HeH+離子飽和吸收光譜量測系統的建立

Establishment of HeH+ Saturation Absorption Spectroscopy Measurement Setup

蕭中芸(Chung-Yun Hsiao)

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/碩士(2010年)

摘要

本論文以偵測HeH+離子飽和吸收光譜為目標，建立一套量測系統，包含產生HeH+離子的放電管，以及偵測光源–光參量振盪器(OPO)兩部分。HeH+是一個由兩個電子、兩個原子核組成的四體系統，因為結構簡單，可經由量子力學計算出高準確度的能階，其振動轉動躍遷頻率目前理論計算不準度為30 MHz，實驗結果不準度可達7 MHz，主要為Doppler線寬造成。我們希望偵測HeH+離子飽和吸收光譜以提高躍遷頻率的量測精準度，但在低壓放電下很難產生高密度的HeH+離子，以及我們實驗室的中紅外可調式差頻雷射光源(DFG)功率不足(約5 mW)，使得直接利用DFG光源偵測HeH+離子飽和吸收光譜的方法難以實現。
為了降低放電管氣壓，並提高HeH+離子密度，我們參考加拿大滑鐵盧大學T. Amano教授的架構，架設了一個放電管系統，此放電管可以將高密度正離子的區域–負輝光區延伸以提高離子密度，並且在低氣壓放電的環境下維持和高壓環境中一樣的離子密度。我們的方法是將He:H2為99:1的混合氣體填入全長約160 cm，直徑4 cm的放電管中，在氣壓約為80 mTorr下放電，並外加300 Gauss的軸向磁場延伸負輝光區，來達到提高HeH+離子密度的目的。我們以5 k的鎮流電阻維持abnormal glow放電模態，在放電電壓達到2.2 kV時會導通放電管，產生6 mA的放電電流。
HeH+飽和吸收光譜光源部分，我們架設了一套光學參量振盪器(OPO; optical parametric oscillator) 系統，其穩定的高輸出功率與可調整的中紅外波長，為理想的偵測光源。我們的OPO採共振signal光的環形共振腔設計，使用MgO:PPLN晶體作光參量增益介質，以及-DFB雷射作為pump光源。-DFB雷射波長為1062 nm，可經由改變雷射驅動電流來改變波長，並維持單模，此雷射可在不跳模的狀況下連續掃描30 ~ 40 GHz，我們預計以改變pump光波長的方式來調整idler光波長。由之前使用1064 nm Nd:YAG雷射作為pump的結果，可預期當pump光超過3 W時，OPO會出光，且在5 W的pump下可以得到> 200 mW的idler光輸出。
我們預計將OPO idler光頻率調整到HeH+離子R(1)躍遷頻率(f = 90788382 MHz)附近，以至少100 mW的idler光偵測放電產生的HeH+離子，希望可以首次偵測到HeH+離子飽和吸收光譜訊號。

關鍵字

放電管 ； 光參量振盪器 ； HeH+ ； 飽和吸收光譜

並列摘要

In this dissertation, we establish a measurement setup for saturation absorption spectroscopy of HeH+. It consists a discharge tube which can produce HeH+ molecular ion, and an optical parametric oscillator (OPO). Because of the simple structure of HeH+, its transition frequency can be calculated accurately by quantum mechanics. At present, the theoretical accuracy in ro-vibrational transitions is about 30 MHz, and the experimental accuracy is 7 MHz which is limited by the large Doppler line width (~ 600 MHz). We expect to improve the accuracy using the saturation absorption spectroscopy. However, there are difficulties from low ion concentration in the discharge and low power of our DFG light source.
In order to increase the ion concentration of HeH+, we have constructed an extended negative glow discharge tube, following the design of Professor T. Amano at University of Waterloo. In the negative glow region, positive ions have the highest concentration. However, this region is relatively short in the normal glow discharge. The negative glow region can be greatly extended by using a longitudinal magnetic field of up to 300 Gauss on a discharge tube with 4 cm inner diameter and 160 cm in length. We use a 99:1 He-H2 gas mixture at 80 mTorr, and 5 k ballast resistor for maintaining the abnormal discharge mode. When the discharge voltage rises to 2.2 kV, we can obtain stable extended negative glow discharge with 6 mA discharge current.
For the light source, we are constructing a singly resonant optical parametric oscillator (OPO). Compare to our DFG, OPO has stable high output power over the mid-IR wavelength, it’s an ideal light source for probing the saturation absorption spectroscopy signal of HeH+. The OPO is a bow-tied ring cavity configuration, based on a MgO:PPLN crystal, and pumping by an -DFB laser. The pump laser can be continuously tuned over 30~40 GHz without mode hop, and maintains single mode. We expect that the OPO provides over 200 mW idler power, and the idler wavelength can be tuned by tuning the wavelength of pump.
In the near future, we plan to search the saturation absorption spectrum of HeH+ at R(1) transition, by using the extended negative glow discharge tube and OPO.

並列關鍵字

extended negative glow discharge ； optical parametric oscillator ； OPO ； HeH+ ； saturation absorption spectroscopy

參考文獻

[2] I. Dabrowski and G. Herzberg, Trans. N. Y. Acad. Sci. Ser. II 38, 14 (1977).

[3] J. H. Black, Astrophys. J. 222, 125 (1987).

[4] C. Cecchi-Pestellini and A. Dalgarno, Astrophys. J. 413, 611 (1993).

[6] V. P. Gaur, G. C. Joshi, and M. C. Pande, Astrophys. Space. Sci. 197, 57 (1992).

[9] T. Amano and A. Maeda, J. Mol. Spectrosc. 203, 140-144 (2000).