| 摘要: | 近數十年來天文物理學家致力研究海王星外天體(TNOs)之古柏帶天體(KBOs)，欲了解來自太陽系早期的衍化過程與尋找相關證據。太空觀測發現KBOs表面不同顏色與性質的差異反映了與距太陽遠近而造成的溫度差異，進而去分析KBOs與太陽系衍化的關係。本論文選擇冥王星做為探討有關KBOs表面的相關衍化機制，有別於先前使用高能粒子來模擬冥王星冰晶的實驗研究，我們選用真空紫外光源(VUV)與極致紫外光源(EUV)來探討不同能量的光子對類冥王星冰晶的光化作用機制之影響。
真空紫外光源部分使用微波氫氣放電管，而極致紫外光源使用同步輻射研究中心(NSRRC) High Flux 光束線所提供之同步輻射光，並使用N2:CH4: CO (10:1:1)混合冰晶作為類冥王星的表面冰晶，利用不同能量分布的光源來探討光子能量對類冥王星冰晶之光化作用的影響。同時搭配紅外線光譜儀及四極質譜儀來分析光化產物的產量與生成機制。研究結果發現在VUV與EUV光子作用下皆能產生CO2、HCO、H2CO與CH3CHO等含碳氧鍵結的光化產物，和CH2N2與HCN等含碳氮鍵結的光化產物。然而只有在EUV光子作用下才能觀測到N3、CN-、HNCO與OCN-等光化產物生成。
本論文所觀測到的含碳氧鍵結之光化產物的生成量皆非常微小，分別為混合冰晶中CO分子總量的千分之一至六的量。這或許可以說明現今在冥王星表面上為何較難看到這些產物的吸收特徵。特別是在光源Ly-α 比例約高達80 %的條件下，CO2與HCO的生成量相對的較少，而H2CO與CH3CHO的生成量並沒有隨光源條件的不同而有太大的差異。此外，本實驗可以明顯的區分CH2N2與HCN含碳氮鍵結的光化產物的吸收特徵，且在VUV光子作用下，CH2N2的光化產量隨CH4被光解的消耗量愈多而愈多，HCN的生成經由CH2N2與CH2自由基交互作用生成的效率遠大於CH2N2結構重新排列。在EUV光子作用下，HCN可經由CH與N的結合而形成，然而在EUV光子作用下N原子較傾向反應生成CN鍵結的產物，如HNCO與OCN-，因此在EUV光子照射條件下CH2N2與HCN的總產量相對於VUV小了許多。
在本實驗的回溫過程中OCN-的熱脫附行為與Moore和Hudson在2001年、黃柏渝於2010年的實驗結果一致，更證實我們有OCN-的生成物產生。此外回溫的紅外光譜顯示不論是VUV或是EUV光子作用皆有許多新的吸收訊號產生，可提供未來天文觀測結果如：2015年的新視界號 (New Horizon)，或往後相關的大分子模擬實驗研究一個參考的依據。
To understand the evolutionary process from the early solar system and find its relevant evidences, astrophysicists have been dedicated to investigating Kuiper Belt Objects (KBOs) of Trans-Neptunian Objects (TNOs) in recent decades. Through astronomical observation, some astrophysicists discovered that the surfaces of KBOs have different colors and properties. Temperature differences might be caused by its distance from the sun
thus, further analysis of the evolution between KBOs and solar system is required. In this research, we choose Pluto to investigate the relevant photochemical mechanism on surfaces of KBOs. Unlike previous studies in using the high energy particles as the energy sources, we use vacuum ultra-violet (VUV) and extreme ultra-violet (EUV) as two different light sources to discuss the effect of photon energies on photolysis of Pluto.
We used (1) a microwave discharged hydrogen-flow lamp (MDHL) as the VUV light source, and (2) the High Flux beamline at National Synchrotron Radiation Research Center (NSRRC) as the EUV light source. We chose N2:CH4:CO (10:1:1) ice mixtures as the simulated surface material of Pluto, and used different energy distributions of light sources for studying the photolysis on it. Fourier Transform Infrared spectroscopy (FTIR) and Quadrupole Mass Spectrometry (QMS) were employed to analyze the formation mechanism and production yield of the products simultaneously. The results showed that the C-O bearing photoproducts as CO2, HCO, H2CO and CH3CHO and N-C bearing photoproducts such as CH2N2 and HCN could be detected under both VUV and EUV irradiation. However, photoproducts of N3, HNCO and OCN- were only observed in EUV irradiation experiment.
Based on this study, the column density of C-O bearing photoproducts (CO2, H2CO and CH3CHO) are only 0.1-0.6 % compared to that of CO molecules. This might suggest why these absorption spectral features in Pluto are hardly to be found. Especially under the light condition containing about 80% Ly-α photon, the production quantities of CO2 and HCO are relatively less. However, the production quantities of H2CO and CH3CHO are not much different under different irradiation light conditions. In addition, we could distinguish clearly the N-C bearing photoproducts between CH2N2 and HCN. Under the VUV irradiation, the production quantities of CH2N2 are directly correlated to the consumption of CH4 while the production quantities of HCN are produced easily by the interaction of CH2N2 with CH2 radicals rather through than the CH2N2 decomposition. Under the EUV irradiation, HCN could be produced by the combination of CH and N. The N atoms tend to produce the molecules with CN bearing, such as HNCO with OCN- in EUV irradiation
therefore, the production quantities of HCN and CH2N2 are much less in EUV irradiation.
In this study, the process of OCN- thermal desorption is in agreement with those reported in Moore and Hudson (2001) and Po-yu Huang (2010) and it supports that OCN- is produced in photolysis. As shown in IR spectra of warm-up process, some complex molecules can be in the substrate identified after either VUV or EUV irradiations. This study could be important for coming astronomical exploration in the future, such as the New Horizon spacecraft in 2015 or provide worthwhile reference for the formation study of complex molecules in laboratory experiments. |
