H2S+CO與H2S+CO2混合冰晶光化作用下含硫分子之生成機制

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题名:	H2S+CO與H2S+CO2混合冰晶光化作用下含硫分子之生成機制
作者:	莊可儒;Ko-ju Chuang
贡献者:	物理研究所
关键词:	光化作用;質譜儀;W33A;紅外光譜;二氧化碳;一氧化碳;真空紫外光;羰基硫;星際冰晶;硫化氫;QMS;W33A;Hydrogen sulfide;interstellar ice;Carbonyl sulfide;Carbon dioxide;VUV;infrared spectrum;photolysis;Carbon monoxide
日期:	2011-07-27
上传时间:	2012-01-05 14:42:32 (UTC+8)
摘要:	天文學家觀測星際物質中，唯獨觀測到含硫化合物OCS與SO2分子以固態形式存在，然而在其他星體如彗星與木星衛星上的固態H2S分子，卻沒有觀測到其存在證據，此外天文上也尚未能解釋消失硫原子究竟是形成那些含硫化合物存在於星際冰晶中。本論文即探討H2S+CO與H2S+CO2混合冰晶，接受真空紫外光化作用下含硫分子之生成機制。本論文採用H2S+CO與H2S+CO2兩種混合冰晶利用不同碳原子來源，討論含硫冰晶之光衍化產物差異。此外亦討論混合冰晶比例差異上所造成的不同生成機制，如H2S:CO=1:7 & 1:20與H2S:CO2=1:7 & 1:20混合冰晶。真空紫外光源部分使用微波氫氣放電管分別搭配氟化鎂與氟化鈣光窗及同步輻射研究中心High Flux光束線所提供之同步輻射光，分析不同光子能量對星際冰晶光衍化之影響。本論文實驗中同時使用紅外線光譜儀及四極質譜儀，對光化產物的生成機制與產量加以分析。其結果發現OCS的產量在H2S+CO混合冰晶中大於H2S+CO2混合冰晶。且OCS的產量只和混合冰晶的比例相關，在CO高濃度的混合冰晶中OCS的產量大於CO低濃度的混合冰晶，此外OCS的生成速率只和所選用的光子能量相關，然而OCS生成總量和所照射之真空紫外光子的能量沒有關係。本論文實驗中觀測到的含硫光化產物有H2S2、OCS、CS2與SO2，其中證實SO2是需要在富含氧原子環境下才可能生成，所以唯有在H2S+CO2混合冰晶中才可觀測到光化產物SO2存在。此外也觀測到CS2在不同光子能量下，有著不同的生成機制，在使用同步輻射30.4 nm與58.4 nm光子做為真空紫外光源下，光子能量較微波氫氣放電管高，能直接光解CO分子的鍵結能(11.1 eV)，形成C及O原子，而O原子再與HS自由基結合為SO分子，進一步形成CS2產物，而使用微波氫氣放電管做為真空紫外光源下，則是透過OCS與H2S結合形成CS2與H2O，在其缺乏O原子環境下，兩個S原子會互相結合形成S2殘留物質，然而因為其對稱結構致使紅外光譜無法觀測到其吸收特徵，本論文推斷在外太空消失的硫原子可能是以Sn (n=2-8)的形式存在於星際冰晶當中，在本論文中可藉由質譜儀觀測到S2 (64 amu)熱脫附訊號，然而更大分子的結構因具有較高之熱脫附溫度，超過實驗系統現有規格，無法直接觀測到更大的Sn分子從冰晶殘渣中熱脫附的證據。在探討OCS的生成機制的同時，也觀測到OCS的紅外光譜吸收位置會隨著H2S+CO混合冰晶比例而改變，在CO高濃度混合冰晶中，OCS吸收位置坐落在較高頻率上，在CO低濃度冰晶中則偏向低頻率。然而OCS的吸收特徵並不受到CO2濃度的影響，所以在H2S+CO2混合冰晶中，並不隨著混合冰晶比例而有明顯偏移量。總結這次實驗結果得知 H2S+CO與H2S+CO2混合冰晶在光化作用下各種產物的種類與生成機制，並與天文觀測結果以及相關利用高能離子撞擊冰晶實驗結果相互比對，得出許多一致的結果。更希望這次的實驗結果能提供往後有關含硫星際冰晶模擬演化實驗之基礎。 Astronomers observed the evidence of solid phase S-bearing molecules, OCS and SO2, in the Interstellar Matter (ISM). However, in the other astronomical object like comet and moons of the Jupiter they also observed the solid H2S molecule, but H2S was absent in the ISM. So far the incomplete inventory of S atoms in the interstellar ice is yet an unsolved puzzle. We choose the H2S+CO and H2S+CO2 as ice mixtures and compare their photolysis products and formation mechanism by VUV photon in order to explore the formation mechanism of S-bearing molecules. Also we will try to understand how were the Sn molecules played the role of missing inventory of sulfur atoms. In this study, different source of the C atom from H2S+CO and H2S+CO2 ice mixtures were evaluated and their difference in producing complex molecules in the photolysis process were investigated. Also the relation between the ice mixture composition such as H2S: CO=1:7 & 1:20 and H2S:CO2=1:7 & 1:20 and its correlation to the product were studied. We used a Hydrogen Microwave Discharge Lamp(HMWDL) with MgF2 & CaF2 window and High Flux beamline of the Synchrotron Radiation Research Center (NSRRC) as VUV source