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研究生(外文):Po-yu Huang
論文名稱(外文):The photolysis difference between Polar and non-polar molecule in CH4-ice mixture
指導教授(外文):Tai-Sone Yih
外文關鍵詞:Interstellar icephotolysisInfrared SpectrumIR SpectrumMethanenon-polarPolarNitrogenWaterUltravioletVUVTransneptunian objectTNO
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近年來,根據天文學家對 TNOs (海王星外星體)的觀測結果,判定這類星體主要含有H2O、CH4、N2 等物質。此論文中以上述氣體做為冰晶之混合成分,探討極性分子(H2O)與非極性分子(N2)對含有CH4 混合冰晶之光化作用有何差異。

本論文的實驗使用三種不同成分的混合冰晶:N2:CH4=1:1、CH4:H2O=1:1 與 N2:CH4:H2O=1:1:1,並以同步輻射研究中心的 High Flux 光束線做為光源。搭配紅外線光譜儀及四極質譜儀,來對光化產物的生成機制加以分析。我們發現當冰晶內含有N2分子時,其CH4的光解速率相對顯得較慢。這是由於 Matrix-isolated 的現象,導致CH4分子光解後產生的CH3自由基受到N2的隔絕而不易反應,因而C2H6與C3H8等烷類的產量也相對來得少。除此之外,從 CH4 大量光解出的H原子,也會影響含氮光化物的產量,使CCCN的產量相對於HNCCC來得小。

本論文同時也比較同步輻射研究中心的High Flux光束線與實驗室的氫氣微波放電管做為真空紫外光源時,所造成N2:CH4:H2O=1:1:1混合冰晶光化產物的差異。其結果顯示同步輻射中心光源的光子能量4~20 eV,已具有將N2分子的鍵結打斷9.76 eV並產生含碳氮鍵之光化產物的能力。然而N2分子在氫氣微波放電管(10.2 eV)的照射下卻無法被光解而產生含碳氮鍵光化產物。然而當我們採用同時長冰與照光的實驗方式時卻能觀測到含碳氮鍵光化產物的生成,因而推測 N2分子在固態時的鍵結能應該不同於氣態下的鍵結能。

此外在同步輻射光照射的含氮冰晶光化實驗中,發現CN^-光化產物在低溫狀態下形成(18 K),這與 Moore 及 Hudson 在回溫過程中才發現的結果有所不同。然而回溫過程中CN^-的紅外線吸收變化行為與 Moore 及 Hudson 的實驗一致。N2+CH4混合冰晶的光化實驗中發現多種 Nitrile(R-CN)光化產物,然而在極性分子 H2O的混入後,便無法在紅外光譜上偵測到 Nitrile 或 Isonitrile 等光化產物的形成,而是偵測到明顯的OCN^-吸收訊號。

由實驗結果發現非極性分子N2與極性分子H2O會導致CH4冰晶在光化過程中有不同的演化機制。本論文的研究重點主要是觀測其中的變化來探討光化過程中,其光化產物之相互競爭與催化效應,並與天文觀測結果或與已發表於國際期刊上之相關冰晶模擬實驗結果相互比對,提供往後有關 TNOs 模擬實驗研究之參考。
The astronomical observation in resent years shows that the main composition of those TNOs ice are H2O, CH4 and N2. We choose these species as the ice composition and
compare their difference in reaction products after exposed to and altered by VUV photons.

N2 : CH4 = 1 : 1, CH4 : H2O = 1 : 1 and N2 : CH4 : H2O = 1 : 1 : 1 mixtures were prepared in gas-phase before condensation on to a precooled (16K) substrate. We
use the High Flux beamline at Synchrotron Radiation Research Cente as the VUV light source. New molecules produced during photolysis were identified on the basis of their characteristic infrared feature. Quadrupole mass spectrometer was employed to detect the desorbed products during ice warming.

CH4 molecules being trapped within a comparably stable N2 species in the N2-containing ices. This will induce Matrix-isolated phenomenon which blocks the radicalradical interaction between CH3 molecules and reduces the destruction rate of CH4 durung photolysis. Therefore, less C2H6 and C3H8 reaction products were detected. Moreover,
lots of H atoms detached form the CH4 molecures will recombine with CCCN molecules to form HNCCC. That’s why we can only find a weak Mid-IR absorbance band of CCCN at 2194 cm^-1, but a clear absorbance feature of HNCCC at 2205 cm^-1 wavenumber.

We also used the hydrogen microwave discharge lamp as the VUV light souce in the laboratory(10.2 eV). And compare the reaction products difference with the High Flux beamline (4 ∼ 20 eV ) after solid N2 + CH4 + H2O mixtures were photolysized by them. However, we didn’t find any N-containing reaction products after the ice was photolyzed
by hydrogen lamp. Supposedly, the hydrogen lamp already have enough photon energy to destruct the bonds in the N2 molecules which bonding energy is about 9.76 eV. After
changing the experimental methodes by photolyzing and condensing the gas-phase mixtures onto the substrate at the same time, lots of new species and products yields
are found by their IR feature. And the N-containing products also shown up in those products. So we infer that the bonding energy of N2 ice is higher than 10.2 eV at least.

N2-containing ice was processed by UV-photons from the High Flux beamline light source. And we find the CN^-’s IR feature at 2082cm^-1 wavenumber. However, the product was only found during ice warming process in Moore and Hudson’s experiment. By comparing the behavior of the CN^-’s IR feature during warming, we confirmed that it’s the same species that also detected in Moore and Hudson’s experiment, and believe that the CN^- will forms after VUV photolysis, but the photochemistry procedurs still needs further study to confirm. There is also lots of Nitrile (R-CN) reaction products synthesized during N2 +CH4 mixtures were processed by UV-photons. However, if there are H2O in the ice mixtures. We can’t find any Nitrile or Isonitrile yields but OCN^- will show up instead.

From the above results, we found that the non-polar molecules–N2 and the polar molecules–H2O will have different influences on the products of the CH4-containing ice after photolysis. By comparing to the astronomical observations and the related publications. We confirmed that those reactions and phenomenons occuring on the TNOs. The discoveries and analysis results in this work can be used as one of reference for TNOsrelated research and other ice simulated experiments in the future.



5.NSRRC-High Flux zero order light 與 氫氣微波放電管1216A(Lyman-alpha)對N2+CH4+H2O混合冰晶之光化作用差異-77-
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