- 學位論文
H3+分子離子ν2-band的精密光譜量測(Precision Measurement of the ν2-band of Triatomic Hydrogen Molecular Ion H3+)
Precision Measurement of the ν2-band of Triatomic Hydrogen Molecular Ion H3+
摘要
H3+分子離子是由三個質子和兩個電子所組成。由於是最簡單的多原子分子，H3+的理論計算可以視為研究其他多原子分子的基礎。此外，H3+的光譜量測在天文上也被廣泛使用在星雲、星際化學以及行星科學方面的研究。目前實驗上振動轉動頻率ν2-band的準確度為150~300 MHz，我們的實驗目的是要提高頻率測量的準確度，對於理論計算及天文上Doppler shift的測量都有很大的助益。 在實驗中，我們建立了一個可調的中紅外差頻光源(DFG)(Difference frequency generation)。波長可調範圍是2.66 ~ 4.77 μm，當波長調至 3 μm時，輸出功率 > 1 mW。產生中紅外光源的方法是將功率大於1.5 W、波長涵蓋760~870 nm 鈦藍寶石雷射(Ti:Sapphire)及功率大於1 W、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)(Periodically poled lithium niobate)產生差頻，為了提高中紅外光功率達到H3+分子離子的飽和強度，我們利用10 W摻鐿光纖放大器來增加Nd:YAG雷射的功率。Ti:Sapphire雷射的頻率鎖在Fabry-Perot的共振腔上，並用光頻梳系統(OFC)(Optical frequency comb)測量頻率。1 W Nd:YAG雷射以offset locking線路穩在另一台碘穩頻的450 mW Nd:YAG雷射。藉由Nd:YAG和Ti:Sapphire雷射穩頻和OFC頻率量測，DFG光源頻率的準確度可達30 kHz。 我們利用DFG光源和離子濃度調制(Concentration modulation)的方法觀察12條H3+ ν2-band的吸收譜線，頻率的準確度約10~20 MHz，比先前的研究提高了一個數量級。同時，Doppler shift以及Drift velocity也可以得知。我們希望可以利用power-booted DFG光源以及White cell結合hollow cathode放電方式，觀察H3+ ν2-band飽和吸收光譜，將頻率準確度提高到 1 MHz。然而，我們的目標並沒有達成。 此外，我們也測量了碘分子在波長750~780 nm範圍內的27條譜線躍遷頻率。測量的絕對頻率，可以改善目前碘分子光譜的理論計算。Dr. H. KnÖckel 和 Dr. E. Tiemann利用我們量測的結果，改善了碘分子光譜在755~815 nm波長範圍的理論模型。這個理論模型可以將原本理論預測與實驗測量的差值從38 MHz減少到4.4 MHz。
並列摘要
H3+ consists of two electrons and three protons in an equilateral triangle configuration. Because it is the simplest stable polyatomic molecules, theoretical calculations of H3+ serve as benchmarks for calculations on other polyatomic molecules. Moreover, H3+ spectroscopy has been widely used for investigation of interstellar clouds, interstellar chemistry and planetary science. The main purpose of this dissertation is to improve the frequency accuracy of H3+ ν2-band transitions. The results of this study should be useful to not only refine the theoretical calculation but also improve Doppler shift measurement in astrophysics. In this experiment, we constructed a tunable mid-IR difference frequency generation (DFG) source with a wavelength tuning range of 2.66 ~ 4.77 μm and an output power of ~ 1 mW at 3 μm by mixing the radiation from a Nd:YAG laser of power ~ 1W at 1064 nm and a Ti:Sapphire laser of power ~ 1.5 W in 760 ~ 870 nm in a PPLN (periodically-poled lithium niobate) crystal. The Ti:Sapphire laser was stabilized to a Fabry-Perot cavity and its frequency was measured by an optical frequency comb (OFC). The 1 W Nd:YAG laser was offset locked to a 450 mW Nd:YAG laser stabilized to a hyperfine component of iodine transition at 532 nm. The accuracy of our DFG source was better than 30 kHz. Twelve absorption lines in the fundamental ν2-band of H3+ were observed with the DFG source and a positive column discharge using the concentration modulation technique. The accuracy of our measurements on the transition frequency was about 10 ~ 20 MHz which is one order of magnitude better than previous results. The Doppler shift and drift velocity of H3+ ion in a H2 discharge were also determined. Finally, we attempt to search for the infrared saturation spectrum of H3+ to achieve an accuracy better than 1 MHz. To increase the saturation effect, the DFG power was boosted to 6 mW by seeding the Nd:YAG laser output into a fiber amplifier and the positive column discharge was replaced by a hollow cathode discharge to produced sufficient H3+ concentration at few tens mtorr pressure. In addition, the hollow cathode discharge tube was placed inside to White type multipass cell to increase the signal. Up to now, our research has not succeed due to some problems in our discharge tube. In addition, the absolute frequencies of 27 hyperfine transitions of the bands (0-12)and (0-13) of I2 in the wavelength region from 750 to 780 nm were also presented in this work. The results of frequency measurements have an accuracy of 200 kHz. Using our measurements, Dr. H. Knockel and Prof. E. Tiemann of University Hannover propose an improved model description of the iodine spectra in the range from 755 to 815 nm. The new model reduces the differences between measurements and predictions from 38 MHz to 4.4 MHz.