H2O 冰晶光子作用之溫度效應研究

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题名:	H2O 冰晶光子作用之溫度效應研究
作者:	邱俊銘;Qiu,Jun-Ming
贡献者:	物理學系
关键词:	水;星際冰晶;光化作用;光脫附作用;紅外光譜;真空紫外光;質譜儀;伽利略衛星;water;interstellar ice;photolysis;photo-desorption;infrared spectrum;VUV;QMS;Galilean moons
日期:	2014-01-29
上传时间:	2014-04-02 15:26:23 (UTC+8)
出版者:	國立中央大學
摘要:	天文學家對太空環境觀測結果中，普遍發現構成星際冰晶最主要的成份為H2O冰晶，並在極低溫(<10 K)的冷星雲環境中仍可觀測到氣相的H2O，表示並非熱效應致使H2O冰晶脫附，其主要成因為光子或是高能粒子作用導致H2O冰晶的脫附行為。本論文即探討純H2O冰晶在不同生成溫度條件下，在真空紫外光照射後之光脫附作用及光化生成機制的差異。本論文使用微波氫氣放電管模擬太空環境中的真空紫外光源，搭配四極質譜儀及紅外光譜儀，對真空紫外光照射H2O冰晶的光脫附行為及光化產物的生成機制和產量加以分析。並利用雷射干涉系統觀察冰晶厚度和隨溫度變化之光學性質。實驗結果中光脫附量會隨著冰晶生成溫度上升而增大。而光化產物則因為冰晶生成溫度提高反而衰減了。實驗結果發現當H2O冰晶生成厚度超過30 ML後其厚度已不會影響H2O光脫附量之變化，其光脫附量約為0.052 molecules/photon 在真空紫外光照射低溫的H2O冰晶情況下，觀測到光脫附物種有 H2、O、OH、H2O與O2，其中光脫附量最大的為H2，而OH和H2O隨著冰晶生成溫度上升而光脫附量提高，但變化不若O和O2明顯。O和O2隨著冰晶生成溫度上升至50 K後光脫附量大幅的增加。而從O和O2的光脫附量質譜訊號比例和純氧實驗時的裂解比例比較之，我們發現隨著冰晶生成溫度上升，氧原子會更容易結合成氧分子再脫附離開H2O冰晶表面。 OH dangling bond隨著冰晶生成溫度或照光時間的上升而衰減，H2、H2O2、HO2、O2等光化產物的生成情形皆隨著冰晶溫度上升而下降。從光化路徑及H2O2、O2生成的情形，配合光脫附實驗的結果，可歸結出O原子因為溫度增加使得機動力上升，並以競爭者的角色抑止了H2O2的生成情形。而HO2則需要含有大量O2和H2O的環境下較易生成。從光脫附的實驗中，當H2O冰晶生成溫度上升至高溫時，O2的光脫附量大幅上升暗示了留存在冰晶的O2光化產物減少。 H2O冰晶隨著溫度的從低溫(14 K)上升至高溫(150 K)有著孔隙多寡、光學性質或結構上的變化，而在雷射干涉系統、質譜及紅外光譜的實驗中，觀測在溫度區間147–155 K中光程差的變化暗示了H2O冰晶在此溫度區間光學性質或結構正在進行異變。; The astronomical observations have shown that the main composition of interstellar ice is water. Astronomers observed H2O in gas phase in cold clouds (<10 K), it means that the ultraviolet photons and cosmic rays can induce H2O ice desorption in cold clouds, because the thermal desorption is negligible in those cold regions. In our study, we tried to understand the vacuum ultraviolet (VUV) photo-desorption process and VUV photolysis of pure H2O ice at different temperature conditions. In this study, we used microwave-discharge hydrogen-flow lamp (MDHL) to mimic the interstellar UV field. A quadrupole mass spectrometer (QMS) was employed to detect the desorbed species during irradiation and warm-up periods. The Fourier transform infrared spectroscopy (FTIR) was used to monitor the variation of absorption intensity of H2O ice and products during VUV irradiation period. The laser interference system was used to monitor the ice thickness and optical properties. Experimental results show that the photodesorption yield of H2O ice increases while the temperature of deposited H2O ice increases, but the trend of production yield of products as a function of temperature is contrary to photodesorption yield of H2O ice. When the thickness of H2O ice is thicker than 30 ML, the photo-desorption yield is independent with thickness, and the photo-desorption yield of H2O ice is about 0.052 molecules/photon. The ice photo-desorption of H2O ice was studied at astronomy relevant temperatures (14 - 110 K). The most abundant desorbed species is H2. The ion signal of desorbed OH increases while temperature increases as well as desorbed H2O. O atom is active while temperature increases, the ion signals of O and O2 are obvious stronger than H2O while temperature increases, and ion signals of O & O2 increase rapidly at temperature above 50 K. Because the O atom is more active at higher temperature and easy to react with H2O2 to form O2. The absorption intensity of OH dangling bond becomes weaker at higher deposition temperature and after VUV irradiation. We observed the productions of H2, H2O2, HO2, O2 decrease while deposition and irradiation temperature increases. From chemical reaction pathway and the experimental results, we found that O atom as a competitor to reduce the production of H2O2. The results also imply that HO2 will be produced by photon irradiation of H2O ice containing O2 in high abundance. H2O ice can increase its average kinetic energy, and the structure of H2O ice transfers from amorphous phase to cubic phase at higher temperature. The comparison of the results of laser interference system, QMS and FTIR, the structure or optical property of H2O ice is changing at 147 – 155 K range.
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