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    题名: CO2冰晶光脫附之溫度效應研究
    作者: 吳尚叡;Wu,Shang-ruei
    贡献者: 物理學系
    关键词: 二氧化碳;星際冰晶;光化作用;光脫附;真空紫外光;四極質譜儀;carbon dioxide;interstellar ice;photolysis;photodesorption;VUV;QMS
    日期: 2014-08-18
    上传时间: 2014-10-15 14:49:19 (UTC+8)
    出版者: 國立中央大學
    摘要: 摘要
    在太空環境中,CO2為星際間分子含量最豐富的三者之一,在原恆星中亦為追蹤化學與物理進程的重要分子,近年來對於CO2冰晶在光子的作用下如何衍化以及其中基本的衍化機制,更是積極地在進行當中,本論文遂以CO2純冰晶作為樣品的選擇,並且在溫度14 K,50 K,70 K之下完成長冰及照光的過程,目的為探討CO2冰晶在真空紫外光場照射下基本的光脫附機制以及溫度效應對於光脫附的影響;真空紫外光子則選以微波氫氣放電管作為能量來源。CO2冰晶光脫附量測上則利用兩種不同性質的量測儀器分別在兩套系統中進行研究,分別為石英晶體微量秤與傅立葉紅外光譜儀,並且都搭配四級質譜儀分析脫附物質的行為。

    實驗結果顯示CO2冰晶極易在真空紫外光子的光化作用下產生CO與O2,並且脫附物質以CO為主,次之為O2,最後才是CO2。並且發現光脫附量在較低溫的CO2冰晶(14 K)中與高溫(50 K,70 K)的差異可達一個數量級之多。不管在何種溫度下CO2冰晶的光脫附行為深受光化產物所影響,由於溫度14 K下的CO2冰晶中,僅有接近表層的CO分子能脫附,因此當CO2冰晶中累積的CO增加時,脫附的CO分子數量也隨著緩緩地增加,爾後才穩定維持。隨著溫度條件改變為50 K與70 K時,光化產物CO分子除了由冰晶表層脫附外,還從CO2冰晶內層經由熱擴散至冰晶表層後脫附。然而高於50 K的CO2冰晶其結構為有序結構, 需在光化產物逐漸地生成累積下致使CO2冰晶結構破壞殆盡時,CO才得以大量的由冰晶中脫附而出。

    此外,在CO可自由地從CO2冰晶中脫附的高溫實驗下(50 K,70 K),不同溫度所造成的溫度效應使CO在CO2冰晶中具有不同的最大可移動距離。


    關鍵字:二氧化碳、星際冰晶、光化作用、光脫附、真空紫外光、四極質譜儀
    ;Abstract
    Carbon dioxide (CO2) is one of the most abundant molecules present in interstellar ice after H2O and CO. In the high mass protostars, CO2 represents an important tracer of the chemical and physical history. Further, the photoprocess of CO2 ice has been being the main subject of experimental studies quite recently.

    In this study, we present CO2 ice deposited and VUV irradiated at different temperatures (14 K, 50 K, 70 K), that aims to understand the thermal effect on the mechanism of photodesorption of CO2 ice. We used a microwave-discharge hydrogen-flow lamp to mimic the interstellar UV field (114−170 nm). A quartz crystal microbalance (QCM) and a fouier transform infrared spectroscopy (FTIR) were selected to be the principal instruments in two different experimental systems, and both systems equipped with a quadrupole mass spectrometer (QMS) to measure the desorbed species during the whole experimental process.

    Mass spectra show that desorbed molecules mostly originated from the event, CO2 ice decomposed to CO. Secondary desorbed molecules come from the photolysis of CO2 ice as VUV irradiation proceeding to form O2. Finally, a few CO2 molecules directly desorbed from CO2 ice. According to our results, photodesorption yield increased with the increased accumulation of photolysis products, reached a limited value due to the photolysis product only can desorb the surface layers at the temperature below sublimation of CO (14 K). After the ice temperature was raised up to 50K and 70K, Photo-product CO will either desorbed from the surface and also from the deep layers of ice. The amorphous ice structure will turn into crystalline above 50K and beyond, so CO will struggle to migrate to the surface gradually. Until the crystalline structure of CO2 was fully destroyed, CO will sublimate freely within the ice interior.

    Moreover, when CO2 ice was deposited and VUV irradiated at the temperature which is above the sublimation temperature of CO, we determined the maximum mobile distance of CO molecule in CO2 ice at 50 K and 70 K.











    Keyword:carbon dioxide,interstellar ice,photolysis,photodesorption,VUV,QMS
    显示于类别:[物理研究所] 博碩士論文

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