UV/EUV光子對星際冰晶的光化作用; UV/EUV photolysis of interstellar ice analogs

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题名:	UV/EUV光子對星際冰晶的光化作用;UV/EUV photolysis of interstellar ice analogs
作者:	陳俞融;Yu-Jung Chen
贡献者:	物理研究所
关键词:	光化作用;星際冰晶;UV photon;photolysis;interstellar ice;EUV photon
日期:	2007-03-26
上传时间:	2009-09-22 10:53:48 (UTC+8)
出版者:	國立中央大學圖書館
摘要:	近數十年關於星際冰晶演化機制的研究，主要以高能粒子或是真空紫外光子的影響探討為主，其中真空紫外光子的部分又以121.6 nm（Lyman α）（10.2 eV）的光子作為探討的重點。在本論文中我們藉由使用國家同步輻射研究中心High-Flux光束線所提供之同步輻射光作為實驗中所需之光子的來源。High-Flux光束線所能提供的光子能量範圍介於4-45 eV (27.5-310 nm)，因此我們能夠探討不同能量的光子對於星際冰晶演化過程所造成的影響。在H2O+CO2+NH3混合冰晶的研究中，我們利用同位素、紅外光譜與質譜分析等方法確認了carbamic acid（NH2COOH）的存在，並證明在光子的作用下其結構為符合量子化學計算之中性分子，不同於離子撞擊下所產生之離子結構。在H2O+CO2+NH3混合冰晶殘餘物質的分析中，我們首次發現無機的混合冰晶經由光子的光化作用後能夠產生有機的胺基酸，其中包含有多種蛋白質型胺基酸。多環芳香烴碳氫化合物（PAHs）是一種散佈於星際空間中的物質，而Naphthalene（C10H8）是結構最簡單的PAHs。我們利用4-20 eV與13-45 eV能量的光子照射H2O+C10H8+NH3混合冰晶，發現光化產物的產量與照射的光子能量有著正比的關係。此外也首次在光子照射含C10H8的混合冰晶實驗中，觀測到含氮多環芳香烴碳氫化合物（PANHs）的生成。在H2O+C10H8+NH3混合冰晶殘餘物質的分析中也量測到了胺基酸的存在。在H2O+N2+CH4混合冰晶的實驗中，我們分別使用HⅠ（121.6 nm）、HeⅠ（58.4 nm）與HeⅡ（30.4 nm）三種不同能量的光子來探討其光化產物的差異性。在使用58.4 nm與30.4 nm光子的情況下，皆觀測到了具有CN鍵的分子形成。不論在有機或是無機混合冰晶的光化實驗中，我們皆能觀測到胺基酸的生成。這樣的結果也說明了組成生命的一些基本有機分子，極可能是在外太空環境中形成並藉由隕石攜帶至早期的地球環境上。 In the last decades, many results were obtained from laboratory experiments of ion and photon irradiation of ice mixtures at low temperature. Photon irradiation studies were almost all focused on the effects of 121.6 nm (Lyman α) photons. In this report, we used synchrotron light provided by the high-flux beamline of the National Synchrotron Radiation Research Center (NSRRC) as an ultraviolet (UV)/extreme ultraviolet (EUV) photon source. Since this high-flux beamline can provide photons with energies ranging from 4 to 45 eV, we can study the photolysis of interstellar ice analogs with different photon energies. The irradiation of H2O+CO2+NH3 ice mixtures could allow us to confirm that small organic compounds were formed after photolysis and warming up to room temperature thanks to infrared spectroscopy, mass spectrometry and isotope labeling of the starting mixtures. For instance, carbamic acid (NH2COOH) was observed to be formed in its neutral form after a 4–20 eV photon irradiation around 250 K, unlike what was reported previously after energetic proton bombardment. We also irradiated ice mixtures containing polycyclic aromatic hydrocarbons (PAHs). These compounds are common throughout the Universe, naphthalene being the smallest PAH since it contains only two aromatic cycles. Two broad-band energy ranges of UV/EUV photons (4–20 eV and 13–45 eV) were used for the irradiation of H2O+NH3+C10H8 ice mixtures at 15 K. We observed that polycyclic aromatic nitrogen heterocycles (PANHs) such as quinoline (C9H7N) and phenanthridine (C13H9N) might have formed, and could estimate the production yields of the photo-products CO, CO2 and OCN- and see that they are significantly higher in the 13–45 eV irradiation experiment than when 4–20 eV photons are used. We finally irradiated H2O+N2+CH4 ice mixtures in order to study the difference of photo-products formed after different irradiation photon energies, namely HI (121.6 nm), HeI (58.4 nm) and HeII (30.4 nm) molecular and ion lines. We observed CN bearing molecules were produced in 30.4 nm and 58.4 nm experiments. In all the experiments mentioned before, amino acids were formed after UV/EUV photons (4–20 eV) irradiation at 15 K and subsequent warming up to room temperature. This result, obtained in experiments simulating astrophysical environments, is important since such organic molecules are probably formed in extraterrestrial environments and are believed to have played a significant role for the origin of life, after having been delivered to the early Earth by comets, asteroids and their interplanetary dust particles.
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