to explore the photolysis processes and the variation of products under different UV energy. The Fourier Transform Infrared spectroscopy (FTIR) and Quadrupole Mass Spectrometer (QMS) were employed to analyze the formation mechanism and yield of the products. The results indicate that the OCS yield in the H2S+CO ice mixture is higher than that in the H2S+CO2 ice mixture. It is noted that the OCS yield is only dependent on the initial composition of the ice mixture but not the photon energy. On the other hand, the OCS formation rate only depends on the photon energy but not the initial composition of ices. We observed the S-bearing molecules including H2S2、OCS、CS2 and SO2 in this work. We also confirmed that the SO2 only could be produced in rich O atom environment, i.e. in the H2S+CO2 ice mixture. While studying the formation of CS2, it was found that there were two different reaction paths of the CS2 in the different photon energy. In the NSRRC with 30.4 nm and 58.4 nm, the UV energy is greater than the HMWDL energy. Therefore, the CO molecule can be dissociated by the photon into C atom and O atom, then the O atom and the HS radical combined to form SO molecule, and finally react with CS to produce CS2. While using the HMWDL with MgF2 and CaF2 window, the OCS molecule and H2S combined to form CS2 and H2O. Due to lacking the O atom formation in the HMWDL experiment, the S atom combined each other to form the S2 molecule. But the S2 molecule has a symmetrical structure and consequently will not have absorption feature in the IR spectrum. The missing S atom in the interstellar ice could be proposed to be present in the Sn (n=2-8) structure. We report the S2 molecule thermal desorption evidenced in our mass spectrum during the sample warming up process. Unfortunately, the larger structure molecules have the higher thermal desorption temperature than 300K. It is beyond our experimental capability at this moment to directly observe the thermal desorption of larger Sn molecules should they present in our ice residue. It is worth to mention that observed peak position of the OCS absorption feature will be affected by the H2S+CO ice mixture composition. In the higher CO concentration ice mixture, the OCS peak is shifted to higher energy. This blue shift of photolysis produced COS molecule feature is found absent in H2S+CO2 ice mixture. In summary, we report the photolysis products and their possible formation mechanism in the H2S+CO and H2S+CO2 ice mixture. Comparison with the astronomical observations and the related experimental results using ion bombardment were discussed and found in good agreement. Wish the experimental results in this work can lay down a milestone for the S-bearing interstellar ice related research in the future.
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