)小兩個階數(order)。本論文也呈現了H3+ R(1,0)的飽和吸收光譜譜線變寬參數的研究結果。
此外，我們也量測了23條CO2 [1001,0201]II←0000 band譜線的躍遷頻率，其量測的精確度可達7 ~ 17 kHz(精確度隨著各譜線的訊噪比減少而增加)，這個量測結果也定義出更精準[1001,0201]II←0000 band的分子常數(molecular constant)，其精確度比目前分子常數的精確度小一個階數(order)。

關鍵字

延伸負光輝區放電 ； 光學參量放大器 ； 飽和吸收光譜 ； 光頻梳 ； 碘分子穩頻Nd:YAG雷射 ； H3+ ； HeH+ ； 中紅外光譜儀 ； 高精密躍遷頻率量測 ； 二氧化碳

並列摘要

This dissertation present a versatile mid-IR molecular ion saturated absorption spectrometer capable of measuring rovibrational transition frequencies with sub-MHz accuracy. An extended negative glow discharge tube was used to produce molecular ions. It has the advantages of higher concentration of positive ions and near field-free. The molecular ion transition is probed with sub-Doppler spectra enabled by an optical parametric oscillator (OPO). To determine the transition frequency, the OPO signal frequency was locked to an optical frequency comb. A tunable offset locking system was used to lock the OPO pump frequency to an iodine-stabilized Nd:YAG laser. With this offset locking system, the OPO idler frequency could scan precisely and obtain the saturated absorption profile. The accuracy of the OPO idler frequency is 7 kHz, demonstrated by measuring the absolute frequency of the F2(2) component of the P(7) transition in the ν3 band of methane, which is recommended by CIPM as a frequency standard. Furthermore, intensity modulation and ion concentration modulation were employed to increase the signal-to-noise ratio (SNR) of the saturated absorption signal.
Using this spectrometer, we measured 16 ν2 fundamental band transitions of H3+ and 9 fundamental band transitions of HeH+. The transition frequencies with acceptable SNR were able to determine to sub-MHz accuracy, which is better than the current theoretical calculations by two orders of accuracy (~300 MHz) for these two molecular ions. In addition, the homogeneous linewidth broadening parameters of H3+ R(1,0) transition influenced by discharge conditions were studied.
In addition, absolute frequencies of 23 carbon dioxide transitions ranging from J = 2 to 70 for both the P and R branches in the [1001,0201]II ← 0000 band near 2.7 μm had been measured to the uncertainties varying from 7 to 17 kHz by using this spectrometer. A refined set of molecular constants were obtained which gave the differences between measured and calculated values of less than 7 kHz.

並列關鍵字

Extended Negative Glow Discharge ； Optical Parametric Oscillator ； Saturated Absorption Spectroscopy ； Optical Frequency Comb ； Iodine-Stabilized Nd:YAG Laser ； H3+ ； HeH+ ； Mid-IR Spectrometer ； Precise Transition Frequency Measurements ； CO2

參考文獻

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