| 摘要: | 天文觀測發現星際空間不是空無一物。通常星際區域的特徵是低密度和低溫度，儘管在某些位置可能會出現極高的密度和溫度。星際介質暴露於各種輻射源中，例如宇宙射線和恆星光(紫外線和X射線)，並且包含磁場。
在壓力高於一般星際物質(稱為彌散雲或密集分子雲)平均壓力的區域中，漂浮著許多簡單分子(如氫氣或是一氧化碳)，擁有足以被現代設備所量測到的濃度。這些分子因細小且由矽酸鹽和碳質所構成的固態粒子(稱為星際塵埃)—稀疏地填充星際物質內。塵埃遮蔽部分恆星光使得分子能穩定存在。因此，在分子雲內部的分子能夠避免紫外線的光解並擁有較長壽命。複雜的化學物質也因此能夠在分子雲中形成。然而，許多的研究指出氫分子只能透過在塵埃顆粒的表面上的反應形成，無法在氣相反應中取得。
黑暗星雲，恆星光被高度屏蔽的區域，塵埃上被氣相沉積的冰幔所覆蓋。與觀測的結果相符，在高密度區域、大質量核、小質量核、和恆星形成的區域中，許多化學物質如水、一氧化碳、二氧化碳、甲醇、甲醛、甲烷及氨等，以保存在塵埃上的形式被觀測到。擁有更複雜結構的分子如乙醇、醋酸及乙醇醛等也在這些高密度的區域中觀測到相對較高的含量。這些分子包含了重要的有機元素 (氫、碳、氮及氧) 是主要的有機物質，其中有些被天文學家認為與天文生物學有關。這些大型天文分子，被稱為複雜有機分子。這些分子形成的基本概念被認為是透過簡單的氫化分子如甲醛、甲醇及氨在冰冷的塵埃顆粒表面上所形成。然後透過固態化學反應大幅的提升化學複雜度。
恆星是由分子雲的重力坍縮所形成。在最初的階段，通過吸積和射流的共同作用，新生恆星（稱為原恆星）逐漸將坍塌的星雲轉變為稱為原行星盤的平盤，因為它是行星形成的原料。
本論文主要以天文學實驗，研究在星際區域和星周盤區域中，電子和X射線的能量的作用下，簡單的星際冰晶可能如何形成複雜有機分子，並且了解複雜性如何在冰晶中提升並透過非熱效應脫附貢獻至氣相中。
本研究中所呈現的部分實驗在新建置的實驗系統上所進行。其命名為星際能量作用系統或以英文稱之為Interstellar Energetic-Process System，為一超高真空系統。建置目的是研究不同輻射源作用下星際冰晶的演化。電子束由電子槍提供，而X射線則由新竹同步輻射中心八號光束線(BL08B)提供。同步輻射光源有著高亮度及連續光譜的優點是良好的光源設施。X射線光譜覆蓋250至1250電子伏特，其光譜與年輕且與太陽相同型態的恆星X射線光譜相似。在電子束的研究中，我使用帶有150至1000電子伏特的電子。這個能量範圍相似於金牛座T星所發射的X射線或是宇宙射線與物質作用後所產生的主要電子能量相近。此論文中使用三種冰晶樣本，分別是純一氧化碳冰晶、加入水的雙成分混合冰晶及加入氨的三成分混合冰晶。冰晶樣本分別進行電子束及X射線的研究，除三元樣本暫時只對X射線完成相關研究。
純一氧化碳冰晶與加入水的混合冰晶在X射線及電子束的作用下皆生成了相同的化學物質。冰晶在X射線的照射下同時伴隨著光子與粒子(電子)的作用。X射線被原子吸收後游離並發射內殼層的電子(稱之為光電子或是主要電子)。主要電子留下的空缺由高能階的電子所充填，並發射出第二顆電子(歐傑電子)。這兩種電子透過與物質的作用不斷的釋放能量到物質中並產生瀑布般的二次電子，驅動著冰晶內的化學反應。另一方面，電子束的電子能夠透過游離或是激發分子。當電子所攜帶的能量足夠高時，能像X射線一般游離原子的內殼層電子，但此狀況不在本論文探討的能量範圍內。如前面X射線的例子所述，電子透過與物質的交互作用釋放能量並產生大量二次電子。因此不論在X射線或是電子束的照射下，冰晶內的化學反應皆由二電子所推動。
在一氧化碳與水的混合冰晶中因為豐富的氫原子與羥基，一氧化碳的氫化反應 (例如生成甲醛與甲醇)與氧化反應(例如二氧化碳)是主要的反應。X射線照射下的三成分混合冰晶產生許多與生命可能的來源相關的有機化合物，如異氰酸、甲醯胺及最簡單的氨基酸，甘胺酸。X射線的照射過程中，偵測到許多質量從冰晶上脫附即使溫度在11 K遠低於這些大質量的揮發溫度。二氧化碳在所有的冰晶樣本及兩種輻射源的照射下，都能觀察到脫附。在電子束的研究中發現純一氧化碳冰晶所產生的二氧化碳大部分參與脫附，參入水後的冰晶則參與的比例下降，此比例與入射電子能量有關。
三成分冰晶(水:一氧化碳:氨)在X射線照射下，四極質譜儀偵測到許多與複雜有機分子相符質量的光脫附，像是異氰酸甲酯、甲酸及甲醯胺。若是這些相應的質量真的為複雜有機分子，這些分子化學起源的爭議將會傾向以固態反應來解釋。;Astronomical observations show that the interstellar space is not empty. Typically, interstellar regions are characterized by low density and low kinetic temperature, although at some locations extreme densities and temperatures can occur. The interstellar medium is exposed to various radiation sources, such as cosmic rays and starlight (ultraviolet and X-rays), and contains magnetic fields.
