Electron-induced diffusion within astrophysical layered ices

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题名:	Electron-induced diffusion within astrophysical layered ices
作者:	李俊逸;Lee, Chun-Yi
贡献者:	物理學系
关键词:	星際混和冰晶;電子照射實驗;擴散作用;Interstellar ice mixture;Electron irradiation;Diffusion
日期:	2024-08-15
上传时间:	2024-10-09 15:48:43 (UTC+8)
出版者:	國立中央大學
摘要:	在分子雲當中，宇宙冰晶常常會形成雙層結構，由CO:CH3OH的混和冰晶覆蓋在H2O:NH3:CH4混和冰晶之上。在寒冷的星際環境中熱擴散作用十分微弱，而無處不在的宇宙射線顯得格外重要，宇宙射線除了會引發的化學反應和分子脫附作用外，輻射誘導的擴散作用更會使得冰晶分子的距離縮短並促進化學反應。在本研究中我們使用1 keV和2 keV的電子照射雙層冰混合物。底層中的13CH4作為示蹤劑用來生成13CO。隨著照射時間的增加，底層的產物擴散到覆蓋層中，導致13CO的脫附比例逐漸增加。我們透過線性擬合中期的13CO脫附比例，估算定義出擴散時間和擴散距離，並根據布朗運動中的擴散時間和擴散距離關係式評估出擴散係數。從我們的實驗結果顯示，使用較高能量的電子照射會導致較大的擴散係數，然而，比較X-ray與電子照射下的擴散係數時，2 keV電子仍然會低於X射線源(250-1250 eV)，這是由於X射線具有較高的穿透力並在下層冰晶中產生大量的二次電子使下層產物的擴散作用更為旺盛。這一系列的實驗驗證了輻射誘導在雙層冰混合物中發生的擴散現象，同時也能評估冰晶的擴散係數。在未來，這方面的研究仍需要更深入探討其擴散過程的機制以及會影響擴散係數的因素，像是下層所進行化學反應時造成的熱，或是上層冰晶不同裂解效率的影響。;In molecular clouds, the icy mantles are anticipated to form a bi-layered structure, CO:CH3OH/H2O:NH3:CH4 (Ciaravella et al. 2020 [1]). Cosmic rays, which are ubiquitous, stimulate various chemical reactions and molecular desorption. In the cold interstellar environment, radiation-induced diffusion is crucial for bringing molecules closer together and facilitating chemical reactions, rather than relying on thermal diffusion. In this study, we irradiated the bi-layered ice mixtures with 1 keV and 2 keV electrons. The 13CH4 in the bottom layer was used as a tracer to produce 13CO. As the irradiation time increased, the products in the bottom layer diffused deeper into the cap layer, resulting in a gradual increase in the fraction of 13CO. The desorption signals of 13CO were linearly fitted to calculate the diffusion time and length. Our results revealed that the diffusion coefficient in the 2 keV experiments was nearly 50% higher than in the 1 keV experiments, indicating that higher energy electrons result in higher diffusion coefficients. However, the diffusion coefficient in 2 keV electron irradiation was still approximately three orders of magnitude lower than in X-ray sources (250-1250 eV) due to the higher penetrative power and secondary electron generation by X-rays inside the bottom layer. These experiments unveiled the electron-induced diffusion in bi-layered ice mixtures. Further investigations are needed to delve deeper into the mechanisms underlying this diffusion process, including the role of chemical reaction in bottom and a simpler, more controlled environment in the cap layer.
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