本論文主要探討嘧啶加水的混合冰晶在天文環境中(14K，< 10-9 torr)，於不同光子能量作用下的光化機制。光源分別以氫氣微波放電管產生的真空紫外光(VUV)以及同步輻射研究中心提供之極致紫外光(EUV)，並挑選不同的光子能量範圍進行實驗：VUV(114−180 nm)，白光(25−300 nm)，He I (58.4nm)，He II (30.4 nm)。使用紅外光譜儀量測嘧啶的光解速率以及光化產物的種類與生成速率。並以液相及氣相層析圖譜分析受光化作用後回溫至室溫的殘餘物質。
實驗結果中發現嘧啶相當容易受光子作用而被消耗，並且生成諸多已被觀測到的星際分子:CO、CO2、HNCO、OCN-、CN-、H2CO、HCONH2。並於所有實驗結果中明顯地偵測到尿嘧啶以及其前體4(3H)-pyrimidone的生成。而不同入射能量的光子作用對嘧啶光消耗以及光化產物的產率是相當地不同。這些結果說明原生有機分子是可以在VUV/EUV光子作用下生成，並且在被水包覆之下存留而抵達地球表面。 ;Carbon-rich meteorites and carbonaceous chondrites are found to contain many biologically relevant organic molecules. The presence of nucleobases (building block of DNA and RNA) in carbonaceous chondrites indicates that molecules of biological components can be formed in non-terrestrial environments via abiotic pathways. Recent laboratory experiments and ab initio calculations had already shown that the VUV irradiation of pyrimidine in H2O ices leads to the formation of oxidized pyrimidine derivatives, including the nucleobase uracil.
In this study, we focus on the different photo-energy effect on photon irradiation of pyrimidine in H2O-rich ice mixtures under astrophysical conditions (<10-9 torr, 14K).The vacuum ultra-violet (VUV) and extreme ultra-violet (EUV) photons provide by microwave-discharged hydrogen-flowed lamp (MDHL) and synchrotron radiation in several ranges: VUV (114−180 nm), 0th order light of synchrotron radiation beamline (25−300 nm), He I (58.4 nm), He II (30.4 nm). The photo-destruction rate of pyrimidine and photo-production rate of products were measured by FTIR spectroscopy. Afterwards, the photo-irradiated samples were warmed up to room temperature and the residues were analyzed by liquid and gas chromatography all together.
The results show that pyrimidine was easy to be depleted by VUV/ EUV photons, and formed many interstellar molecules such as: CO, CO2, HNCO, OCN-, CN-, H2CO, HCONH2, etc. All the photo-products in our experiments evidenced that 4(3H)-pyrimidone and uracil could be conclusively identified. The photo-destruction rate of pyrimidine and photo-production rate of products are found photon energy dependent. These results illustrate that the prebiotic components can be formed and survive in astrophysical environments which is subjected to VUV/EUV irradiation constantly. Finally they can be wrapped by H2O-rich ice matrix and fall down to Earth’s surface.
