本論文的目的是二氧化碳氣體分子在中紅外波段的高精密光譜量測，期望提供最準確的躍遷頻率作為理論計算驗證的基礎。我們利用絕對頻率的實驗數據並結合分子常數擬合方法，得到新的分子常數，並且進一步的修正二氧化碳的理論模型及理論預測的頻率位置。

我們建立一套窄線寬、可調、高功率的中紅外差頻雷射光源，進行兩種不同波段(4.3、2.7 μm) 的飽和光譜實驗。此光源是將功率大於1.5W、波長涵蓋760~870 nm 鈦藍寶石雷射 (Ti:sapphire laser) 及功率大於1W、波長1064 nm摻銣釔鋁石榴石雷射 (Nd:YAG) 通過週期性反轉非線性晶體 (PPLN)，產生波長2.66 ∼ 4.77 μm的差頻，為了提高中紅外光功率達到飽和強度，利用10W摻鐿光纖放大器來增加Nd:YAG雷射的功率。

在4.3μm 0001 ← 0000實驗，結合頻率調制技術及飽和吸收光譜的實驗架構，觀測了56條高訊雜比之飽和吸收譜線。頻率測量的方法是將Nd:YAG雷射以碘分子穩頻，Ti:sapphire 雷射鎖在二氧化碳飽和吸收譜線的中心，並且以光頻梳(OFC) 來校正其頻率。在4.3μm 0001 ← 0000躍遷譜線的頻率準確可達30 kHz。

在2.7μm [1001,0201]I ← 0000實驗，我們利用飽和螢光光譜的方法測量了19條飽和吸收譜線。1.2 W Nd:YAG雷射以offset locking線路穩在另一台碘穩頻的Nd:YAG雷射，藉著兩台差頻的調整可以改變1.2 W Nd:YAG雷射的頻率。Ti:sapphire 雷射以thermal stabilized Fabry-Perot cavity和自組的光頻梳 (OFC) 來測量其頻率。在2.7μm [1001,0201]I ← 0000實驗躍遷譜線的頻率準確可達40 kHz。

我們已經利用所建立的中紅外可調光源來觀測二氧化碳的飽和吸收光譜在0001←0000及 [1001,0201]I ←0000的躍遷譜線，並且藉由光頻梳準確地測量譜線的頻率。此外，使用分子常數擬合方法，新的分子常數的準確度改進了一個數量級。

The aim of this dissertation is to accomplish high precision spectroscopic measurements on 12C16O2 spectra in the mid-infrared region. The research involves two spectroscopic methods and focuses on two different absorption bands. We report the application of a CW mid-infrared difference frequency generation (DFG) source generated in periodically poled lithium niobate (PPLN) nonlinear crystal by mixing a Nd:YAG laser and a Ti:sapphire laser to the saturation spectroscopy of 12C16O2 at 4.3 μm and 2.7 μm region. This DFG source has a characteristics of high power (> 6 mW) and narrow linewidth (< 100 kHz). Optical frequency comb (OFC) system is the second niche of this work. It plays a significant role in absolute frequency measurement of Ti:sapphire laser. The operating spectrum of our OFC system is from 500 nm to 1450 nm and the frequency accuracy is better than 10−12.

First, 56 transition lines of sub-Doppler profile of the 0001-0000 band at 4.3 μm region are observed by means of a pump-probe scheme with frequency modulation
spectroscopy. The Nd:YAG laser is stabilized onto the frequency standard line, 127I2 hyperfine transition R(56)32-0 a10 component. The Ti:sapphire laser is locked onto the center of CO2 transition and then beat with OFC for frequency measurement. The absolute frequencies of those transition lines are derived by the formula, fline = fTis − fYAG. The measurements of absolute frequencies for transitions up to J’ = 62 of the 0001-0000 band are accomplished for the first time with an given uncertainty of 30 kHz.

The second work is to observe the sub-Doppler spectra of the [1001,0201]I-0000 band at 2.7 μm region by adopting saturated 4.3 μm fluorescence approach. Offset locking method is employed for stabilizing and precise tuning Nd:YAG laser. Ti:sapphire laser is stabilized onto a ultra-stable Fabry-Perot cavity and frequency counted by OFC. After fitting the saturated profile, the transition frequency can be determined. We successfully carry out absolute frequency measurements on transitions up to J’ = 30 of the [1001,0201]I-0000 band with an uncertainty of 40 kHz which is 15 times better than previous results.

In addition, a new set of molecular constants of 0000 and [1001,0201]I states have been determined. The STD errors of our fitting are 29 kHz (9.7 × 10−7 cm−1) and 46 kHz (1.5 × 10−6 cm−1) respectively which are one order of magnitude better than previous results. With the help of our constants, one can provide more accurate linepositions to revise the current HITRAN database and construct more accurate molecular model of 12C16O2.

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