In regions over-pressured with respect to the average pressure of the general interstellar medium (called diffuse and dense molecular clouds) are present a number of free floating simple molecules (such H2 and CO), able to reach concentrations high enough to be detected. These molecules owe their existence to tiny solid particles (the so called cosmic dust) ~\textendash~ sparsely populating the interstellar medium ~\textendash~ composed by silicate and carbonaceous materials. The primary role of dust in molecular survival consist in the (partial) shield of clouds from ambient starlight. Therefore, molecules in cloud interiors are protected from photodissociation by UV starlight and have a long lifetime. A complex chemistry is therefore able to build up in cloud interiors. However, hydrogen molecules cannot be formed by gas-phase reactions and are formed in reactions at the surfaces of dust grains, as demonstrated by a number of appropriate laboratory experiments.
In highly shielded regions (dark clouds) where the starlight is largely excluded, dust grains should be coated with icy mantles of condensed gas. This is indeed what is observed in very dense regions, in high- and low-mass cores in star-forming regions, where water, carbon monoxide and dioxide, methanol, formaldehyde, methane, ammonia, and other species are observe to be resident on dust grains. More complex species, such as ethanol, acetic acid, and glycolaldehyde, are detected at relatively high abundances in these regions. These molecules, composed by the most important biogenic elements (hydrogen, carbon, nitrogen, and oxygen) are mainly organic species, and are considered by some astrochemists to be related to the emerging subject of astrobiology. These larger astronomical molecular species are called complex organic molecules, or COMs, for short. The basic idea for the formation of these molecules is that simple hydrogenated molecules like H2CO, CH3OH and NH3 are formed on the cold grain surfaces. Then, the chemical complexity can be dramatically enhanced, through some form of solid-state chemistry.
Stars are formed by the gravitational collapse of molecular clouds. In the very initial phases, through the combined action of accretion and jet flows, the nascent star (called protostar), gradually transforms the collapsing cloud into a flat disk called the protoplanetary disk, because it constitutes the raw material from which planets will form.
In this work, I will perform laboratory experiments, investigating how simple interstellar ices may form COMs under the transforming action of energetic sources characteristic of interstellar and circumstellar regions, electrons and X-rays, and I shall also address the specific problem on how chemical complexity arising in the ices could be ejected into the gas-phase without thermal desorption.
The experiments presented in this dissertation have been performed with the new Interstellar Energetic-Process System, an ultra-high vacuum facility specifically designed for the study of the irradiation of interstellar and circumstellar ices. Electrons are produced trough an electron gun, while X-rays have been collected at the at National Synchrotron Radiation Research Center. Synchrotron light sources are ideal because of their high intensity and wide wavelength coverage. The X-ray spectrum ranges between 250 and 1250 eV, with a shape roughly resembling the X-ray spectrum of a young solar-type star. In the electron radiolysis I use energies from 150 to 1000 eV. Such a range is similar to the primary electron spectrum produced by X-rays emitted by T-Tauri stars or cosmic-rays interacting with the gaseous interstellar medium. In this thesis I study three kinds of samples, pure CO ices, a binary mixtures of H2O:CO, and a ternary mixture of H2O:CO:NH3. In the first cases, irradiation has been performed with both X-rays and electrons, while the ternary mixture is irradiated with just X-rays.
Pure CO ice and H2O:CO ice mixture irradiated by X-rays and electrons produce the same chemical species. X-rays irradiation of ices is the result of photons and particles processing. The absorption of X-rays results in the ionization of an inner shell of atoms composing a molecule, in which a photo-electron (the primary electron) is ejected. As an electron from the higher energy levels fills the core vacancy a second electron, the Auger electron, is ejected into the continuum. These two electrons will deposit and degrade their energies interacting with the ice materials, and creating a cascade of secondary electron that drives the chemistry in the ices. On the other hand, in the electron radiolysis, electrons impinging on the ice can excite and ionize the molecules or if sufficiently energetic can (not the case in our experiments), as X-rays, ionize inner-shell electrons of the atoms in the molecules. As in the previous case involving X-rays, the electrons slow down loosing their energy interacting with the ice, and producing secondary electrons. Therefore, the chemistry promoted in X-ray irradiation or electron radiolysis are mainly promoted by the secondary electrons.
In H2O:CO ice mixture, because of the rich environment in H atoms and OH radicals, hydrogenation of carbon monoxide (e.g., formaldehyde, and methanol) and oxidation (e.g., carbon dioxide) are the main reactions. X-rays irradiated H2O:CO:NH3 produces many organic compounds of prebiotic relevance, such as isocyanic acid, formamide, and the simplest amino acid, glycine.
During X-rays irradiation, several masses have been observed to leave the ice, despite the very low ice temperature, 11 K, well below its sublimation temperature. The desorption of carbon dioxide have been detected in all samples, and for all radiolysis. In electron irradiated CO ice experiments, it is found that almost all of the CO2 formed in the ice desorbes, while only partial CO2 desorption occurred in water mixtures. The desorbing ice fraction is related to the energy of the impinging electrons.
In X-rays irradiated H2O:CO:NH3 ice mixtures, QMS detections are consistent with photo-desorption of COMs such as methyl isocyanate, formic acid, and formamide. If this would be actually the case, the controversy on the chemical origin of such species would be definitely solved in favor of solid state reaction channels. |
