| 摘要: | 在太空環境中許多易揮發性分子已被觀測到以固態冰晶形式存在，但是在溫度極低的冷星雲與星系際空間（< 20 K），天文學家卻觀測到易揮發性分子以超乎預期豐富含量的氣態形式存在，這樣的觀測結果與吸積模型的預測大相徑庭，因此天文學家提出非熱效應導致易揮發性分子脫附的可能性。在溫度極低的天文環境中，星際冰晶受到不同能量來源的作用（宇宙射線與能量光子），除了誘發冰晶分子的化學作用外，也能導致冰晶分子的脫附效應。根據天文學家的模擬計算，真空紫外光子誘發冰晶分子脫附的過程，對於星際冰晶是一個相當重要的非熱脫附來源。本論文著重於研究星際冰晶在真空紫外光與X光作用下所引發的光脫附與光化學反應，其中更致力於了解主宰光脫附效率的參數。此外，冰晶分子生長溫度條件對於冰晶的光物理特性也是本論文探討的重點之一。
本論文的研究方式是利用一套低溫超高真空系統模擬天文環境的低溫與低壓。藉由傅立葉轉換紅外光譜儀與四極柱質譜儀監控冰晶分子在能量作用下所產生的化學演化與脫附過程。在冰晶成分上，我們選擇了天文學家相當關注的一氧化碳、二氧化碳、氮與甲烷等分子，以單一分子冰晶或是混和冰晶的方式進行能量作用實驗。在此研究中能量來源分為微波氣體放電管搭配氟化鎂光窗所產生的真空紫外光源（114--170 nm;7.9--10.9 eV）與同步輻射研究中心TLS 08B光束線所提供的X光光源（250--1250 eV）。本論文嘗試改變冰晶分子的生長溫度、生長角度與、混和冰晶比例與光通量等參數，進而討論上述參數對冰晶分子的化學衍化與光脫附效應的影響。
光脫附的研究結果顯示一氧化碳冰晶與二氧化碳冰晶對於生長溫度的反應截然不同。一氧化碳冰晶隨著生長溫度越高其光脫率值越低，尤其是在臨界相變溫度的區域會有更加顯著的冰晶結構影響光脫附率。二氧化碳冰晶的光脫附率不論是在勻相結構或是晶質結構下，幾乎沒有差異。不論是溫度對於一氧化碳冰晶的光脫附影響，或是二氧化碳冰晶光脫附率與文獻的差異，皆能在考慮冰晶分子的真空紫外光吸收截面與光源能量譜分布的影響下獲得完美的詮釋。此外，在一氧化碳與氮分子及二氧化碳的混和冰晶，實驗結果證實冰晶中的能量傳遞是經由間接電子躍遷造成冰晶分子的脫附現象。
在能量光子照射冰晶的研究中，根據不同成分的冰晶樣本與能量源可以從紅外光譜上偵測到各式各樣的光化產物，其中質量較大的複雜有機分子能同時經由紅外光譜與質譜來標定分子的種類。除了照光過程外，一些大質量的分子也能在回溫過程中被偵測到，而這些複雜有機分子增添了天文環境中化學衍化的多樣性。冰晶的紅外光譜對於天文觀測中的分子標定是不可或缺的，也能提供星際冰晶生成溫度歷史的參考。本論文所設計的研究實驗，是追求對於天文物理與天文化學能有更完整的認知。;Numerous solid molecules have been observed in various interstellar regions, and the abundant gaseous molecules are also detected in some environments with low temperatures beyond expectation, like the interstellar medium (ISM) and cold dense clouds. The radiation of the energetic cosmic rays, VUV photons, electrons, and X-rays impinge on the dust grains, resulting in the desorption of the ice mantles. The non-thermal photodesorption process has been seen as an indispensable origin of the overabundance of molecules in the gas phase in the cold regions.
Hence, energetic radiation experiments are of significance for studying the details of photodesorption and photochemical processes. The VUV and X-ray photo-processing on the pure and ice mixtures are focused to figure out the governing parameters on the photodesorption yield, as well as the photochemistry happening simultaneously during irradiation. The fundamental physical properties of the ice are involved in the discussion.
The experiments are conducted through the ultra-high vacuum (UHV) chamber, equipped with a closed-cycle helium cryostat to mimic the low pressure and low temperature near the cold dense clouds and the interstellar medium. The evolution of the solid ice is observed by a transmission Fourier transform infrared spectrometer (FTIR), while a quadrupole mass spectrometer (QMS) is used to detect the gaseous desorbing signals. Different kinds of ice samples are selected to be the sample, such as pure CO2, CO, CH4, the mixtures with N2, and the H2O:CO:NH3. The samples are irradiated by the VUV photons, generated by the Microwave-Discharge Hydrogen-flow Lamp (MDHL) produced in the laboratory with the wavelength 110--180 nm. The other energetic source performed in this thesis is the X-ray, provided by the National Synchrotron Radiation Research Center (NSRRC) with the energy 250 to 1250 eV. Various parameters are designed to investigate the photon-induced desorption and chemical reactions, including the different deposition temperatures, deposition angles, ice mixing ratios, and photon fluxes.
The parameters affecting photodesorption are revealed in detail. In the case of deposition temperature issue on the pure CO2 and CO ice, only the latter one shows the temperature dependency, and the desorption factors are developed for determining the photodesorption yields. Another important role is the absorption cross section of the solid ice, which should be considered when we discuss photodesorption. Moreover, we found that the angle between gas inlet and the substrate falls at 70$\degree$ can create a maximum photodesorption yield of CO ice, along with a columnar structure of ice. The energy transfer between CO2/CO ice with N2 is discussed, confirming that the indirect desorption induced by electronic transition (DIET) dominates the photodesorption processes. During irradiation, the solid products are observed on the IR spectra according to the ice samples and the radiation sources. There are Complex Organic Molecules (COMs) identified by both the IR spectra and the mass spectra, and the chemical formation routes of these molecules are represented. Some larger molecules are detected during the temperature-programmed desorption (TPD) processes, supporting the molecular diversity in the interstellar regions. The infrared spectra of the ice at different temperature environments are required not only to identify the peak position of the molecules but also to provide information to track the temperature history of the ice mantles on the dust grains. With the experiments performed in this thesis, we hope to contribute to a more complete knowledge of astrophysics and astrochemistry. |
